JPH0758613B2 - Battery separator, method of manufacturing the same, and lithium battery using the battery separator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a battery separator using an improved porous polypropylene film, a method for producing the same, and a lithium battery using the same.

[Prior Art] As a battery separator, a synthetic resin non-woven fabric or a glass fiber non-woven fabric is generally used. 2. Description of the Related Art In recent years, electronic devices have been rapidly made portable, miniaturized, lightened, thinned, and made into cards, and a battery as a power source thereof is also required to have high energy density and thinning. As one of the methods for that purpose, the separator in the battery element has been improved. That is, a method of using a porous film as a battery separator has been reported in order to meet the demands for improving battery performance and reducing battery thickness by increasing the amount of active material and reducing internal resistance. For example, JP-A-62-1
222562, JP-A-63-126159 and the like disclose a method of impregnating a liquid electrolyte or a semi-solid electrolyte using a microporous polypropylene film as a separator.

[Problems to be Solved by the Invention] However, the microporous polypropylene film used in these examples has a porosity of at most about 45%. Methods for producing these conventional porous polypropylene films include, for example, Japanese Examined Patent Publication No. 46-40119.
It is disclosed in, for example, Japanese Patent Publication No. 50-2176 and Japanese Patent Publication No. 55-32531. Most of the porous thermoplastic resin films and the method for producing the same disclosed in the above publications have a temperature of not less than room temperature or the second-order transition temperature of the thermoplastic resin to be used after first heat treating the molded polypropylene film. The skeleton is a method in which the pores are generated by the stretching treatment to form a porous body, and the pores formed by re-stretching at a higher temperature are heat-treated again by heat-treating again. However, the porous polypropylene film obtained by the above method cannot retain the electrolyte sufficiently due to its low porosity, and since the region capable of ionic conduction is small, the internal resistance becomes high, and this was used for the separator. The battery has a problem of low performance.

Further, when it is attempted to increase the porosity in the conventional method for producing a porous polypropylene film, the formed pores become nonuniform, and when used as a battery separator, fine powder in the electrode active material passes through the separator. There was a risk of contacting the counter electrode and impairing battery performance.

Therefore, an object of the present invention is to provide a battery separator using an improved porous polypropylene film having a uniform pore size distribution even if the porosity is increased and showing excellent properties, and a lithium battery using the same. It is in.

[Means for Solving the Problems] That is, according to the present invention, there is provided a battery separator comprising a porous polypropylene film having a large number of fine pores obtained by stretching polypropylene, the porous polypropylene film comprising: A group of unstretched plate-like planes that run at a substantially predetermined interval perpendicular to the stretching direction of the film and are formed substantially parallel to a cross section perpendicular to the stretching direction of the film, and between the plate-like planes of the film. It is formed by a group of stretch-oriented relatively thin fibrils that run substantially parallel to each other in the stretching direction and that are connected to each other between plate-like planes, and the gaps between the thin fibrils that connect between the plate-like planes expand approximately two-dimensionally. A large number of micropores having a substantially uniform shape are formed, and most of the micropores form through-holes and have a substantially uniform pore diameter, and the porosity of the film. Battery separator, characterized in that 45% or more, and a lithium battery using the battery separator, is provided.

Further, as a method for manufacturing the battery separator, a polypropylene stretching step, in a medium selected from the group consisting of nitrogen, oxygen, argon, carbon monoxide, methane and ethane, and the stretching temperature is -70. A method (first method) in which the temperature is not higher than ℃ and the temperature is 50 ° C. higher than the boiling point of the medium below the freezing point of the medium (first method), or the polypropylene stretching step is performed at room temperature in advance. And a stretching strain rate of less than 10% / min in a high temperature range of 110 ° C. to 155 ° C. (second method).

The configuration example of the lithium battery of the present invention is as follows. Note that examples of the constituent materials are given, but the materials are not limited to these.

As the electrolyte membrane, the above-mentioned porous polypropylene film impregnated with a liquid electrolyte or a gel electrolyte is used.

As the liquid electrolyte, a liquid electrolyte in which a lithium ion salt is dissolved is used.

