JP2000348703A - Separator for battery and lithium battery using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a battery having an excellent fuse function of quickly thermally blocking in a low-temperature region, and a lithium battery using the same. SOLUTION: This separator for a battery is made of a laminated porous film obtained by drawing, into pores, a laminated film in which at least two kinds of polyethylene having different melting points are laminated in three layers or more. The separator for a battery has a maximum pore diameter of 0.02-3 μm, a porosity of 30-58%, a film thickness of 5-100 μm, a permeability of 30-2000 sec/100 cc, and a heat shrinkage in a film width direction of +3% or less after heat treatment at 105 deg.C for 2 hours. A lithium battery uses the separator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator useful as a constituent material of a lithium battery and a lithium battery using the same.

[0002]

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

Taking a battery as an example, a non-aqueous electrolyte battery such as a lithium battery having a high energy density, a high electromotive force and little self-discharge, particularly a lithium secondary battery has been developed and put into practical use in recent years. Examples of the negative electrode of a lithium battery include metallic lithium, an alloy of lithium and another metal, a carbon material capable of adsorbing or intercalating lithium ions such as carbon and graphite, and a conductive material doped with lithium ions. Polymer materials and the like are known, and as the positive electrode, for example, (CF _x ) _n
Fluorinated graphite, MnO ₂ , V ₂ O ₅ , CuO, A
Metal oxides such as g ₂ CrO ₄ , TiO ₂ , LiCoO ₂ and LiMn ₂ O ₄ , sulfides and chlorides are known.

As a non-aqueous electrolytic solution, an organic solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane, tetrahydrofuran or the like is used. ₆ , LiBF ₄ , LiCl
O ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN
A solution in which an electrolyte such as (SO ₂ C ₂ F ₅ ) ₂ is dissolved is used.

[0005] The role of the separator, which is a constituent material of such a lithium battery, is to prevent short-circuiting of the positive and negative electrodes and not to hinder the battery reaction, and to prevent heat generation and ignition of the battery due to thermal obstruction due to abnormal heat. The following various porous films have been proposed.

A single-layer porous film of a thermoplastic resin such as polyethylene or polypropylene (JP-B-46-4011)
No. 9, JP-B-55-32531, JP-B-59
-37,292, Tokai Sho 60-23954,
JP-A-2-75151, U.S. Pat.
No. 8, etc.). Porous films made of a mixture of polyethylene having different molecular weights or a mixture of polyethylene and polypropylene (JP-A-2-21559, JP-A-5-21559)
-331306). A porous film using a thermoplastic resin or a non-woven fabric as a support (JP-A-3-245457, JP-A-1-258358, etc.). Laminated porous films in which a plurality of porous films made of polyethylene and polypropylene are laminated (Japanese Patent Application Laid-Open Nos. 62-10857, 2-77108, 6-5556)
9, JP-A-6-20671, JP-A-7-3
No. 07146).

[0007] These methods for making a porous film porous are roughly classified into a stretching method (dry method) and an extraction method (wet method). In the extraction method, a thermoplastic resin composition containing high-density polyethylene as a main component and a filler and a plasticizer blended are extruded into a film, and then the filler and plasticizer are extracted from the film to make it porous. I do. The film is
Since biaxial stretching is performed before or after the extraction of the filler or the plasticizer, there is a problem that the film contracts not only in the machine direction but also in the width direction of the film. On the other hand, in the stretching method, the crystal structure is controlled in the process of melting and extruding the thermoplastic resin, and thereafter, porosity is formed by generation and growth of craze accompanying the stretching. Since stretching is uniaxial stretching, there is a problem that the anisotropy of mechanical strength is large, but there is no thermal shrinkage in the width direction of the film.

[0008] One of the important roles of the separator incorporated in the lithium battery is to close the hole at the time of abnormality and cut off the current,
This is a fuse function that prevents battery heat and ignition. When a porous film is used as a battery separator, it is necessary that the film that has been made nonporous by closing holes in an appropriate temperature range and has no heat resistance so as not to short-circuit the positive and negative electrodes. However, when a porous film manufactured by the extraction method is used as a battery separator, the film shrinks in the width direction of the film as the temperature rises, and the size in the width direction is reduced. There is a concern that this will cause

It is desirable that the heat blocking temperature at which the porous film is heat-blocked and made nonporous is low as long as the battery does not malfunction during normal use. Further, the heat blockage needs to be performed more quickly in order to effectively prevent heat generation and ignition of the battery. As a method of lowering the heat closing temperature, a method of manufacturing a porous film from a composition of polyethylene having a high melting point and polyethylene having a low melting point is disclosed in JP-A-8-34873.
And JP-A-8-64194, and JP-A-10-17702 in which a low molecular compound such as wax is mixed with polyethylene to produce a porous film are known. In addition, the porous film has a multi-layer structure, and a layer responsible for the shape retention of the film and a layer responsible for the thermal blockage are provided for each function, so that only the layer responsible for the thermal blockage, not the entire porous film, is locally reduced in calorific value. And the heat can be quickly closed.

