JPH10302747A - Production for separator of battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the response of a SD function of a separator to temperature rise by improving a method for elongating a separator. SOLUTION: A film containing a low melting point resin component of such as polyethylene and high melting point resin component of such as polypropylene is uniaxially elongated at least two times within a temperature range defined by the following expression (1) and then elongating in the same direction as that of the uniaxial elongation within a temperature range defined by the following expression (2): (1) from -20 deg.C to (Tmb-30) deg.C; (2) from (Tmb-30) deg.C to (Tmb-2) deg.C wherein Tmb stands for the melting point of the low melting point resin component.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a battery separator, and more particularly, to a method for manufacturing a battery separator having a porous structure and improving battery safety.

[0002]

2. Description of the Related Art In recent years, lithium batteries have attracted attention as batteries for responding to cordless electronic devices because of their high energy density, high electromotive force, and low self-discharge.

The negative electrode material of a lithium battery is formed of lithium metal, an alloy of lithium and a metal such as aluminum, a material that adsorbs or occludes lithium ions such as carbon or graphite, or a conductive polymer doped with lithium ions. Are known. As the positive electrode material, fluorinated graphite generally represented by (CF _x ) _n , CoLiO ₂ , MnO ₂ , V ₂ O ₅ , CuO, A
Metal oxides such as g ₂ CrO ₄ and sulfides such as TiO ₂ and CuS have been proposed.

In such a lithium battery, lithium as a constituent material of the negative electrode has strong reactivity,
LiPF _{6 in} an organic solvent such as ethylene carbonate, propylene carbonate, acetonitrile, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran,
Since a non-aqueous electrolyte using LiCF ₃ SO ₃ , LiClO ₄ , LiBF ₄ or the like as an electrolyte is used, an external short circuit or a positive electrode,
When an abnormal current flows due to incorrect connection of the negative electrode or the like, heat may be generated due to the resistance of the electrolyte.

[0005] In order to prevent such abnormal heat generation, a film used as a separator, when an abnormal rise in temperature due to an external short circuit occurs, shuts off current by increasing electric resistance to suppress a rise in temperature (hereinafter, referred to as a "shutdown"). It is required to have a function. Further, from the viewpoint of ensuring safety, the electric resistance increased by the SD function is a temperature at which the SD function starts (hereinafter, referred to as “SD start temperature”).
That. It is desirable that the temperature be maintained even at a high temperature exceeding ()).

It has been proposed to use a porous film of a synthetic resin such as polypropylene or polyethylene as a separator having an SD function (Japanese Patent Application Laid-Open No. 60-23954). According to such a porous membrane, when the temperature rises abnormally and becomes equal to or higher than the melting point of the resin, the resin melts, and this melt blocks the pores in the membrane, shuts off the electric current, and further raises the temperature. Can be suppressed. However, at a temperature equal to or higher than the melting point, the separator itself shrinks due to melting, and the original shape cannot be maintained. As a result, the electric resistance is reduced to further increase the temperature, and the positive and negative electrodes are brought into contact with each other to cause an internal short circuit. In other words, there is a problem that the difference between the heat resistance temperature of the separator and the SD start temperature is small in the porous film made of a single resin.

Therefore, a mixture of a resin having a different melting point (a material having a melting point at the SD starting design temperature and a material having a melting point higher by about 30 ° C.) (Japanese Patent Laid-Open No. 4-206257) or a multilayer structure (Japanese Patent Laid-Open No. 4-206257). No. 181651, JP-A-62
It has been proposed to use a porous membrane having the following. These become porous membranes at high temperatures.
The low melting point resin melts and closes the holes, blocking the flow of current. At this time, since the resin with a high melting point does not melt,
The shape of the film is maintained and the increased electrical resistance can be maintained.

[0008]

However, a mixture of a high-melting-point resin component and a low-melting-point resin component or a laminate containing these resins does not always exert the SD function while reliably maintaining the shape of the film. Absent. In particular, the air permeability is 600
In a high-permeability product of (sec / 100 cc) or less, it takes a long time to close the hole because the hole diameter is large, and during this time, the current continues to flow through the hole that is not completely closed, and the temperature of the battery rises. there were.

It is an object of the present invention to provide a battery separator having a high response of the SD function to a temperature rise.

[0010]

Means for Solving the Problems To achieve the above object, a method for producing a battery separator according to the present invention comprises forming a film containing a low-melting resin component and a high-melting resin component having a higher melting point than the resin component. After uniaxially stretching at least twice in the temperature range of the following formula (1), stretching is performed in the same direction as the direction of the uniaxial stretching in the temperature range of the following formula (2).