Examples of non-aqueous solvents are propylene carbonate, γ-
Examples thereof include butyrolactone, dimethoxyethane, dioxolane, tetrahydrofuran, dimethyl sulfoxide, ethylene carbonate, acetonitrile, sulfolane, low molecular weight liquid polyethylene oxide and its terminal alkyl etherified products. These may be mixed and used.

In the present invention, as an example of the lithium ion salt, LiCl
Examples include O ₄ , LiB (C ₆ H ₅ ) ₄ , LiCF ₃ SO ₃ , and LiPF ₆ . These may be mixed and used.

The gel electrolyte is a gel obtained by mixing a polymer material with the above liquid electrolyte. Examples of polymeric materials include
Examples thereof include polyethylene oxide, polymethacrylate, acryloyl-modified polyethylene oxide polymer, polyvinylidene fluoride, polyvinyl methyl ether, polyacrylonitrile, and copolymers thereof.

Examples of the negative electrode active material in the present invention include lithium and lithium alloys, for example, alloys of lithium with aluminum, mercury, zinc and the like.

Further, as the positive electrode active material, for example, manganese dioxide,
Examples include molybdenum trioxide, vanadium pentoxide, titanium, or niobium sulfide, chromium oxide, and copper oxide.
Graphite as a conductive agent and, if necessary, a binder such as polytetrafluoroethylene are mixed with these active materials and pressure-molded to be used as a positive electrode plate.

Since the separator of the present invention is made of a polypropylene film and has excellent chemical resistance, it can be widely applied to, for example, alkaline dry batteries and lithium batteries. Also,
It is cut into various shapes according to the shape of the battery used. An electrolytic solution, a gel electrolyte, a fixed polymer electrolyte, etc. are impregnated or compounded between the positive electrode and the negative electrode of the battery to prevent a short circuit between both electrodes and realize the function of ion conduction.

The porous polypropylene film of the present invention has a higher porosity, a uniform shape and a smaller pore size than conventional ones, and has many through holes, and therefore has the above-mentioned function as a separator.

When the porosity of the film is less than 45%, the ion permeability decreases and the internal resistance increases. Further, if the porosity is to be increased, in the conventional production method, the pore size becomes non-uniform, but in the porous polypropylene film of the present invention, the uniformity of the pore size is maintained, and it functions as an excellent separator. Show.

Below, the manufacturing method of the porous polypropylene film in this invention is demonstrated.

In the production method of the present invention, since the conditions for the porosity thereof are completely different from those of the conventional method, the polypropylene to be used is not particularly limited, and a homopolymer of propylene and a block copolymerization of polypropylene with other monomers or oligomers. Coalescence, random copolymer of propylene and other monomer or oligomer (in the present invention, when described as polypropylene without any particular limitation, the expression polypropylene means a generic term for these) Etc. can be used. The other monomer or oligomer that can be used is not limited as long as it can be copolymerized, and examples thereof include ethylene and an oligomer derived from ethylene.

In addition, polypropylene containing an additive (material) such as a plasticizer, a colorant, a flame retardant, and a filler can also be used.

First, the polypropylene resin as described above is molded by a known film manufacturing method to obtain an unstretched polypropylene film. Examples of film manufacturing methods that can be used include an inflation film molding method and a T-die film molding method. The molding conditions in such a molding method can be appropriately selected from known techniques. For example, the film forming temperature may be a temperature higher than the temperature at which the polypropylene used can be discharged and lower than the thermal decomposition temperature of the polypropylene. Usually 170-300 ℃, preferably 190-2
70 ° C.

The elastic recovery rate (or draft ratio) of the unstretched polypropylene film obtained by molding is also not particularly limited. However, when an unstretched polypropylene film having an elastic recovery rate (or a draft ratio) of zero (%) to extremely low, that is, an unstretched polypropylene film having extremely low crystal orientation is used, the stretching step of the present invention However, in some cases, it may be difficult to provide the resulting porous film with satisfactory properties. Therefore, it is preferable to set the molding conditions for the unstretched film in consideration of the porosity of the resulting porous polypropylene film and the average pore diameter of the fine pores.