However, in the production process by the extraction method, even if a low-molecular compound such as wax is blended for the purpose of lowering the heat blocking temperature, the low-molecular compound is extracted by a solvent at the time of making porous, so that it is added. There is a problem that it is not easy to design the blending amount of the agent, and even when blending polyethylene having a low melting point, it is necessary to heat block the entire porous film, so that the heat of fusion cannot be reduced.

[0011]

On the other hand, in a method for producing a porous film by a stretching method, a laminated porous film in which polypropylene and polyethylene are alternately laminated and a porous film of polyethylene single layer are known. It is difficult to lower the heat blocking temperature of the polyethylene layer that is responsible for the heat blocking due to the selection of the material polymer and the restrictions on the manufacturing conditions in the laminated porous film using polypropylene, and the heat of fusion required for the heat blocking in the polyethylene single layer porous film. There is still room for improvement because a small one has not been obtained.

Furthermore, it has been pointed out that a laminated porous film in which polypropylene is disposed in the surface layer and polyethylene is disposed in the intermediate layer has poor wettability and permeability of the nonaqueous electrolyte when used as a battery separator. . Poor wettability of the separator is not preferable because the time required for the non-aqueous electrolyte to permeate during battery production becomes longer or the non-aqueous electrolyte tends to be depleted. SUMMARY OF THE INVENTION An object of the present invention is to provide a high-reliability and high-safety polyethylene-based separator for a lithium battery separator, which exhibits an excellent fuse function by rapidly heat-sealing in a low temperature range and has extremely small heat shrinkage in the film width direction. An object of the present invention is to provide a laminated porous film and a lithium battery using the same.

[0013]

Means for Solving the Problems As a result of diligent studies, the present inventors have found that a three- or more-layer laminated film in which at least two or more types of polyethylene having different melting points are laminated is made porous by a stretching method, thereby obtaining a battery. It has been found that when used as a separator for polyethylene, a polyethylene-laminated porous film that can be thermally closed quickly in a low temperature range can be obtained. That is, the present invention is a battery separator comprising a laminated porous film obtained by making a laminated film of three or more layers in which at least two or more types of polyethylene having different melting points are laminated by a stretching method. 0
2-3 μm, porosity 30-85%, film thickness 5-100 μm
m, air permeability 30 to 2000 sec / 100 cc, heat shrinkage in the film width direction after heat treatment at 105 ° C. for 2 hours is +3
% Or less. The present invention also relates to a lithium battery including the battery separator as a constituent material.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The battery separator according to the present invention comprises at least one kind of low melting point polyethylene having a melting point of 105 ° C. or more and less than 133 ° C. and at least one kind of high melting point polyethylene having a melting point of 133 ° C. or more. Preferably, Further, the battery separator is preferably made of a laminated porous film in which high-melting-point polyethylene and low-melting-point polyethylene are alternately laminated. Further, when the laminated porous film is laminated so that the surface layer is made of high-melting-point polyethylene and the inner layer is made of low-melting-point polyethylene, stretching and porousness are particularly suitably performed, and heat is rapidly applied when the battery is abnormal. It is preferable because it can be closed. As the high melting point polyethylene used for the battery separator of the present invention, those having a melting point of 133 ° C. or more are preferable because of excellent film shape retention when the battery separator is thermally closed. In particular, high-density polyethylene is preferable, and the number average molecular weight of polyethylene is 10,000 or more, more preferably 20,000 or more, and the melting point is 133 ° C.
The above-mentioned materials are preferable because they have excellent film shape retention when the battery separator is thermally closed.

The low-melting polyethylene used in the battery separator of the present invention may be any of high-density polyethylene, medium-density polyethylene, linear low-density polyethylene, etc., but those having a melting point of 105 ° C. or more and less than 133 ° C. Is preferable because it is thermally blocked in a low temperature range. The polyethylene preferably has a number average molecular weight of 8,000 to 30,000. Further, as the low melting point polyethylene, 80 to 99% by weight of high density polyethylene and 20 to 1% of low density polyethylene are used.
A polyethylene composition consisting of% by weight can also be used. If the weight ratio of the low-density polyethylene in the polyethylene composition is more than 20% by weight, it is difficult to make the polyethylene porous by the stretching method, and if it is less than 1% by weight, the effect of lowering the heat blocking temperature is lost, so the above range is appropriate. . Furthermore, as long as the melting point is in a range of 105 ° C. or more and less than 133 ° C., a composition in which low melting point polyethylene contains another low melting point resin such as polyethylene or rubber can be obtained.
In the battery separator, the heat blocking temperature at which the porous structure of the porous layer made of low melting point polyethylene disappears and becomes nonporous is 132 ° C. or less, and the heat of fusion associated with the heat blocking is 0.12 J /. cm ² or less.

