JP3050021B2 - Battery separator and lithium battery using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator having good safety and good mechanical properties, and a lithium battery using the same.

[0002]

2. Description of the Related Art Porous films or sheets have been widely used for various purposes. Various methods for producing such a porous resin molded article have been proposed. Porous resin moldings for use as battery separators are:
Generally, from a resin composition containing ultra-high molecular weight polyethylene and a plasticizer, a film or sheet is once produced by melt-extrusion molding, and then the plasticizer contained in the film or sheet is converted into an organic solvent such as isopropanol, ethanol, hexane or the like. After being dissolved and removed with a solvent, it is produced by stretching with a stretching machine such as a roll stretching machine or a tenter transverse stretching machine, that is, by solid-phase deformation in order to improve mechanical strength.

[0003] If the separator has insufficient high-temperature film shape retention characteristics, a large current flows in a short time due to a short circuit accident or the like, and the battery generates heat, which may cause internal short circuit due to breakage of the separator due to the heat. For this reason, the separator has a property that the pores of the separator are automatically closed by heat when the internal temperature of the battery rises (self-closing property) and a property that maintains the membrane shape and separates the electrodes even at high temperatures (high-temperature membrane). (Shape maintaining property) is required.

[0004]

However, the film or sheet obtained by the solid-phase deformation as described above essentially has a high entanglement of polyethylene molecular chains of ultra-high molecular weight polyethylene and a high residual stress. When a temperature higher than a certain level is applied in a fixed state such as being incorporated, there is a problem that the film is broken, and there is a limit in improving the high-temperature film shape maintaining characteristics and the like.

[0005] Polypropylene separator membranes have excellent shape retention at high temperatures, but especially when used as lithium battery separators, the temperature at which self-closing occurs is about 175 ° C, which is close to the ignition temperature of lithium of 180 ° C. There is a problem. In the separator membrane, stretching is usually performed to improve the strength. However, the stretched membrane has a low high-temperature membrane shape maintaining property and is 150 to 160 in polyethylene.
For polypropylene and polypropylene, the film breaks at around 180 ° C., which causes a problem in electrode insulation.

[0006]

SUMMARY OF THE INVENTION Some of the present inventors have previously proposed a polyethylene membrane for a battery separator having excellent gas, liquid and ion permeability and excellent membrane shape maintaining properties at high temperatures ( Japanese Patent Application No. 4-111820). The present inventors have further focused on the characteristics of ultra-high-molecular-weight polyethylene, in which orientation relaxation after melt deformation is unlikely to occur, in order to obtain a battery separator having a smaller residual stress and excellent surface strength, particularly, excellent pin puncture strength, After careful examination,
A film or sheet obtained by melt-extrusion molding is deformed by applying deformation stress while maintaining it in a molten state, and is melt-deformed (molded). That a sheet can be obtained, in particular, by being melt-deformed under specific conditions, that a film or sheet with improved surface strength can be obtained, and further, a film or sheet obtained by being melt-deformed and cooled The inventors have found that a porous film or sheet can be produced industrially advantageously without further stretching treatment as in the prior art, and have completed the present invention.

That is, the gist of the present invention is to provide an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 to 4,000,000, and an optional component having a viscosity average molecular weight of 100% by weight or less based on the ultrahigh molecular weight polyethylene is less than 500,000. And (a) a thickness of 10 to 100 μm, and (b) an air permeability of 20 to
2,000 seconds / 100 cc, (c) porosity 15 to 80
%, (D) Pin sting strength of 120 g / 25 μm or more, (e)
Thermal closure temperature 90-150 ° C and (f) thermal rupture temperature 16
The present invention relates to a battery separator formed from a porous film or sheet having a characteristic of 0 ° C. or higher and a lithium battery incorporating the same.

Hereinafter, the present invention will be described in more detail. The resin composition of the present invention contains at least ultrahigh molecular weight polyethylene and a plasticizer. The ultra-high molecular weight polyethylene used in the present invention includes a viscosity average molecular weight (hereinafter simply referred to as “molecular weight”).
That. ) Is 500,000 to 4,000,000, preferably 1.5 million to
3,000,000 or more linear polyethylenes.
When used as a battery separator, if the molecular weight is less than 500,000, the separator incorporated in the battery may generate heat when heated to 180 ° C. by self-heating in the battery or external heat. Is difficult to maintain. Further, if the molecular weight is too large, for example, exceeding 4 million, the pores of the separator will not be blocked when heated due to too low fluidity,
It is not preferable because the battery is ignited due to a short circuit.