-20 ° C to (Tmb-30) ° C (1) (Tmb-30) ° C to (Tmb-2) ° C (2)

Here, Tmb is the melting point of the low melting point resin component.

[0013] With such a configuration, it is possible to reduce the pore diameter of the pores in the separator, and to produce a separator in which the pores are rapidly closed while maintaining the strength when the temperature is abnormally increased.

In the above method, the film is preferably composed of two or more layers having different contents of the low melting point resin component. With such a configuration, it is possible to manufacture a separator that reliably maintains its shape even near the SD start temperature.

In the above-mentioned manufacturing method, it is preferable to use a film containing polyethylene as a low melting point resin component and polypropylene as a high melting point resin component. With this configuration, the SD start temperature and the heat resistance temperature can be set to values suitable for a lithium battery separator.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In practicing the present invention, first, a film containing a low melting point resin component and a high melting point resin component having a higher melting point than this resin component is prepared. The melting point difference between the low melting point resin component and the high melting point resin component is 5 ° C. or more, preferably 1 ° C.
The type of resin is not particularly limited as long as it is 0 ° C or higher,
It is preferable to use a polyolefin such as polyethylene or polypropylene.

This film may have a single layer or a laminated structure. In the case of having a laminated structure, each layer may be composed of a simple substance of the low melting point resin component or the high melting point resin component, or may be a mixture of both resin components, but the content of the low melting point resin component is different. It is preferred to have two or more layers.

An embodiment of the present invention using a laminated film will be described below. The laminated film can be formed by a known extrusion molding method or the like. The extrusion temperature (die temperature) when employing the extrusion molding method is not particularly limited, but the melting point (Tma) of the high melting point resin component is considered from the viewpoint of workability. ) Is preferably performed at a temperature higher by 10 to 150 ° C. After the extrusion, the draft ratio (the value obtained by dividing the film winding speed by the resin extrusion speed) is 10 to 40.
A long laminated film can be obtained by winding on a core so that the winding speed is 0 to 200 m / min, preferably 10 to 200 m / min in a range of 0, preferably 20 to 200.

In the method of the present invention, the laminated film is stretched at a low temperature.
Annealing can be performed by heating in a temperature range of (mb-30) ° C. to (Tma-2) ° C. for a predetermined time (typically, several seconds to several hours). By performing this annealing, the porosity of the porous film obtained through two stretching operations performed later can be increased.

The laminated film or the annealed laminated film is first subjected to -20 ° C. to (Tmb-3
0) In a low temperature region of 0 ° C., the film is uniaxially stretched by a method such as roll stretching or tenter stretching. If the temperature at this time is too low, the film is likely to break and stretching is difficult, and if the temperature is too high, it is possible to obtain the desired porous separator even by performing high-temperature stretching in the next step. Can not. This low temperature stretching is performed in the same direction twice or more.

The stretching ratio during the first low-temperature stretching is usually from 1 to
It is 100%, preferably 2 to 50%, and the stretching speed is usually 10 to 1000% / min. This stretching ratio is a value calculated by the following equation (3) using the length L ₀ of the laminated film before stretching and the length L ₁ after the first low-temperature stretching.

Stretch ratio (%) = (L ₁ −L ₀ ) / L ₀ × 100 (3)

After the completion of the first low-temperature stretching, the second and subsequent low-temperature stretching are performed. Although the interval between each low-temperature stretching is not particularly limited, it is usually performed for 1 second to 10 minutes. The total stretching ratio of the low-temperature stretching performed in several times is usually 5 to 150%, preferably 20 to 100%, and the stretching speed of each stage after the second time is usually 100 to 1000% / min. Note that this stretching ratio is a value calculated by the following equation (4) using the length L ₀ of the laminated film before stretching and the length L ₂ after all the low-temperature stretching is completed.

Stretch ratio (%) = (L ₂ −L ₀ ) / L ₀ × 100 (4)

In the method of the present invention, after this low-temperature stretching, uniaxial stretching is performed in the high-temperature region of (Tmb-30) ° C. to (Tmb-2) ° C. in the same direction as the low-temperature stretching. By this high-temperature stretching, the micropores generated in the laminated film during the low-temperature stretching are enlarged, and the porosity is increased. If the temperature during high-temperature stretching is too low, the porosity cannot be increased,
If it is too high, the low melting point resin melts and only a resin having a high resistance value can be obtained.

This high-temperature stretching can be performed by roll stretching, tenter stretching or the like, similarly to low-temperature stretching. The stretching ratio during high-temperature stretching is usually 10 to 600%, preferably 20 to 300%, and the stretching speed is usually 10 to 400%.
/ Min. Note that the stretching ratio is based on the length L ₀ of the laminated film before the low-temperature stretching, using the length L ₂ of the laminated film that has been subjected to the low-temperature stretching (the length before the high-temperature stretching) and the length L ₃ after the high-temperature stretching. Is a value calculated by the following equation (5).