In the present invention, the number average molecular weight of polyethylene was determined in terms of standard polystyrene using a 150C gel permeation chromatograph manufactured by WATERS. Use two Shodex HT-806M columns, 0.3wt / vol%
Was measured at 135 ° C. in ortho-dichlorobenzene prepared as described above. The melting point of polyethylene was measured using Perkin-Elmer DSC-7. The sample was held at 230 ° C for 10 minutes to completely remove the heat history, and then completely melted. After cooling to room temperature at 10 ° C / min, the maximum value of the melting curve was measured at a heating rate of 10 ° C / min. And

Furthermore, low-molecular-weight compounds such as plasticizers, lubricants, and solvents can be appropriately blended with the low-melting-point polyethylene used in the battery separator of the present invention. Specific examples include waxes, plasticizers such as paraffin, lubricants such as stearic acid monoglyceride and palmitamide, and polyethylene solvents such as biphenyl, dodecanol, and long-chain hydrocarbons. The compounding amount of these low molecular compounds is 0.1 to 2
0% by weight, preferably 0.3 to 10% by weight,
Those that are hardly eluted in the non-aqueous electrolyte in the battery are preferable because they do not hinder the charge / discharge characteristics of the battery. If the amount of the low-molecular compound is more than 20% by weight, it is difficult to make the film porous by the stretching method, and if it is less than 0.1% by weight, the effect of lowering the heat blocking temperature will not be obtained.

The method of blending low-density polyethylene or a low-molecular compound such as a plasticizer, a lubricant or a solvent with polyethylene is not particularly limited, but it can be blended by kneading using a usual kneader. For example, pellets can be obtained by melt-kneading using a single-screw extruder, a twin-screw extruder, a mixing roll, or the like. Also, Henschel mixer,
It may be blended by dry blending using a tumbler or the like. The battery separator of the present invention may appropriately contain a flame retardant, a coloring agent, or a reinforcing material such as a glass fiber or a silicon-based fiber as long as the characteristics of the battery separator are not impaired.

Specific methods for producing the battery separator of the present invention include, for example, a method in which a polyethylene film having a different melting point is co-extruded and then stretched and porous to obtain a laminated porous film. There is a method of obtaining a laminated porous film by separately melt-extruding and laminating and then stretching and making it porous. Further, in the stretching and porosification step, if the length of the film in the width direction is greatly reduced and the performance of the porous film such as air permeability, porosity and maximum pore diameter is impaired, the invention of the present application and the like are first described. As in the method described in Japanese Patent Application No. 10-99633, a method of stretching while fixing both ends in the width direction of the film with a chuck, a pinch roll, or the like, at the time of uniaxial stretching after stretching the film uniaxially vertically. The resulting reduction in the film length in the width direction can be improved by a method such as a method of restoring the film length by transverse stretching. Either method can produce the battery separator of the present invention.

The melt extrusion method is performed by a melt extrusion molding method using a T-die, an inflation method, or the like. For example, when a film is melt-molded with a T-die, generally at a temperature 20 to 60 ° C. higher than the melting temperature of polyethylene,
The drafting is performed at a draft ratio of 5 to 500, preferably 50 to 300. The take-up speed is not particularly limited, but the molding is usually performed at 10 to 50 m / min. The melt extruded film is heat treated to increase its crystallinity and its orientation. The heat treatment temperature is 100 to 130C, preferably 110 to 125C. If the heat treatment temperature is low, it is not sufficiently porous, and if it is too high, polyethylene is melted, which is not suitable. The heat treatment time is not particularly limited,
This is performed within a range of 3 seconds to 180 seconds.

The heat-treated polyethylene film has a birefringence of 25 × 10 ^{−3 to} 48 × 10 ⁻³ , preferably ³ × 10 ⁻³ .
The elastic recovery at 50% elongation at 0 × 10 ^{−3 to} 45 × 10 ⁻³ is in the range of 40 to 80%, preferably 50 to 75%. If the birefringence and elastic recovery are out of these ranges, the degree of porosity becomes insufficient, which affects the pore size and pore size distribution, porosity, delamination strength, mechanical strength, etc. of the porous film after stretching. The above range is appropriate because the quality tends to vary.

In the present invention, the birefringence is a value measured using a Bellec compensator under crossed Nicols using a polarizing microscope. Further, the elastic recovery rate is based on the following equation (1). In addition, polyethylene film is 25 degreeC,
At a relative humidity of 65%, the sample was set on a tensile tester with a sample width of 15 mm and a length of 2 inches and a speed of 2 inches / min.
%, The sample was kept in the extended state for 1 minute, and then relaxed at the same speed.