Further, in the present invention, ultra-high molecular weight polyethylene having a melting temperature of 11 is used because of easy melting deformation.
Ethylene homopolymer at 0 to 140 ° C. can be suitably used. Further, in the present invention, as an optional component, 100% by weight based on the above ultrahigh molecular weight polyethylene
Below, the viscosity average molecular weight of preferably 2 to 80% by weight is 5
Polyethylene polybutene-1, polypropylene having a molecular weight of less than 10,000 or a polyethylene having a viscosity average molecular weight of less than 500,000 can be used in combination.

As such polyethylene, branched or linear low-density polyethylene (molecular weight: 5,000 to 10)
10,000), high-density polyethylene (molecular weight 10,000 to 500,000) and polyethylene wax (molecular weight 5,000 or less). When the polyethylene is used in combination with 100% by weight or less, preferably 30 to 90% by weight, particularly preferably 40 to 80% by weight based on the ultrahigh molecular weight polyethylene, the heat blocking temperature can be lowered.

As the plasticizer, various known plasticizers can be used as long as they have a good compatibility with ultra-high molecular weight polyethylene and do not evaporate during melt-kneading or molding, for example, those having a boiling point higher than the melting temperature of ultra-high molecular weight polyethylene. Can be used. Specifically, for example, paraffin wax which is solid at room temperature, or higher aliphatic alcohol such as stearyl alcohol or ceryl alcohol, or n which is liquid at room temperature
-Decane, n-alkane such as n-dodecane, liquid paraffin, kerosene and the like.

The proportion of the ultrahigh molecular weight polyethylene and the plasticizer is usually 5 to 60% by weight, preferably 10 to 50% by weight of the ultrahigh molecular weight polyethylene and 40 to 50% by weight of the plasticizer.
It is selected from the range of 95% by weight, preferably 90 to 50% by weight. In the resin composition of the present invention, known various additives, for example, an antioxidant and the like in the resin composition, 0.01
You may use together about 5 weight%.

The respective components of the resin composition are uniformly kneaded by a known single-screw or twin-screw extruder, and are melt-extruded.
A twin-screw extruder is preferably used in terms of extrusion amount, extrusion stability, and kneading strength. Melt extrusion is usually carried out at 140-2.
Perform at a temperature of 40 ° C, 5-50μ or 50-300μ
Extruded into a film or sheet with a thickness of In the present invention, the thus extruded film or sheet is melt-deformed. That is, since the molecular weight is extremely large,
The mechanical strength of the porous film or sheet finally obtained by melting and deforming utilizing the properties of ultra-high molecular weight polyethylene, which is less likely to relax the orientation of the molecular chains due to deformation and is easy to orient in the direction of deformation To improve.

The melt deformation is performed by applying a deformation stress while keeping the resin composition constituting the extruded film or sheet in a molten state. Usually, the temperature of the resin composition is about 130-240 ° C., preferably 1
Deformation stress is applied while maintaining the temperature in the range of 60 to 200 ° C. At this time, the deformation can be performed not only in one direction but also in multiple directions. When used as a battery separator, if it is deformed in only one direction, the pores in the film or sheet will be elongated in the deformation direction, the flow path will be slightly narrowed, the air permeability will be reduced, and the separator Since the ionic resistivity tends to increase, it is preferable to apply a well-balanced deformation in multiple directions so as to widen the pores.

More specifically, for example, in the case of applying a deformation in one direction, in a T-die or an inflation molding method, preferably in an inflation molding method, the die gap is increased, and the take-up speed is increased. In other words, by increasing the draft rate, MD (machine)
Apply deformation in the direction. When deformation is applied in multiple directions, MD and TD are increased by increasing the draft rate and blow ratio in the inflation molding method.
Melt deformation is applied in the (width) direction. Furthermore, in the case of multi-directional deformation by the T-die molding method, the width direction end of the molten film or sheet is fixed to a caterpillar with a pin tenter, and the width of the two caterpillars is increased in the flow direction to thereby reduce TD. In the MD direction by simultaneously increasing the take-up speed.