The elastic recovery rate of the unstretched polypropylene film is not particularly limited as described above, but for the above reason, the unstretched polypropylene film represented by the following formula has a temperature of 25 ° C. and a relative humidity of 65%.
In 50% elongation, the elastic recovery rate is preferably 20% or more, and when considering the productivity when using an ordinary molding apparatus, it is in the range of 30 to 95%. Particularly preferred.

Elastic recovery rate (%) = [length after extension-length after extension] /
[Length at extension-length of original film] × 100 Further, considering factors such as the above requirements and productivity, the draft ratio of the unstretched polypropylene film used in the present invention (unstretched thermoplastic resin film) It is desirable that the ratio of the take-up speed and the discharge speed from the die: take-up speed / discharge speed) be in the range of 10 to 6000.

The unstretched polypropylene film may be heat treated before being subjected to the stretching step. By performing the heat treatment before the stretching, the crystallinity of the unstretched polypropylene film can be increased, so that the properties of the porous polypropylene film obtained by stretching are further improved.

The heat treatment is carried out by a method of heating the unstretched polypropylene film in the air heated to 100 to 155 ° C. for 3 seconds or more.

The stretching step of the present invention is performed by any of the following methods.

The stretching step is carried out in a medium selected from the group consisting of nitrogen, oxygen, argon, carbon monoxide, methane and ethane, and the stretching temperature is −70 ° C. or lower, and the stretching point is from the freezing point of the medium. The temperature should be lower than the boiling point of the medium by 50 ° C or lower, or 110 ° C without stretching at room temperature.
The stretching strain rate is less than 10% / min in the high temperature range from ˜155 ° C.

The first method will be described.

In the cryogenic stretching step, the above-mentioned media can be used alone or as a mixture.

An example of a preferred stretching temperature when using the above medium, when using nitrogen, the range of -209 ℃ ~ -146 ℃,
When oxygen is used, the range is −218 ° C. to −132 ° C., when argon is used, the range is −189 ° C. to −140 ° C., when carbon monoxide is used, −205 ° C. The range is -141 ° C, the range is -182 ° C to -111 ° C when methane is used, and the range is -183 ° C to -70 ° C when ethane is used. When the stretching temperature is higher than -70 ° C, for example, when an unstretched film having a low elastic recovery rate is used, the rate of formation of effective through holes by stretching becomes low.

In addition, in the method, the temperature 50 ° C. or higher above the boiling point does not mean a temperature range accurately lower than the temperature 50 ° C. higher than the boiling point, but does not mean a temperature higher than the boiling point by approximately 50 ° C. or lower.

At such an extremely low temperature, the medium exhibits a liquid state, a liquid / gas state, or a gas state, and this stretching step can be carried out regardless of the state of the medium.

It is presumed that the above-mentioned stretching occurs because the crazing action appears when the medium is stretched at an extremely low temperature. In ordinary media other than the above, the polypropylene film becomes a glass state at an extremely low temperature, and is cut without showing elongation to cause a crazing action.

The cryogenic stretching temperature is a temperature of −70 ° C. or lower, and from the freezing point of the medium to be used, it can be carried out in a range of a temperature 50 ° C. higher than the boiling point, but generally, the stretching is near the boiling point of the low temperature liquid. It is advantageous to carry out the treatment at a temperature of 10 to control the production and keep the properties of the obtained porous polypropylene film constant.

The stretching ratio in the above-mentioned cryogenic stretching step is generally in the range of 1 to 200% with respect to the unstretched polypropylene film. However, the preferred stretching ratio is a value in the range of 10 to 150%. With a draw ratio within these ranges, the number of through holes tends to increase as the draw ratio increases, and this tendency is utilized to adjust the average through hole diameter and porosity of the resulting porous polypropylene film to the purpose. It is also possible to do so.

The above-mentioned cryogenic stretching step can be repeated twice or more until the desired average pore diameter and porosity are obtained.

In the above-mentioned specific medium, the polypropylene film is made porous by utilizing a stretching step under cooling at an extremely low temperature, unlike the case of a conventional stretching step near room temperature, for example, from a strain of 50% at 25 ° C. It also effectively acts on a polypropylene film having an elastic recovery rate of less than 40%, and can be an excellent porous polypropylene film having uniform pores and a high porosity.