Stretch rate (%) = (L ₃ −L ₂ ) / L ₀ × 100 (5)

The separator obtained by sequentially performing the low-temperature stretching and the high-temperature stretching may have a built-in stretching strain.
For the removal, the separator can be kept in a tensioned state or a relaxed state after the high-temperature stretching, and can be heated at a predetermined temperature (usually the same temperature as the high-temperature stretching). The heating time for removing the distortion is set in accordance with the temperature, the amount of distortion remaining in the separator, and the like, and is usually 2 seconds to 10 minutes.

The battery separator obtained by such a method is a laminate having a high melting point resin component and a low melting point resin component, and each layer is made porous. The porous structure of such a separator can be confirmed by an electron microscope or the like, and its pore size and porosity can be adjusted according to manufacturing conditions. However, the pore size is preferably about 0.02 to 1 μm, and the porosity is Preferably, it is about 20-60%. The electric resistance measured in the organic electrolyte is preferably about 10 to 30 Ωcm ² .

In the battery incorporating the separator obtained by the present invention, when the internal temperature rises due to an abnormal current during use, the low melting point resin component of the separator quickly melts and closes the pores, while the high melting point resin component Maintains the shape of the separator without melting up to its melting point. Then, due to the blockage of the holes due to the melting of the low-melting-point resin component, the electric resistance sharply increases and the electric current is interrupted, so that an excessive rise in temperature is controlled and the safety is maintained. In the case of a laminated film composed of a plurality of layers having different contents of the low-melting resin component, a layer having a high content of the low-melting resin component closes the pores as soon as possible to cut off the current. The layer having a low ratio functions as a support for the separator, and can reliably maintain the shape.

The safety of the separator obtained by the present invention will be described more specifically. The electrical resistance of a lithium battery separator measured in an organic electrolyte solution (conductivity of about 10 mS / cm) at around room temperature is generally about 10 Ωcm ² at a thickness of 25 μm. It has been recognized that in order to control an excessive rise in temperature due to an abnormal current, it is necessary to increase the electric resistance by at least two to three orders of magnitude over a value near room temperature in order to cut off the current. When the present inventors tested a separator manufactured by the method of the present invention using a laminated film containing polyethylene and polypropylene, the electrical resistance of the separator near room temperature was 25%.
It is about 10 Ωcm ² at μm, but it is about 100,000 to 1,000,000 Ωcm ^{2 at a} temperature higher than the melting point Tmb of the low-melting resin component, and it has been found that the resistance increases by 4 to 5 digits. This large increase in resistance is due to the electrical cutoff function of this separator,
That is, it indicates that the SD characteristics are excellent and the safety of the battery incorporating the separator is excellent.

The reason why the SD characteristics are particularly improved by performing the low-temperature stretching twice or more is that the air permeability decreases by performing the low-temperature stretching twice or more, in other words, the pore diameter is reduced and the pores are securely closed. It seems to be because it becomes.

The separator obtained by the present invention is subjected to a hydrophilic treatment such as a corona discharge treatment, a surfactant impregnation / drying treatment, and a draft polymerization treatment of a hydrophilic monomer in order to improve the wettability to the electrolyte. It may be incorporated in

[0034]

EXAMPLES The characteristics of the separators produced in the following examples and comparative examples were measured in the following manner.

(Air permeability) The air permeability is the time required for passing 100 cc of air, and the measurement is made by a Gurley type densometer NO. 323.

(Average pore diameter) The density (ρ ₀ ) of the unstretched laminated film is determined using DENSIMETER-H manufactured by Toyo Seiki Seisakusho as a specific gravity meter. Next, the apparent density (ρ ₁ ) of the separator is determined from the thickness, area and weight of the separator obtained from the laminated film. Then, the porosity is calculated by the following equation (6).

Porosity = (1− (ρ ₁ / ρ ₀ )) × 100 (6)

(SD Characteristics) As shown in FIG.
m platinum plate electrodes 1 and 1 ′ are opposed to each other, and a separator 2 is disposed therebetween, silicone rubbers 3 and 3 ′ are used as packing, and polytetrafluoroethylene plates 4 and 4 ′ are further provided.
The whole was fastened from both sides. As the electrolyte, propylene carbonate and dimethoxyethane were mixed in equal amounts,
A solution prepared by dissolving LiBF ₄ at a concentration of 1 mol / liter was used, and this was mixed with a nonwoven polypropylene fabric 5 filled between the electrodes 1, 1 ′ and the polytetrafluoroethylene plates 4, 4 ′. 5 'was impregnated. Although not shown, an ohmmeter was connected to the platinum electrode, and a thin thermocouple was arranged between the electrode and the separator. The measurement cell having such a structure was set in a dryer, the temperature was increased at a rate of 5 to 7 ° C./min, and the electric resistance at each temperature was measured. The electric resistance is a resistance meter manufactured by Kokuyo Electric Industries, LCR
Using a meter KC-533 type, it was measured at an alternating current resistance of 1 kHz and converted by the following equation (7).

