JPH0722014A - Separator for battery - Google Patents

Abstract

PURPOSE:To realize an excellent property at low temperatures and hold the shape of a sealing film even at high temperatures by laminating the porous film composed of micro molecular weight polyethylene having a specific viscosity average molecular weight, and nonwoven fabric including multiple layer fiber provided with a core made of a resin having a melting point higher than that of polyethylene. CONSTITUTION:A composition composed of micro molecular polyethylene having a viscosity average molecular weight (MV) of 500,000 or more and a plasticizer is fused and extruded, thus obtaining a sheet. A plasticizer is extracted from the sheet, thereby obtaining a porous film. Nonwoven fabric consisting of an outer layer of polyethylene and a core of polyester and including multiple layer fiber made of a resin having a melting point higher by 20 deg.C than that of polyethylene is laminated on the porous film by heat lamination or the like, thus obtaining a separator. It is preferable that the porous film has a thickness of 10-50mum, air permeability of 20-3000sec/100cc, and a hole ratio of 25-80%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator, and more particularly to a battery formed by laminating a non-woven fabric and a porous film having excellent gas, liquid and ion permeability and excellent film shape retention at high temperature. Regarding the separation letter.

[0002]

2. Description of the Related Art With the development of small portable devices, small size and high performance batteries have been required. The lithium battery is highly useful as an electrode material for a small high-performance battery because it has a high electromotive voltage due to the use of the base metal, lithium. However, lithium is highly reactive and will cause battery rupture if mishandled. Even in the case of lithium batteries, cases such as ignition have occurred in the past, and prevention of shortage is an important issue.

[0003]

The separator of a lithium battery is made of a porous film and is interposed between the positive electrode and the negative electrode to allow ions to pass between the positive electrode and the negative electrode but prevent physical contact between the positive electrode and the negative electrode. belongs to. The separator is usually composed of a porous film such as polypropylene.However, if the high-temperature film shape retention property of the separator and the heat blocking property of the pores at high temperature are insufficient, a large current will be generated in a short time due to an external short-circuit accident. When flowing, the lithium battery generates heat, the separator cannot maintain the film shape due to the heat, the positive electrode and the negative electrode physically contact with each other to cause an internal short circuit, and an accident such as rupture may occur. Therefore, when the internal temperature of the battery rises in the separator, the property that the pores of the separator are automatically closed by heat (self-closing property) and the property that the film shape is maintained and the electrodes are separated even when the temperature becomes high (high temperature film) Shape retention properties) are required.

A separator made of a polypropylene porous film is excellent in shape retention at high temperatures, but when it is used as a lithium battery separator, the temperature at which it self-closes is about 175 ° C., which is the ignition temperature of lithium. 1
The temperature is close to 80 ° C, and there is a problem such as bursting unless self-closing is surely caused in the case of a short circuit accident. Further, the separator film is usually stretched to improve the strength, but the stretched film has low high-temperature film shape retention property, and shrinkage or rupture occurs in the vicinity of 150 to 160 ° C. for polyethylene and 180 ° C. for polypropylene, so that the electrode Creates a problem with the isolation of the.

[0005]

The inventors of the present invention have conducted extensive studies to solve such problems, and as a result, have a high low-temperature occluding property due to a laminate in which a special porous membrane and a special nonwoven fabric are laminated, The inventors have invented a membrane having a high temperature membrane shape maintaining property, which is extremely satisfactory as a separator. The gist of the present invention is a laminate in which a porous membrane made of ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 500,000 or more and a non-woven fabric are laminated, wherein the non-woven fabric is polyethylene as an outer layer and the polyethylene is a core. A separator for a battery, which is a non-woven fabric containing multi-layered fibers made of a resin having a higher melting point of 30 ° C. or higher.

The present invention will be described in detail below. The polyethylene porous film (porous sheet) constituting the laminate of the present invention is a porous film made of ultra-high molecular weight polyethylene having a viscosity average molecular weight (Mv) of 500,000 or more. The thickness of the porous membrane is 1
0 to 50 μm is preferable, and more preferably 15 to 30
μm. Films thinner than 10 μm have small absolute strength,
It is easy to break during film formation or lamination processing, or film breakage after battery processing. Further, when the film thickness exceeds 50 μm, the amount of water permeation may be small, or the proportion of the separator in the battery may be large, resulting in a decrease in battery capacity. The air permeability is preferably about 20 to 1000 seconds / 100 cc, and preferably 50 to 300 seconds / 100 cc. If the air permeability is less than 20 seconds / 100 cc, the ratio of the pores to the surface area of the film (open area ratio) becomes too large, and the strength of the film tends to decrease. If it is more than 1000 seconds / 100 cc, the permeation resistance of ions becomes large, which is not preferable as a separator.

The polyethylene porous membrane of the present invention has a thickness of 130-1.
At 45 ° C, the pores close and block the ionic current. But,
Since it is laminated with a non-woven fabric, the film shape is maintained even if the temperature is raised to about 200 ° C. Therefore, the electrode reaction can be safely stopped even if the battery generates heat such as a short circuit.

The porosity is preferably about 25 to 80%. If the porosity is less than 25%, the pore structure becomes too dense and the ion permeability decreases. If it is more than 80%, the amount of polyethylene occupying the unit volume becomes too small and the strength is lowered. The breaking strength is preferably 100 kg / cm ² or more in both the longitudinal and lateral directions. If it is less than this range, the film is likely to be broken during production, resulting in poor workability. Bubble point is 2-5kg
/ Cm ² or more is desirable. If the bubble point is less than 2 kg / cm ² , the pore structure becomes sparse and it is not practical. On the other hand, if it is larger than 5 kg / cm ² , the pore structure becomes too dense, resulting in resistance to filtration and ion permeation.