The polypropylene film which has been made porous through a drawing process at an extremely low temperature in the above-mentioned specific medium is preferably then subjected to a heat setting treatment. The main purpose of this heat setting treatment is heat setting for holding the formed fine through holes. This heat-setting treatment is a method of heating a polypropylene film which has been made porous while maintaining the stretched state at an extremely low temperature in air which is usually heated to 110 to 165 ° C, preferably 130 to 155 ° C for 3 seconds or more. It is carried out by.

When the heating temperature is significantly higher than the upper limit of the stated temperature, the formed fine pores may be closed, and
If the temperature is significantly lower than the lower limit or the heating time is shorter than 3 seconds, heat fixation tends to be insufficient, the formed pores may be closed later, and heat shrinkage may occur due to temperature change during use. Easier to do. The above-mentioned cryogenic stretching and heat setting treatment can be repeatedly performed until the desired average pore diameter and porosity are obtained. That is, the temperature of the film can be returned to room temperature and repeatedly subjected to a cryogenic stretching (and heat setting treatment). The number of through holes formed by repeating the cryogenic stretching can be increased, and the average through hole diameter can be increased.

Porous polypropylene film prepared as described above has a large average pore diameter formed, and also shows good characteristics with high porosity, but further subjecting the above-mentioned porous thermoplastic resin film to a heat stretching step Thereby, the characteristics are further improved.

The thermal stretching step of the polypropylene film which has been made porous through the above-mentioned ultra-low temperature stretching step at least once is carried out as follows. This hot drawing step is mainly performed for the purpose of expanding the diameter of the fine through holes formed at an extremely low temperature. This hot stretching step is carried out by stretching the porous polypropylene film in air heated to 80 to 160 ° C, preferably 110 to 155 ° C. When the heating temperature is higher than the upper limit of the above temperature, the formed fine pores may be closed, and when the temperature is lower than the lower limit, the expansion of the through-hole diameter by stretching may be insufficient. There is.

The stretching ratio in this hot stretching step is usually 10 to 700%, preferably 50 to 550%, with respect to the film length (initial length) before being subjected to the cryogenic stretching step. Draw ratio
If it is lower than 10%, the expansion of the through holes may be insufficient,
If it is higher than 700%, the film may be cut.

The hot stretching step is performed alternately with the cryogenic stretching step described above, or after at least one cryogenic stretching step is completed.

Next, the stretching method of the present invention will be described.

The stretching step in this case is performed at a stretching strain rate of 10% / in a temperature range of 110 to 155 ° C, preferably 110 to 145 ° C.
Do in less than a minute.

When stretching is carried out at a temperature of less than 110 ° C., only those having a small through-hole diameter may be obtained, or the film may be cut at a small stretching ratio and only one having a small porosity may be obtained.

When the film is stretched at a temperature higher than 155 ° C, the thickness or width of the film may be reduced, polypropylene may be melted or partially melted, and pores may not be formed, or only small pores may be obtained. Occur.

On the other hand, when the stretching strain rate is 10% / min or more, only small pores may be obtained or no pores may occur at all.

When the stretching strain rate is less than 10% / min, the average pore diameter of the through holes,
The porosity increases according to the draw ratio.

The draw ratio can be changed corresponding to the average pore diameter of the through holes according to the intended use of the intended porous polypropylene film. The stretch ratio is 100 to 700%, preferably 150 to 600%, relative to the initial length of the unstretched polypropylene film.
Is. If the draw ratio is higher than 700%, the film may be cut.

It is preferable that the polypropylene film made porous through the above stretching step is then subjected to heat treatment.
The main purpose of this heat treatment is heat fixation for holding the formed fine through holes. This heat treatment is carried out by heating the porous polypropylene film while maintaining the stretched state in air heated to 110 to 155 ° C, preferably 130 to 155 ° C for 3 seconds or more.
If the heating temperature is higher than 155 ° C, the formed micropores may close, and if the temperature is lower than 110 ° C or the heating time is shorter than 3 seconds, heat fixation tends to be insufficient. After that, the through holes are closed, and thermal contraction easily occurs due to temperature change during use.