The degree of melt deformation is not particularly limited as long as the effect of the present invention is not impaired, but is usually from 10 to 1000, preferably from 3 to 3 in terms of a melt deformation rate represented by the following formula.
It is preferable to apply a deformation stress so as to be in the range of 0 to 800, particularly preferably 50 to 400. For example, when a deformation stress is applied vertically and horizontally, the aspect ratio of the deformation ratio is DR /
BUR ≦ 50, preferably ≦ 20, particularly preferably
It is better to perform so as to be ≦ 10.

[0017]

## EQU1 ## Melt deformation rate = (D × ρ ₁ ) / (t × ρ ₂ ) D: Die gap (mm) ρ ₁ : Melt density of resin composition (g / cm ³ ) t: Molded film or sheet Film thickness (mm) ρ ₂ : solid density of molded film or sheet (g / cm
³ )

It is known that in an inflation molding method, a polyethylene film is produced by melting and deforming using an annular die having a die gap of 0.5 mm (Japanese Patent Application Laid-Open No. 62-223245). Conventionally, 10-100μ
When forming a film or sheet having a thickness of m, the die gap is usually 1 mm or less, and at most 1.5 mm or less. However, a film or sheet obtained by molding in the range of a conventional die gap does not always have sufficient strength, particularly pin puncture strength. In order to obtain a battery separator having good strength, the range of the die gap is important together with the cooling rate described below. In the present invention, the die gap is preferably 2
It is good to carry out in the range of 20 mm, particularly preferably 3 mm to 10 mm.

After cooling the melt-deformed film or sheet, the plasticizer contained in the film or sheet is removed to make the film or sheet porous. At this time, the cooling is preferably performed so that the cooling solidification time defined by the following formula is 50 seconds or less, preferably 20 seconds or less. If the cooling and solidifying time is too long, phase separation proceeds, and the pore size increases and the strength decreases. Definition of cooling solidification time

[0020]

(Equation 2) τ: Cooling solidification time (seconds) L: Distance between lip exit and cooling solidification line (cm) (Inflation corresponds to frost line height; T die corresponds to air gap) V ₀ : Rip exit Moving speed of resin (cm / sec) V ₁ … Moving speed of resin in cooling and solidification line (cm / sec)

The plasticizer is removed by, for example, a known organic solvent method in which the plasticizer in the film or sheet is dissolved in an organic solvent such as isopropanol, ethanol, or hexane and extracted and removed by solvent replacement. be able to. The film or sheet from which the plasticizer has been removed to make it porous is stretched uniaxially or biaxially to improve its mechanical strength, or 100 to 1
The heat setting may be performed at about 80 ° C. In the present invention, it is preferable to use a film or sheet that has not been subjected to a stretching treatment because the stretching treatment tends to reduce the ability to maintain a high-temperature film shape.

In the present invention, those having a shrinkage stress of 20 gf or less, preferably 1 to 10 gf, both in the MD (machine direction) and TD (in the direction perpendicular to the machine direction) are excellent in shape maintaining ability. Is preferred. The shrinkage stress was measured using an INTESCO high temperature tensile tester, width 2
1 mm from room temperature under the condition of 5 mm and 50 mm between chucks
The temperature is raised to 50 ° C. (5 ° C./min), and the shrinkage stress at 150 ° C. is determined as a value converted into a film or sheet thickness of 25 μm.

According to the present invention, a porous film or sheet having a thickness of 10 to 100 μm, preferably 15 to 60 μm can be obtained. The average pore size of the battery separator of the present invention is usually 1 μm or less, preferably 0.01 μm or less.
１1 μm, and air permeability is 20-2,000 sec / 10
0 cc, preferably 100-700 sec / 100 cc
And the porosity is 15 to 80%, preferably 30 to 70%.
Range.