Electric resistance = resistance (Ω) × electrode area (cm ² ) (7)

Example A laminated film having a three-layer structure in which the low melting point resin component was polyethylene (melting point 130 ° C.) and the high melting point resin component was polypropylene (melting point 165 ° C.) was used. Intermediate layer part: polyethylene: polypropylene = 1: 1
And a layer of polypropylene alone was laminated on both sides of this layer. The thickness ratio of each layer was 1: 1: 1. Extrusion molding is performed at a die temperature of 230 ° C.
A μm laminated film was obtained. 145 of this laminated film
After heating at 5 [deg.] C. for 5 minutes to perform annealing, the first uniaxial stretching is performed at 30 [deg.] C. so that the stretching ratio becomes 10% in the machine direction, and after 10 seconds, the total low temperature stretching ratio becomes 40%. The same low-temperature stretching was performed as described above. Next, the separator was uniaxially stretched at 125 ° C. in the same direction as the above so that the stretching ratio became 200%, to obtain a separator.

The thickness of this separator is 25 μm,
Is 650 (sec / 100 cc), and the average pore size is 0.042 μm.
m, porosity is 41%, electric resistance at 25 ° C. is 12Ω
cm ^TwoMet. This SD characteristic is as shown in FIG.
You. As can be seen from FIG.
The resistance value increases rapidly by 4 to 5 orders in the near future, and this large resistance value
Is maintained at around 160 ° C. Resistance value around 130 ° C
The rapid increase in
This phenomenon occurs when the resistance increase phenomenon occurs around 160 ° C.
This separator has excellent SD characteristics because it is maintained at
It can be understood that

(Comparative Example) A laminated film produced under the same materials and extrusion conditions as in the example was subjected to the same annealing as in the example. This was subjected to a single low-temperature stretching at 30 ° C. so that the stretching ratio was 40% in the machine direction. Then 1
The separator was uniaxially stretched at 25 ° C. in the same direction as the above so that the stretching ratio became 200%, to obtain a separator.

The separator had a thickness of 25 μm, an air permeability of 600 (sec / 100 cc), and an average pore diameter of 0.044 μm.
m, porosity is 42%, and electric resistance at 25 ° C. is 12.
It was 5 Ωcm ² . This SD characteristic is as shown in FIG. As can be seen from FIG.
In the vicinity, the resistance value increases by 4 to 5 digits, and this large resistance value becomes 1
The temperature is maintained up to around 60 ° C., but the resistance value rise is not sharp.

[0044]

As described above, according to the present invention, a film containing a low-melting resin component and a high-melting resin component is uniaxially stretched twice or more in a predetermined low-temperature region, and then is stretched in a predetermined high-temperature region. By stretching in the same direction as the stretching, it is possible to provide a separator having high responsiveness to the temperature of the SD function and higher safety.

[Brief description of the drawings]

FIG. 1 shows S of a battery separator obtained according to an example.
It is a graph which shows D characteristic.

FIG. 2 shows S of a battery separator obtained according to a comparative example.
It is a graph which shows D characteristic.

FIG. 3 is a cross-sectional view schematically showing an apparatus for measuring SD characteristics of a separator.

[Explanation of symbols]

 1, 1 'platinum electrode 2 separator 3, 3' silicone rubber 4, 4 'polytetrafluoroethylene plate 5, 5' polypropylene non-woven fabric

Claims (3)

[Claims] 1. A film containing a low melting point resin component and a high melting point resin component having a higher melting point than this resin component is uniaxially stretched twice or more in a temperature range of the following formula (1), and then the following formula (2) Wherein the film is stretched in the same direction as the uniaxial stretching in the temperature range described above. -20 ° C to (Tmb-30) ° C (1) (Tmb-30) ° C to (Tmb-2) ° C (2) where Tmb is the melting point of the low melting point resin component. 2. The method for producing a battery separator according to claim 1, wherein the film comprises two or more layers having different contents of the low melting point resin component. 3. The low melting point resin component is polyethylene,
3. The method for producing a battery separator according to claim 1, wherein the high melting point resin component is polypropylene.