When observed with a scanning electron microscope, the porous polypropylene film formed by the above methods and runs in a direction substantially perpendicular to the stretching direction of the film and is substantially parallel to a cross section perpendicular to the stretching direction of the film. Formed by a group of unstretched plate-like planes formed in the above-mentioned manner, and a group of stretch-oriented relatively thin fibrils that run between the plate-like planes in a direction substantially parallel to the stretching direction of the film and at substantially predetermined intervals and connect between the plate-like planes. The gaps between the thin fibrils connected between the plate-like planes form a large number of micropores having a substantially uniform shape that spreads in a substantially two-dimensional manner, and most of the micropores form through-holes. It is extremely preferable from the standpoint of battery performance to use a film having a substantially uniform pore size and having such characteristics as a battery separator.

[Examples] Next, examples and comparative examples of the present invention are shown and described, but the present invention is not limited to these examples.

The water permeability shown in the following examples and comparative examples is AS
According to the method specified in TM-F317, the obtained porous film is hydrophilized by dipping it in alcohol and a surfactant, and the hydrophilized porous film is mounted on a predetermined holder. The pressure was applied to one side of the film in the thickness direction of the film, and the amount of water per unit time permeated from the pressure side of the film to the other side was measured and determined.

(Example 1) An unstretched polypropylene film was molded by using an inflation molding machine equipped with an inflation molding die having a diameter of 50 mm and a slit gap of 0.7 mm. Molding operation, polypropylene is discharged at a resin discharge temperature of 210 ° C while blowing air into the valve so that the blow ratio becomes 0.7,
The target unstretched polypropylene was obtained by blowing air at room temperature onto the outer wall surface of the film discharged at a position of 5 cm on the die to cool it, and then at a position of 1.8 m on the die by a nip roll at a drawing speed of 36 m / min. I got a film.

The polypropylene used and the molding conditions are as follows.

Polypropylene UBE-PP-Y101J (manufactured by Ube Industries, Ltd.) MFI = 1 g / 10 min Resin discharge temperature 210 ° C. Take-up speed 36 m / min Blow ratio 0.70 The thickness of the obtained unstretched film was 10 μm. The elastic recovery rate from 50% elongation of this film was 80%.

The obtained film was heat-treated at 145 ° C. for 10 minutes without tension. The elastic recovery rate of the obtained film was improved to 87%.

This unstretched film was stretched in liquid nitrogen (−196 ° C.) by 10% with respect to the initial length, and heat-set in a heated air tank at 145 ° C. for 10 minutes while maintaining this stretched state.
After the heat setting is completed, 18 is applied to the length stretched in liquid nitrogen.
Stretching was performed at 0%, and heat-setting was performed in a heated air tank at 145 ° C. for 10 minutes while maintaining this stretched state to produce a porous polypropylene film.

The obtained porous polypropylene film was designated as ASTM F316-8.
According to the method specified in 0, the average pore diameter measured by the half-dry method (hereinafter the same) using ethanol was 0.13.
was μm. The porosity was 54.60% as measured by the mercury porosimetry method (measurement was performed by using POROSIMETRO SERIES 1500 made by CARLOERBA (Italy).] The rate was 23.18 l / m ² · min · atm.

When the surface and cross section of this porous polypropylene film were observed by a scanning electron microscope, they were formed at approximately predetermined intervals at a substantially right angle to the stretching direction of the film and formed substantially parallel to the cross section at a right angle to the stretching direction of the film. Formed by a group of unstretched plate-like planes and a group of stretch-oriented relatively narrow fibrils that run between the plate-like planes in a direction substantially parallel to the stretching direction of the film and at substantially predetermined intervals and that connect between the plate-like planes The gaps between the thin fibrils connected between the plate-like planes formed a large number of micropores having a substantially uniform shape that spreads in a substantially two-dimensional manner, and the hole diameter was also substantially uniform throughout. Further, when the cross section of the film was observed, it was confirmed that most of the formed holes were through holes penetrating from one surface to the other surface.

The results of measuring the electrical conductivity of this film are shown in Table 1.

(Comparative Example 1) An unstretched polypropylene film was molded using the same apparatus as in Example 1. The polypropylene used and the molding conditions are as follows.