Formula (1) Elastic recovery rate (%) = ((length at 50% elongation−length at 50% load after elongation at 0%) / (length at elongation−length before elongation) Sa)) x 100

The heat-treated polyethylene film is laminated by thermocompression bonding. The layer structure of the laminated porous film, that is, the battery separator, is preferably such that the high-melting-point polyethylene is arranged on the outside and the low-melting-point polyethylene is arranged on the inside, because the thickness of the layer responsible for heat occlusion can be reduced. . When the thickness of the layer responsible for thermal closure is small and the amount of heat of fusion is small, the battery separator can quickly thermal close with a rise in temperature. The number of laminated polyethylene films is not limited as long as the number is three or more. For example, in the case of laminating three films, the film is unwound from three sets of raw roll stands, nipped and rolled between heated rolls, and laminated. Lamination must be performed so that the birefringence and elastic recovery of each film do not substantially decrease.

The temperature of the heated roll, that is, the thermocompression bonding temperature is 110 to 130 ° C., preferably 115 to 125 ° C.
° C. If the temperature is too low, the adhesiveness between the films is weak and peeling occurs in the subsequent stretching step, and if it is too high, the birefringence and elastic recovery of the film decrease due to the melting of polyethylene, making it difficult to make the film porous. The above range is appropriate. The nip pressure for thermocompression bonding is 1 to 3 kg / cm ² , and the unwinding speed is 0.5 to 8 m / min. The peel strength between the thermocompression-bonded films is 3 to 60 g / 15.
A range of mm is preferred.

The heat-treated and laminated film is made porous by stretching. Stretching is performed in the order of low-temperature stretching and then high-temperature stretching, and is usually performed at a peripheral speed difference of a stretching roll. The temperature for the low temperature stretching is -20 to 50C, particularly preferably 20 to 35C. If the stretching temperature is low, the film is likely to break during the operation, while if it is too high, the porosity becomes insufficient, which is not suitable. Low-temperature stretching ratio is 5-200
%, Preferably in the range of 10 to 100%. If the stretching ratio is too low, only a material having a low porosity can be obtained, and if it is too high, a material having a predetermined porosity and pore size cannot be obtained, so the above range is appropriate. In the present invention, the low-temperature stretching ratio (E ₁ ) complies with the following equation (2). In the formula (2), L ₁ means the film size after low-temperature stretching, and L ₀ means the film size before low-temperature stretching.

Equation (2) E ₁ = [(L ₁ −L ₀ ) / L ₀ ] × 100

The low temperature stretched film is then hot stretched. High temperature stretching is 70-1 in a heated air circulation oven.
The reaction is carried out at a temperature of 30 ° C, particularly preferably 100 to 125 ° C. If the stretching temperature is out of the above range, the porosity becomes insufficient, which is not appropriate. The high-temperature stretching is preferably performed at a temperature 40 to 100 ° C. higher than the low-temperature stretching temperature. The high temperature stretching ratio is in the range of 100 to 400%.
If the stretching ratio is too low, the porosity becomes insufficient, and if it is too high, the desired air permeability, porosity and pore size cannot be obtained, so the above range is appropriate. In the present invention, the high-temperature stretching ratio (E ₂ ) follows the following equation (3). In the formula (3), L ₂ means the film size after high-temperature stretching, and L ₁ means the film size after low-temperature stretching.

Equation (3) E ₂ = [(L ₂ −L ₁ ) / L ₁ ] × 100

[0030] In the process of stretching and porosifying the laminated porous film of the present invention, when the length in the width direction of the film is greatly reduced and the performance of the porous film such as air permeability, porosity and maximum pore diameter is impaired. A method of stretching while fixing both ends in the width direction of the film with a chuck, a pinch roll, or the like, a method of restoring a decrease in the width of the film in the width direction caused by stretching after stretching the film to be porous, and the like, such as a transverse stretching method. Improvements can be made by the technique. For example, in order to restore the decrease in length in the width direction of the film, transverse stretching is performed at a high temperature after stretching at a low temperature and a high temperature.

For the high-temperature transverse stretching, a method in which the film is fixed in a heated air circulation oven with both-side chucks in the width direction and stretched in the width direction by a tenter method, or a method in which stretching is performed according to a lead angle using a spiral roll. Etc. The temperature for high-temperature transverse stretching is 70 to 130 ° C, particularly preferably 100 to 130 ° C.
125 ° C. Outside of this range, high-temperature transverse stretching is not suitable because the air permeability, porosity and maximum pore size are not improved.

Usually, the length in the width direction of the film is 80 to 9 times that of the film before stretching by low-temperature stretching and high-temperature stretching.
It decreases to about 0%. The stretching ratio of the high-temperature transverse stretching is preferably set within a range that appropriately restores the decrease in the length in the width direction. The stretching ratio of the high-temperature transverse stretching is 5 to 40%, more preferably 10 to 30%. If the stretching ratio is too low, the decrease in the film length in the width direction caused at the time of low-temperature and high-temperature stretching cannot be restored, and if it is too high, the thickness of the film will vary widely, and in some cases, the porous film will break. The above range is preferable because a film is formed. In the present invention, the high-temperature transverse stretching ratio (E ₃ ) complies with the following equation (4). L _{4 in the} formula (4) means the length in the width direction of the film after high-temperature transverse stretching, and L ₃ means the length in the width direction of the film after low-temperature stretching and high-temperature stretching.