Further, the porous film or sheet obtained according to the present invention has good mechanical strength, especially good pin puncture strength and heat-resistant film breakage. The pin sting strength is measured according to Japanese Agriculture and Forestry Standard Notification No. 1019 [Measurement device: Rheometer (NRM-2002J manufactured by Fudo Kogyo Co., Ltd.)) Pin diameter 1 m
mφ tip 0.5R, pin piercing speed 300mm / mi
n] is 120 g or more, preferably 140 g (2
5 μm) or more, particularly preferably 170 to 300 g
(25 μm thickness) is obtained. Further, the heat-resistant film rupture property is an index of rupture easiness due to shrinkage stress generated by heat of the film or sheet in a high temperature range, and when used as a battery separator, maintains the film shape even at high temperatures, This is an important characteristic because it is necessary to keep the distance. The air permeability was measured according to JIS P8117. The device used was a B-type Gurley type densometer (trade name) manufactured by Toyo Seiki Co., Ltd.

The method of measuring the thermal closing temperature and the thermal rupture temperature is to prepare a square Teflon film (TF) having a hole with a diameter of 4 cm in the center and a square 8 cm side and an aluminum plate (Al).
A test piece was stacked in the order of Al / TF / porous film / TF / Al and fixed with a clip or the like. Test specimen at 130 ° C
5 ° C / 5min while heating in an oven
Take a sample every minute and measure the air permeability (JIS P811
7) is measured, and the temperature at which the air permeability becomes 0 is defined as the heat blocking temperature.

Similarly, the temperature at which a hole that can be visually recognized and recognized while the temperature is continuously increased is defined as the thermal rupture temperature. The porosity was measured by punching 5 places in the width direction of the film into a circle having a diameter of 3 cm, measuring the thickness of the center of the punched film,
Also, the weight is measured and calculated by the following equation.

[0027]

Porosity (%) = (Vρ−W) / (Vρ) × 100 V: volume of film (for 5 sheets) W: weight (for 5 sheets) ρ: density of material

Further, the porous film or sheet obtained by the present invention has a heat closing temperature, that is, a temperature at which self-occluding property is exhibited when held at a predetermined temperature for 5 minutes, is 100 ° C.
Since it is -150 degreeC, Preferably it is 105-135 degreeC, it is suitable also from battery safety. There is a practical problem if the blockage occurs at 100 ° C. or less, and the block temperature is 150 ° C.
Above, there is a problem that the battery tends to be overheated.

The thermal rupture temperature is 160 ° C. or higher, preferably 180 ° C. or higher, and more preferably 200 ° C. or higher. If the thermal rupture temperature is less than 160 ° C., the temperature rises quickly even when the separator is thermally closed at about 150 ° C. when the battery generates heat, and the inside of the battery may be 160 ° C. or more due to residual heat. Therefore, in order to ensure safety, it is also important that the difference between the thermal blocking temperature of the separator and the thermal rupture temperature is large.

The temperature difference between the two is 30 ° C. or more, preferably 5 ° C.
When the temperature is 0 ° C. or higher, the separator has a safety margin with respect to the temperature rise that continues even after the heat occlusion, and is an excellent separator. The battery separator of the present invention can be suitably used, for example, for lithium batteries, lithium ion secondary batteries, and the like. In the present invention, the porous film or sheet has a bubble point value of 2 to 8 measured by a bubble point method.
kgf / cm ^2, preferably, suitably in the range of 3~7kgf / cm ^2.

The bubble point value is calculated according to JIS K3832.
This value can be measured by the bubble point method (using isopropyl alcohol as a solvent) described in (1) and represents the maximum pore size of the film or sheet.
If the bubble point value is 2 kgf / cm ² or less, the insulating property becomes insufficient, and the closing time at the time of overheating becomes long, which is a safety problem, and the strength is reduced. Conversely, 8kgf
/ Cm ² or more, the internal resistance of the battery increases, so it is preferable to use one within the above range.

The lithium battery (secondary battery) of the present invention comprises a separator comprising the above-described porous film or sheet, an aprotic electrolyte, a positive electrode comprising lithium compound (positive electrode during discharge), and a negative electrode. Is done. First, an aprotic electrolyte is filled into the pores of the separator, and the filling can be easily performed by dropping, impregnation, coating or spraying. This is because the porous film or sheet has an average through-hole diameter of 0.001 to 0.1 μm, so that the aprotic electrolyte having a contact angle of 90 ° or less with the film or sheet is formed by capillary condensation. This is because it is easily taken into the hole by the action.