Polypropylene UBE-PP-Y101J (manufactured by Ube Industries, Ltd.) MFI = 1 g / 10 min Resin discharge temperature 210 ° C. Take-up speed 36 m / min Blow ratio 0.70 The thickness of the obtained unstretched film was 10 μm. The elastic recovery rate from 50% elongation of this film was 80%.

The obtained film was heat-treated at 145 ° C. for 10 minutes without tension. The elastic recovery rate of the obtained film was improved to 87%.

This film is stretched in liquid nitrogen in air (25 ° C)
Stretching and heat setting were performed in the same manner as in Example 1 except that the stretching was changed to 1. to produce a porous polypropylene film.

The average pore diameter of the obtained porous polypropylene film measured by the half dry method was 0.07 μm, and the porosity measured by the mercury porosimetry was 41.50%. The water permeability is 6.66 l / m.
^{It was 2} minutes / atm.

The unstretched polypropylene used was the same as that used in Example 1, but the average pore diameter, porosity and water permeability of the obtained porous film were the same as those obtained in Example 1. The value was lower than the quality polypropylene film. Furthermore, when the surface and cross section of the obtained porous polypropylene film were observed with a scanning electron microscope, pores were formed on the film surface, but when the film cross section was observed, it penetrated from one surface to the other surface. The number of through holes was smaller than that of the film obtained in Example 1. The results of measuring the electrical conductivity using this are shown in Table 1.

(Example 2) An unstretched polypropylene film was molded using the same apparatus as in Example 1. The polypropylene used and the molding conditions are as follows.

Polypropylene UBE-PP-J130G (manufactured by Ube Industries, Ltd.) MFI = 30 g / 10 min Resin discharge temperature 170 ° C. Take-off speed 36 m / min Blow ratio 0.70 The thickness of the obtained unstretched film was 20 μm. The elastic recovery rate from 50% elongation of this film was 30%.

The obtained film was heat-treated at 145 ° C. for 10 minutes without tension. The elastic recovery rate of the obtained film was improved to 72%.

Using this film, the same operation as in Example 1 was carried out to perform stretching and heat setting in liquid nitrogen.

This film was stretched 330% of the length stretched in liquid nitrogen in a heated air tank at 145 ° C, and heat-set in the heated air tank at 145 ° C for 10 minutes while maintaining this stretched state. A porous polypropylene film was manufactured.

The average pore diameter of the obtained porous polypropylene film measured by the half dry method was 0.70 μm, and the porosity measured by a mercury porosimeter was 65.30%. The water permeability was 59.00 l / m ² · min · atm.

When the surface and cross section of this porous polypropylene film were observed with a scanning electron microscope, it was found that pores were formed substantially uniformly on the film surface and the pore diameter was also substantially uniform throughout. Further, when the cross section of the film was observed, it was confirmed that most of the formed holes were through holes penetrating from one surface to the other surface. The results of measuring the electric conductivity using this film are shown in Table 1.

(Example 3) Polypropylene (trade name: UBE-PP-F109K, manufactured by Ube Industries, Ltd., MFI = 9 g / 10 minutes) was placed on an inflation molding machine equipped with an inflation molding die having a diameter of 50 mm and a slit gap of 0.7 mm. An unstretched polypropylene film was molded. Molding operation was carried out by blowing polypropylene at a resin discharge temperature of 200 ° C while blowing air into the valve so that the blow ratio was 0.7, and blowing room temperature air onto the outer wall surface of the film discharged at a position of 5 cm on the die. The film was cooled by immersing, and the target unstretched polypropylene film was formed by a method in which it was taken at a position of 1.8 m on the die by a nip roll at a take-up speed of 35 m / min.

The thickness of the obtained unstretched film was 20 μm. The elastic recovery rate from 50% elongation of this film was 38%.

This unstretched film, at a temperature of 145 ℃, strain rate 8.33% /
The film was stretched by 300% with respect to the initial length, and heat-set for 10 minutes in a heated air tank at 145 ° C while maintaining this stretched state to produce a porous polypropylene film.