Equation (4) E ₃ = [(L ₄ −L ₃ ) / L ₃ ] × 100

In the present invention, after the low-temperature stretching, the high-temperature stretching and the high-temperature transverse stretching, the porous film is heat-set.
In order to prevent the film from shrinking in the stretching direction due to the residual stress applied during stretching, the heat setting is performed by a method in which the length of the film after stretching is reduced by 10 to 50% in advance, or the dimension in the stretching direction is not changed. It is carried out by a method such as heating while regulating the temperature. By this heat setting, it is possible to obtain a porous film having good dimensional stability and satisfying the intended problem.

The battery separator of the present invention has a contact angle of 55 degrees or less, more preferably 50 degrees or less, as evaluated using the following measuring device. The evaluation of the contact angle was performed by using 1: 1 (vol / vol) of dimethoxyethane / propylene carbonate.
l) A non-aqueous electrolyte prepared by dissolving lithium perchlorate in a mixed solution to 1 M / L was dropped on a separator and measured. The smaller the contact angle is, the better the wettability and permeability of the separator to the non-aqueous electrolyte are. Measurement conditions: temperature 23 ° C, humidity 50% Measuring device Image processing type contact angle meter CA-X type manufactured by Kyowa Interface Chemical Co., Ltd.

The porosity of the battery separator manufactured as described above varies somewhat depending on the manufacturing conditions, but the porosity is 30 to 85%, preferably 35 to 75%, and the maximum pore size is 0.02 to 3 μm, preferably. Is 0.03-0.5 μm,
Air permeability is 30 seconds / 100cc to 2000 seconds / 100c
c, preferably 70 sec / 100 cc to 600 sec / 100
cc. If the porosity is too low, the function when used as a battery separator is not sufficient, and if it is too high, the mechanical strength decreases. On the other hand, if the maximum pore diameter is too small, the mobility of ions is deteriorated when used as a battery separator, and if the maximum pore diameter is too large, ion migration is too large, which is not suitable.

The battery separator of the present invention is
The heat shrinkage in the film width direction when heat-treated at 2 ° C. for 2 hours is + 3% or less. If the battery separator thermally shrinks in the width direction of the film more than the above range, the dimension in the width direction is reduced, and there is a concern that the electrode at the end in the width direction is exposed to cause a short circuit, which is not appropriate.

In the battery separator of the present invention, when a high-melting polyethylene is provided for the surface layer and a low-melting polyethylene is provided for the intermediate layer (inner layer), the porous layer made of the low-melting polyethylene of the intermediate layer has a predetermined temperature range. After heat-sealing, the porous layer made of high-melting-point polyethylene disposed on the surface layer is continuously heat-sealed following the heat-sealing of the intermediate layer, so that the laminated porous film, that is, the entire battery separator. Becomes nonporous. This thermal obstruction has a dominant factor in the heat shrinkage of the separator film because the battery separator is manufactured by a stretching method.For example, a porous layer made of polypropylene that does not undergo thermal deformation up to a higher temperature region in the surface layer. When a layer is provided, a problem occurs in that thermal shrinkage of the intermediate layer is hindered, so that thermal clogging does not occur quickly. In other words, by selecting a material to be disposed in the intermediate layer and a laminated structure made of polyethylene having different melting points, excellent heat blocking characteristics of the battery separator are exhibited. Batteries containing the battery separator as a constituent material have a low melting temperature due to the non-porosity of only the middle layer of the battery separator, and a small heat of fusion compared to the heat required to heat block the entire conventional separator. Heat generation and ignition at the time of abnormality can be avoided quickly. Specifically, the heat blocking temperature at which the porous structure of the low melting point polyethylene disposed in the intermediate layer disappears and becomes nonporous is 132 ° C. or less, and the heat of fusion due to the heat blocking is 0.12J /
cm ² or less. Furthermore, the thickness of the battery separator is 5 to 100 μm in terms of mechanical strength, performance, miniaturization, and the like.
Particularly preferably, the thickness is adjusted to 20 to 40 μm.

In the present invention, polyethylene having different melting points is laminated and stretched and porous under appropriate conditions and techniques.
A laminated porous film that quickly and thermally closed in a low temperature range, that is, a battery separator was obtained. This improvement in the heat blocking performance can impart an excellent fuse function to a lithium battery including the battery separator as a constituent material.