As the aprotic electrolyte, propylene carbonate, dimethyl sulfoxide, 3-methyl-
1,3-oxazolidin-2-one, sulfolane, 1,
LiBF is used alone or in a multi-component organic solvent such as 2-dimethoxyethane and 2-methyltetrahydrofuran.
₄ , a solution in which a lithium salt such as LiClO ₄ is dissolved can be used. In particular, propylene carbonate,
1,2-dimethoxyethane, a combination of LiBF ₄ ,
Dimethyl sulfoxide, 1,2-dimethoxyethane, L
iBF ₆ combination, propylene carbonate, 1,
The combination of 2-dimethoxyethane and LiClO ₄ is
It is known that the electrical conductivity at room temperature is 10 ⁻³ to 10 ⁻² s / cm, which is suitable.

By using the separator thus obtained, a lithium battery having excellent reliability and safety can be obtained. The lithium battery of the present invention is a so-called non-aqueous electrolyte type battery using the above-mentioned aprotic electrolyte as an electrolyte, and its structure is basically the same as a normal lithium battery of the same type. Between the positive and negative electrodes, the separator of the present invention described in detail above is provided.

In the case of a primary battery, the negative electrode is lithiated.
Silver chromate, fluorocarbon,
Manganese oxide or the like can be used. Also with secondary batteries
The lithium compound as the positive electrode (positive electrode during discharge)
Or aluminum or fusible gold (Pb, Cd, In
Alloy containing carbon) or carbon with lithium absorbed.
TiS with layered structure as pole_Two, MoS_TwoAnd NbS
e_ThreeMetal chalcogenides and tunnel-like vacancies
CoO_Two, Cr_TwoO_FiveAnd V_TwoO_Five(・ P_TwoO _Five), M
nO_Two(・ LiO_Two) And other metal oxides, polyacetylene
Using conjugated polymer compounds such as polyaniline and polyaniline
Can be

[0036]

The present invention will be described more specifically with reference to the following examples.

EXAMPLE 1 20 parts by weight of ultrahigh molecular weight polyethylene powder having a melting point of 135 ° C. and a molecular weight (viscosity average) of 2 × 10 ⁶ and 80 parts by weight of ceryl alcohol were fed to a φ40 mm extruder and continuously kneaded at 230 ° C. 400mm wide, 2mm die gap
The sheet was extruded from a T-die and taken up at a pulling speed of 2.5 m / min to apply melt deformation in the MD direction (drawing direction) to obtain a sheet having a thickness of 0.05 mm. At that time, the die temperature was 170 ° C., and the linear velocity at the gap was 7.1 cm / mi.
n, the draft rate (Dr) was 35.1. The melt density of the resin composition used in this example was 0.76 g / cm ³
And the solid density of the extruded sheet is 0.867 g / c.
m ³ , the melt deformation rate applied in such melt deformation was 35.1.

The obtained sheet was immersed in isopropyl alcohol at 80 ° C. to extract and remove ceryl alcohol from the sheet, and then heat-treated with a heating pinch roll having a surface temperature of 125 ° C. for 30 seconds to obtain a 27 μm thick film. Was obtained. Table 1 shows the physical properties of this film. The obtained porous resin film (cut to a width of 58 mm and a length of 1 m) was sandwiched between a positive electrode and a negative electrode of a lithium battery (negative electrode: lithium cobalt oxide, positive electrode: carbon, electrolyte solution: propylene carbonate), and rolled up. A lithium secondary battery was housed in a metal container having a length of 60 mm and a diameter of 15 mm.

Table 1 shows the failure rate when the secondary battery was assembled and the result of an external short circuit test. Failure rate at assembly (%)
Is sandwiched between a positive electrode and a negative electrode of a lithium battery with a porous resin film pierced and held with a pin or the like. It is shown. External short circuit test, after assembling into a lithium battery, externally short-circuit the positive and negative electrodes of the battery,
It is the result of observing the state of the battery.