The average through-hole diameter of this film measured by the same method as in Example 1 was 0.4 μm, and the porosity was 67%. The results of measuring the electrical conductivity of this film are shown in Table 1.

Method for Measuring Conductivity A porous polypropylene film was impregnated with a liquid electrolyte (a mixed solution of propylene carbonate in which 1 mol of lithium perchlorate was dissolved and an equal volume of dimethoxyethane), and the conductivity was measured by a conductivity meter.

(Example 4) A metal lithium disc having a diameter of 16 mm and a thickness of 0.75 mm was used as a negative electrode,
68 mg of manganese dioxide and acetylene black 8.
5 mg and Teflon powder as a binder (Daikin Co., Ltd.)
Luflon L-5) 8.5 mg was mixed, pressure-molded at a pressure of 7.5 ton / cm ² , and heat-treated at 300 ° C to obtain a diameter of 13 mm and thickness.
A 1.3 mm positive electrode plate was prepared.

As the porous polypropylene film, the one produced in Example 1 was used.

This film was impregnated with an electrolyte dissolved in an equal volume mixed solvent of propylene carbonate and dimethoxyethane in which 1 molar concentration of lithium perchlorate was dissolved, and sandwiched between the positive electrode and the negative electrode to prepare a battery.

As a battery performance test, we conducted a constant resistance discharge test of 2.8 kΩ,
A battery capacity of 16 mAh was obtained by cutting 2.0 volts.

(Comparative Example 2) Using the film prepared in Comparative Example 1, a lithium battery was prepared in the same manner as in Example 4. As a result of performing a battery performance test in the same manner as in Example 4, the discharge capacity was 52 mAh.
Met.

(Example 5) Using the film produced in Example 2 above, a lithium battery was produced in the same manner as in Example 4. As a result of performing a battery performance test in the same manner as in Example 4, the discharge capacity was 16 mAh.
Met.

Example 6 A lithium battery was produced in the same manner as in Example 4 using the film produced in Example 3 above. As a result of performing a battery performance test in the same manner as in Example 4, the discharge capacity was 17 mAh.
Met.

[Effects of the Invention] As described above, according to the battery separator of the present invention, the majority of the through-holes have a substantially uniform pore diameter and a special form having a porosity of 45% or more. Since the porous polypropylene film having fine pores having the above is used, it is possible to provide a film having excellent characteristics such as ionic conductivity. Therefore, a lithium battery using this battery separator can have excellent battery performance.

Claims (4)

[Claims] 1. A battery separator comprising a porous polypropylene film having a large number of fine pores obtained by stretching polypropylene, wherein the porous polypropylene film has a substantially predetermined interval perpendicular to the stretching direction of the film. And a group of unstretched plate-like planes formed substantially parallel to a cross section perpendicular to the stretching direction of the film, and between the plate-like planes substantially parallel to the stretching direction of the film and at substantially predetermined intervals, and Formed by a group of elongated and oriented relatively thin fibrils connected between plate-like planes, a large number of micropores having a substantially uniform shape in which the gaps between the thin fibrils connected between the plate-like planes spread in a substantially two-dimensional manner are formed. Most of the fine pores form through holes and have a substantially uniform pore diameter, and the porosity of the film is 45% or more. Parator. 2. A method for producing a battery separator comprising a porous polypropylene film having a large number of fine pores obtained by stretching polypropylene, wherein the polypropylene stretching step includes nitrogen, oxygen, argon, carbon monoxide, methane and In a medium selected from the group consisting of ethane, and the stretching temperature is −70 ° C. or lower, and the temperature is 50 ° C. higher than the boiling point of the medium from the freezing point of the medium, and the temperature is 50 ° C. or lower. A method for producing a battery separator comprising a porous polypropylene film. 3. A method for producing a battery separator comprising a porous polypropylene film having a large number of fine pores obtained by stretching polypropylene, wherein the polypropylene stretching step is performed at 110 ° C. to 155 ° C. without performing the stretching step at room temperature in advance.
A method for producing a battery separator comprising a porous polypropylene film, which is carried out at a stretching strain rate of less than 10% / min in a high temperature range of ° C. 4. A lithium battery using the battery separator according to claim 1.