By using the separator of the present invention, it is possible to manufacture a lithium battery having an excellent fuse function of rapidly closing a heat in a low temperature range. The lithium battery of the present invention is manufactured into a cylindrical shape, a square shape, a coin shape, or the like by the usual method using the separator. The constituent members other than the separator constituting the lithium battery are not particularly limited, but the following constituent members are used.

For example, as the positive electrode material (positive electrode active material), a lithium-containing metal compound such as a lithium-containing metal oxide, sulfide or chloride is used. As the lithium-containing metal oxide, for example, a lithium composite oxide of at least one metal selected from the group consisting of cobalt, manganese, nickel, chromium, iron and vanadium and lithium is used. Examples of such a lithium composite oxide include LiCoO ₂ , LiMn ₂ O ₄ ,
LiNiO _{2 and the} like.

The positive electrode is prepared by kneading the above positive electrode material with a conductive agent such as acetylene black and carbon black and a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) to form a positive electrode mixture. The positive electrode material is applied to an aluminum foil or a stainless steel lath plate as a current collector, dried, pressed and molded.
It is produced by performing a heat treatment under a vacuum at a temperature of about 0 ° C. to 250 ° C. for about 2 hours.

As the negative electrode (negative electrode active material), a carbon material containing lithium metal, a lithium alloy, and carbon or graphite capable of occluding and releasing lithium, for example, a carbon material such as coke, natural graphite and artificial graphite, and a composite tin An oxide is used. In particular, it is preferable to use a carbon material having a graphite-type crystal structure in which the plane spacing (d ₀₀₂ ) of the lattice plane (002) is 0.335 to 0.340 nm.
The powdered carbon material is used as a negative electrode mixture by kneading with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), or polyvinylidene fluoride (PVDF).

Examples of the non-aqueous electrolyte include ethylene carbonate, propylene carbonate, butylene carbonate,
A solution in which an electrolyte is dissolved in an organic solvent such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane, and tetrahydrofuran is used. Examples of the electrolyte include LiPF ₆ , LiBF ₄ ,
LiClO ₄ , CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NL
i, (C ₂ F ₅ SO ₂ ) ₂ NLi, LiC (SO ₂ CF ₃ ) ₃
And the like. These electrolytes may be used alone or in combination of two or more. These electrolytes are used after being dissolved in the above non-aqueous solvent at a concentration of usually 0.1 to 3M, preferably 0.5 to 1.5M.

The production of a lithium battery using the above-mentioned constituent members is not particularly limited. For example, a coin-type battery can be produced by the following method. LiCoO
₂ 80% by weight of (positive electrode active material), 10% by weight of acetylene black (conductive agent), and 10% by weight of polyvinylidene fluoride (binder), and 1-methyl-
A mixture obtained by adding and mixing a 2-pyrrolidone solvent is applied onto an aluminum foil, dried, press-molded, and heat-treated to prepare a positive electrode. 90% by weight of natural graphite (negative electrode active material) and 10% by weight of polyvinylidene fluoride (binder) were added, and a 1-methyl-2-pyrrolidone solvent was added thereto. , Dried, pressed, and heat-treated to prepare a negative electrode. Then, the coin-type lithium secondary battery (diameter 20 mm, thickness 3.2 mm) can be manufactured by injecting the non-aqueous electrolyte using the separator of the present invention.

[0046]

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1 As a high melting point polyethylene, a number average molecular weight of 20,000
High-density polyethylene having a density of 0.964 and a melting point of 134 ° C. was melt-extruded using a T-die having a discharge width of 1,000 mm and a discharge lip opening of 2 mm. The discharge film is 117
After being guided by a cooling roll at 25 ° C. and being cooled by blowing cold air at 25 ° C., it was taken off at 20 m / min. The thickness of the obtained polyethylene film was 10.5 μm. This unstretched polyethylene film is heat-treated at 120 ° C. for 60 seconds with the take-off direction fixed,
Cooled to room temperature. The heat-treated unstretched polyethylene film has a birefringence of 35.1 × 10 ⁻³ and an elastic recovery of 5
6%.

High-density polyethylene having a number average molecular weight of 14,000, a density of 0.957 and a melting point of 132 ° C. was used as the low melting point polyethylene.
It was melt extruded using a mm T die. The discharge film was guided to a cooling roll at 117 ° C., cooled by blowing cool air at 25 ° C., and then taken up at 20 m / min. The thickness of the obtained polyethylene film is 10.5 μm.
m. This unstretched polyethylene film was heat-treated at 120 ° C. for 60 seconds with the take-off direction fixed, and then cooled to room temperature. The heat-treated unstretched polyethylene film had a birefringence of 35.8 × 10 ⁻³ and an elastic recovery of 46%.

The heat-treated polyethylene film was laminated in a three-layer structure in which high-melting-point polyethylene was disposed on the surface layer and low-melting-point polyethylene was disposed on the intermediate layer. The lamination is performed by unwinding the polyethylene film from the three roll stands at an unwinding speed of 6.5 m / min, respectively, and guiding the polyethylene film to a heating roll.
Thermocompression bonding was performed at a temperature of 120 ° C. and a linear pressure of 1.8 kg / cm. Winding speed is 6.5m / min, unwinding tension is 0.9kg
Met. The thickness of the obtained unstretched laminated film was 31.
It was 5 μm.