Example 2 In Example 1, the width was 500 mm and the die gap was 6.2.
mm T-die, linear velocity at gap 2.3cm
/ Min, take-up speed 2.3 m / min (Dr98.
8) The film thickness of 0.055 mm
Created a sheet. The melt deformation rate applied in such melt deformation was about 98.8. Then, after removing the seryl alcohol from this sheet in the same manner as in Example 1,
By heat treatment, a porous resin film having a thickness of 28 μ was obtained. Table 1 shows the physical properties of this film. The obtained porous resin film was used as a separator of a lithium battery in the same manner as in Example 1.

Example 3 30 parts by weight of an ultrahigh molecular weight polyethylene powder having a melting point of 138 ° C. and a molecular weight (viscosity average) of 3 × 10 ⁶ and 70 parts by weight of stearyl alcohol were fed to a φ50 mm extruder at 200 ° C.
While being kneaded, the material is continuously extruded from an inflation die having a die diameter of 50 mm and a die gap of 4.5 mm, and a take-up speed of 10 m / min (a die temperature of 180 ° C., a linear velocity in a gap of 25.4 cm / min, Dr.39.3). Take-up, melt deformation is applied at a blow ratio (BUR) of 2.4, and a film thickness of 0.
A sheet of 043 mm was obtained. Since the melt density of the resin composition used in this example was 0.79 g / cm ³ and the solid density of the extruded sheet was 0.876 g / cm ³ , the melt deformation rate applied in such melt deformation Was about 94.4. This sheet was immersed in isopropyl alcohol at 60 ° C. to extract stearyl alcohol, and then heat-treated in the same manner as in Example 1 to obtain a film having a thickness of 24 μm.
Was obtained. Table 1 shows the physical properties of this film.
The obtained porous resin film was used as a separator of a lithium battery in the same manner as in Example 1. Table 1 shows the test results.

Example 4 In Example 3, a die temperature of 170 ° C. was obtained from an inflation die having a die diameter of 40 mm and a die gap of 0.28 mm.
r1.0, melt deformation at BUR 5.5 (melt deformation rate of about 5.5), then removal of plasticizer as in Example 3,
Heat treatment was performed to obtain a 28 μm-thick porous resin film. Table 1 shows the physical properties of this film. The obtained porous resin membrane was used in Example 1
It was used as a lithium battery separator in the same manner as described above. Table 1 shows the test results.

Example 5 A porous resin film having a thickness of 60 μm was obtained in the same manner as in Example 1 except that Dr43 and BUR1.1 were used. Table 1 shows the physical properties of this film. The obtained porous resin film was used as a separator of a lithium battery in the same manner as in Example 1. Table 1 shows the test results.

Example 6 30 parts by weight of ultrahigh molecular weight polyethylene powder having a melting point of 135 ° C. and a molecular weight (viscosity average) of 2 × 10 ⁶ and 70 parts by weight of ceryl alcohol were fed into a φ90 mm extruder and continuously kneaded at 230 ° C. Width 500mm, Die gap 3.5
extruded from a T-die having a width of 12 mm, the temperature of the resin composition was maintained at 170 ° C. by a heating roll and an infrared heater, and the sheet width was reduced to 12 by drawing with a pin tenter at Dr30.7.
After being spread to 50 mm and melt-deformed (melt deformation rate 76.7), it was cooled to obtain a sheet having a thickness of 40 μm. Seryl alcohol was removed from this sheet in the same manner as in Example 1, and heat treatment was performed to obtain a 20 μm-thick porous resin film. Table 1 shows the physical properties of this film. The obtained porous resin film was used as a separator of a lithium battery in the same manner as in Example 1.
Table 1 shows the test results.

Example 7 In Example 3, a 50 mmφ twin screw extruder was used as the extruder, and the die diameter was 40 mm and the die gap was 5.6 m.
m inflation die, die temperature 170 ° C, Dr17.
6. Melt deformation at BUR 5.5 (melt deformation rate about 96.
Except for adding 8), the thickness was 52 μm in the same manner as in Example 3.
Sheet was obtained. Next, the plasticizer was removed in the same manner as in Example 3, and heat treatment was performed using a heat treatment machine of a pinch roller type to obtain a film thickness of 28 μ
Was obtained. This film had a bubble point value of 5.5 kgf / cm ² and a longitudinal shrinkage stress of 12 gf. Table 1 shows other physical properties. The obtained porous resin film was used as a separator of a lithium battery in the same manner as in Example 1. Table 1 shows the test results.