The unstretched laminated film was stretched at a low temperature by 50% between nip rolls kept at 30 ° C. At this time, the distance between the rolls was 350 mm, and the roll speed on the supply side was 2 m / mi.
n. The laminated film stretched at low temperature continues to be 11
It is guided into a hot-air circulation oven heated to 5 ° C., stretched at a high temperature to a total stretching amount of 250% between the rolls by using a difference in roll peripheral speed, and then heated at 115 ° C. by a roll.
The film was heat-set for 72 seconds with a 5% relaxation, and a laminated porous film, that is, a battery separator was continuously obtained.

Table 1 shows the film thickness, porosity, maximum pore size, air permeability, tensile strength, heat shrinkage, and heat closing temperature of the obtained battery separator, and FIG. 1 shows the heat closing behavior. The contact angle of the battery separator with the non-aqueous electrolyte was 46 degrees. FIG. 2 is a cross-sectional view of the battery separator before and after the heat treatment performed at 132 ° C. for 10 minutes to simulate the thermal closure.
(Magnification 4000 times) and FIG. 3 (magnification 5000 times). In the battery separator of Example 1, the low-melting-point polyethylene disposed in the intermediate layer was locally clogged with heat.
, And the temperature range where the electric resistance increases is low. When the intermediate layer responsible for thermal obstruction of the battery separator was taken out and the heat of fusion was measured, it was 0.094 J / cm
^{Was 2} . The above evaluation method was performed as follows. 1) Porosity, maximum pore diameter Measured using a mercury porosimeter manufactured by Yuasa Ionics. After weighing 0.03 to 0.07 g of the sample and evacuating it in a glass cell, mercury is injected and filled. The maximum pore diameter and porosity were determined from the mercury pressure and the amount of mercury injected during the filling. 2) Air permeability 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 fastened to 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 from 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). 3) Tensile strength Measured according to ASTM D-822. The maximum stress was measured using a UTM-III-100 type tensile tester manufactured by Orientec under the conditions of a sample width of 10 mm, an initial sample length (between chucks) of 50 mm, and a tensile speed of 50 mm / min. 4) Heat Shrinkage Scale was marked with an initial length of 200 mm in the width direction of the battery separator, and the dimensions were measured after taking out in an oven set at 105 ° C. for 2 hours in an unconstrained state. The heat shrinkage ratio follows the following equation (5). In the formula (5), L ₆ means the film size after being removed from the oven, and L ₅ means the initial film size. Equation (5) Heat Shrinkage = (L ₅ −L ₆ ) / L ₅ × 100 5) Heat Closure Temperature, Heat Closure Behavior Temperature dependence of electric resistance using a self-made cell for measuring electric resistance (FIG. 4). Was measured. The battery separator is a 1: 1 (vol / v) of dimethoxyethane / propylene carbonate.
ol) Lithium perchlorate was dissolved in the mixed solution, immersed in a non-aqueous electrolyte solution adjusted to 1 M / L and degassed, and the non-aqueous electrolyte solution was contained in the microporous body. This sample (separator) 8 was sandwiched between the nickel-made electrodes 3 and 4, set in a cell for measurement, and heated in an oven at a rate of 10 ° C./min from room temperature. The electric resistance between the electrodes is 352 manufactured by Hioki Electric Co., Ltd.
0 Measured using LCR HiTESTER. The heat closing temperature was a temperature at which the electric resistance became 100 times or more as compared with that before the measurement. In FIG. 4, only one of the four fastening portions in the longitudinal sectional view is shown for convenience. 5) Heat of fusion It was measured using DSC-7 manufactured by PerkinElmer. The sample was cut into a predetermined area, and the heat of fusion was determined from the integral of the melting curve at a rate of temperature increase of 10 ° C./min.

Example 2 A battery separator was obtained in the same manner as in Example 1 except that high-temperature transverse stretching was performed in the width direction of the film after high-temperature stretching.
In the high-temperature transverse stretching, the stretching temperature was 120 ° C., the stretching ratio was 10%, and the relaxation ratio was 4%. Film thickness, porosity, maximum pore size, air permeability, tensile strength, heat shrinkage of the obtained battery separator,
Table 1 shows the heat closing temperature. The contact angle of the battery separator with the non-aqueous electrolyte was 47 degrees. Further, when the intermediate layer responsible for heat blocking of the battery separator was taken out and the heat of fusion was measured, it was 0.093 J / cm ² .

Comparative Example 1 Number average molecular weight 20,000, density 0.964, melting point 134
C. was extruded using a T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm.
The discharge film was guided to a cooling roll at 117 ° C.
20m / mi after being cooled by blowing cold air of
n. The thickness of the obtained polyethylene film was 10.5 μm. This unstretched polyethylene film is heated to 120 ° C. in a state where the take-up direction is fixed.
After the heat treatment for 0 second, it was cooled to room temperature. The birefringence of the heat-treated unstretched polyethylene film was 35.3 × 1
0 ^-3 , the elastic recovery was 58%.

The heat-treated polyethylene film was laminated into a three-layer structure in which three identical polyethylene films were laminated, and then stretched and made porous. Lamination and stretching of the film were performed in the same manner as in Example 1. Table 1 shows the thickness, porosity, maximum pore size, air permeability, tensile strength, heat shrinkage, and heat closing temperature of the obtained laminated porous film, that is, the battery separator.
FIG. 1 shows the thermal clogging behavior. In the battery separator of Comparative Example 1, since the high-melting-point polyethylene thermally clogs the entire battery separator, the increase in electrical resistance in FIG. 1 is moderate and the temperature range in which the electrical resistance increases is also high. The heat of fusion required for heat blockage was 0.26 J / cm ² .

[0055]

According to the present invention, by laminating polyethylenes having different melting points and making them porous by a stretching method, a polyethylene-based material which has a small heat shrinkage in the film width direction, and is quickly heat-sealed in a low temperature region. A battery separator made of the laminated porous film was obtained. By improving the heat closing behavior of the present invention, it is possible to provide a battery separator having an excellent fuse function of rapidly performing heat closing in a low temperature range, and a lithium battery using the same.

[0056]

[Table 1]

[Brief description of the drawings]

FIG. 1 is a view showing a heat blocking behavior in a battery separator of the present invention.

FIG. 2 is a cross-sectional view of the battery separator of the present invention;
It is a figure which shows the cross-sectional structure before the heat treatment of this battery separator heat-clogged by heat-treating at 10 degreeC for 10 minutes simulatedly.

FIG. 3 is a cross-sectional view of the battery separator of the present invention;
It is a figure which shows the cross-section of the said battery separator heat-clogged by heat-treating at 10 degreeC for 10 minutes simulatedly.

FIG. 4 is an electric resistance measuring cell for measuring the temperature dependence of electric resistance.

[Description of References] 1: Top case (made of aluminum) 2: Top current collector (made of stainless steel) 3: Top electrode (made of nickel) 4: Bottom electrode (made of nickel) 5: Bottom current collector (made of stainless steel) 6 : Bottom case (made of Teflon) 7: Spacer (made of Teflon) 8: Separator (sample) 9: Hexagon socket head bolt 10: Wing nut 11: Collar (brass) 12: Spring (SUS)

Continued on front page F-term (reference) 5H021 BB05 CC00 CC04 EE01 EE04 EE31 HH00 HH01 HH02 HH03 HH06 5H024 AA01 AA02 AA03 AA06 AA07 BB00 BB01 CC03 CC17 DD09 EE09 HH01 HH11 HH13 5H029 AJ07 AK01 AM05 A03 DJ04 DJ13 EJ12 HJ00 HJ04 HJ06 HJ14

Claims (7)

[Claims] 1. A battery separator comprising a laminated porous film formed by laminating three or more laminated films in which at least two or more types of polyethylene having different melting points are laminated by a stretching method, and having a maximum pore diameter of 0.02. ３3 μm, porosity 30-85%, film thickness 5-100 μm, air permeability 30-
A battery separator having a heat shrinkage in the film width direction of + 3% or less when heat-treated at 2,000 seconds / 100 cc and 105 ° C. for 2 hours. 2. At least one low melting point polyethylene having a melting point of 105 ° C. or more and less than 133 ° C., and a melting point of 13 ° C.
The battery separator according to claim 1, comprising at least one kind of high melting point polyethylene having a temperature of 3 ° C or higher. 3. The battery separator according to claim 2, comprising a laminated porous film in which high melting point polyethylene and low melting point polyethylene are alternately laminated. 4. The battery separator according to claim 2, wherein the surface layer is made of a laminated porous film laminated so that the high-melting-point polyethylene and the inner layer are low-melting-point polyethylene. 5. The heat blocking temperature at which the porous structure of the porous layer made of low melting point polyethylene disappears and becomes nonporous is 132 ° C.
Below, and the heat of fusion associated with the thermal blockage is 0.12 J
/ Cm < ² > or less, The battery separator of Claims 2-5 characterized by the above-mentioned. 6. The battery separator according to claim 2, wherein the low melting point polyethylene contains at least one low molecular weight compound selected from a plasticizer, a lubricant, and a solvent. 7. A lithium battery comprising a positive electrode made of a lithium-containing metal oxide, sulfide, or chloride, a negative electrode made of a carbon material, a non-aqueous electrolyte, and a separator. Lithium battery containing a separator for components as a constituent material.