Example 8 In Example 7, the melting point was 135 ° C. and the molecular weight was 2.5 × 10
^A porous resin film was obtained in the same manner except that 25 parts by weight of ultra-high molecular weight polyethylene powder of ⁶ , ⁶ parts by weight of stearyl alcohol and 10 parts by weight of polyethylene wax having a molecular weight of 3,500 were supplied to an extruder. 5.2
At kgf / cm ² , the longitudinal shrinkage stress was 7 gf.
Used as a lithium battery separator. Table 1 shows the test results.

Comparative Example 1 A commercially available porous film made of polypropylene (film thickness 25 μ, pin puncture strength 376 g (per 25 μ thickness), air permeability 660 sec.
/ 100cc, thermal closure temperature 177 ° C, thermal rupture temperature 180
C), and a battery was assembled in the same manner as in Example 1. Table 1 shows the battery performance.

Comparative Example 2 70 parts by weight of ceryl alcohol and 30 parts by weight of ultrahigh polyethylene powder having the same melting point of 135 ° C. and molecular weight (viscosity average) of 2 × 10 ⁶ as used in Example 1 were fed to a 40 mmφ twin screw extruder. While kneading at 230 ° C., the mixture was continuously extruded from a die having a width of 500 mm and a die gap of 0.25 mm at a die temperature of 170 ° C. to form a sheet at a draft rate of 2.5. The sheet thickness was 85μ. After cooling, the ceryl alcohol was removed in the same manner as in Example 1, and then stretched 2.0 times (stretching temperature: 125 ° C.) using a roll stretching machine to obtain a 30 μm-thick porous resin molded body. Table 1 shows the physical properties of this molded product and the results of a test performed as a battery in the same manner as in Example 1.

Comparative Example 3 The procedure of Comparative Example 2 was repeated except that the die gap was 0.16 mm and the sheet thickness was 55 μm. Heat treatment was performed in the same manner as in Example 1 without stretching, to obtain a 25 μm-thick porous resin molded body.
Table 1 shows the physical properties of this film and the test results of a battery in the same manner as in Example 1.

[0050]

[Table 1]

[0051]

When the battery separator of the present invention is used for a secondary battery such as a lithium battery, abnormal heating and breakage of the battery itself are prevented, and a highly safe secondary battery can be obtained.

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kyosuke Watanabe 3-10 Ushidori, Kurashiki-shi, Okayama Prefecture Inside Mizushima Plant, Mitsubishi Kasei Co., Ltd. Address Mizushima Plant, Mitsubishi Chemical Corporation (56) References JP-A-3-203160 (JP, A) JP-A-5-234578 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) H01M 2/14-2/18 H01M 6/14-6/16 H01M 10/40

Claims (5)

(57) [Claims] (1) a viscosity average molecular weight exceeding 500,000 to 4,000,000;
High molecular weight polyethylene and the above-mentioned superpolymer as an optional component
Amount of viscosity average of 100% by weight or less based on polyethylene
A porous film or sheet made of polyethylene having a molecular weight of less than 500,000 , and (a) a thickness of 10 to 10
100 μm, (b) air permeability 20 to 2,000 seconds / 100
cc, (c) porosity 15-80%, (d) pin puncture strength 1
20 g (per 25 μm of film thickness) or more, (e) heat closing temperature 9
A battery separator formed from a porous film or sheet having characteristics of 0 to 150 ° C and (f) a thermal rupture temperature of 160 ° C or higher. 2. A battery separator according to claim 1, wherein the difference between the thermal blocking temperature and the thermal rupture temperature of the porous film or sheet according to claim 1 is 30 ° C. or more. 3. The battery separator according to claim 1, wherein the bubble point value of the porous film or sheet according to the bubble point method is ² to 8 kgf / cm ² . 4. A battery separator according to claim 1, wherein the polyethylene having a viscosity average molecular weight of less than 500,000 is selected from polyethylene wax, low density polyethylene and high density polyethylene. 5. A lithium battery incorporating the battery separator according to claim 1 .