The water permeability is 100 to 1500 liters / hr.
・ M ²・ atm is good. Permeability is 100 liters /
If it is less than hr · m ² · atm, the ion permeability is lowered. If it is more than 1500 liters / hr · m ² · atm, the pore structure becomes sparse and there is a risk that the electrodes contact each other when used as a battery separator. It is desirable that 0.091 μm styrene latex particles block 50% or more. If it is less than 50%, there is a risk that the electrodes are in contact with each other due to the uneven size of the holes.

A preferred method for obtaining the porous membrane used in the laminate of the present invention is to melt-extrude a composition comprising ultra-high molecular weight polyethylene and a plasticizer to obtain a sheet, without stretching, and then plasticize the sheet. It is a porous membrane obtained by extracting and removing the agent with a solvent or the like. The porous film can be heat-treated for stabilization, and the heat treatment can be performed by a heating roll method, a tenter method, or the like. The porous film thus obtained has a dense network structure.

The polyethylene used for the porous membrane is a so-called ultra high molecular weight polyethylene having a weight average molecular weight of 500,000 or more, and particularly a viscosity average molecular weight of 1 × 10.
^{6 to} 3.0 × 10 ⁶ is preferable.

If the molecular weight is too low, it is difficult to uniformly knead with the plasticizer, and a porous membrane having a fine pore structure cannot be obtained. Further, stable sheet formation becomes impossible. As a plasticizer used in the production of a porous film, it has good compatibility with ultra-high molecular weight polyethylene, has a boiling point higher than the melt-forming temperature (up to 250 ° C) of the ultra-high molecular weight polyethylene, and does not easily evaporate during sheet forming. A low vapor pressure is a preferable condition. 160-200 good or poor compatibility of plasticizer with ultra high molecular weight polyethylene (matrix resin)
It depends on whether or not the melt-kneaded body of these two components at 0 ° C. is a uniform dispersion system that can be stably melt-extruded. Specifically, when a large number of apparent viscosities of the melt-kneaded product are measured with a flow tester, those having stable fluidity with a swing range of 30% or less, preferably 10% or less are considered to have good compatibility. To do.

Further, in consideration of the molding stability of a sheet comprising an ultrahigh molecular weight polyethylene and a plasticizer obtained in the intermediate stage of the production of the porous membrane and the ease of handling, the plasticizer is specifically liquid paraffin, solid paraffin, Stearyl alcohol and cetyl alcohol are preferable. In particular, those that are solid at room temperature are very useful in handling. These plasticizers and ultra-high molecular weight polyethylene are usually mixed with a mixer or the like, then uniformly kneaded by melt-kneading, then pelletized and then subjected to sheet forming. When stearyl alcohol is used, it can be used in the form of fine granules and can easily be mechanically blended with powdery ultra high molecular weight polyethylene. It is possible to extrude the formed sheet. Further, a heat stabilizer, an antioxidant, a colorant and the like may be added to the composition.

The mixing ratio of ultrahigh molecular weight polyethylene and plasticizer is usually ultrahigh molecular weight polyethylene / plasticizer = 10/90 to 40/60, preferably 15 /
The range is 85 to 35/65. If the proportion of polyethylene is too low, the extruded state in the extruder becomes unstable and a good sheet cannot be obtained. On the other hand, if the proportion of polyethylene is too high, the viscosity becomes too high, and the flow at the die becomes unstable, making it impossible to obtain a stable sheet. Those obtained by melt-kneading these compositions and pelletizing them can prevent the polyethylene and the plasticizer from being separated during the sheet formation, and thus improve the molding stability.

The sheet is formed by feeding pellets obtained by melt-kneading polyethylene and a plasticizer or a mixture obtained by mechanically blending polyethylene and a plasticizer to an extruder, and then bringing the mixture into a uniform molten state, and then a sheet is formed from an appropriately selected die. It is done by extruding into a shape. The thickness of the sheet obtained by sheet forming is usually 0.03 to 0.5 mm, preferably 0.03 to 0.08 mm.

At this time, it is desirable that the sheet is not stretched and molded so as not to cause molecular orientation as much as possible. The next removal (extraction) of the plasticizer dissolves the plasticizer well,
An extraction method using an easily volatile solvent is preferable. As the volatile solvent, hydrocarbon system such as pentane, hexane, heptane,
Examples include halogenated hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride and ethane trifluoride, alcohols such as methanol, ethanol and propanol, and ethers such as diethyl ether and dioxane. Then, the volatile solvent is removed by drying to obtain a porous sheet. The content of the plasticizer remaining in this porous sheet is preferably less than 1% by weight. The plasticizer should be removed at room temperature or higher, usually 5 to improve removal efficiency.
It is desirable to carry out at about 0 ° C to 90 ° C. When using a volatile solvent, it is necessary to pay attention to the ignition temperature.If the removal temperature is close to the ignition temperature, mix the volatile solvent with water etc. and make the ignition temperature sufficiently lower than the removal temperature. good. When removing the plasticizer, the sheet is 5% to 30% in the thickness direction.
It is preferable to carry out the treatment under the condition that the shrinkage is about%.

Although the above-mentioned porous film has sufficient strength and heat resistance, it is laminated with a polyolefin-based nonwoven fabric having a special structure in order to further enhance safety as a battery separator. The porous film may be used as it is by removing the plasticizer as it is, but it is preferable that the porous film is subjected to heat treatment to prevent shrinkage due to temperature and then subjected to lamination. The heat treatment industrially includes a heating roll method, a tenter method, and the like, and it is preferable that the heat treatment temperature is high. However, when the heat treatment temperature is higher than the melting point of the polyethylene, the pores are blocked and the air permeability is significantly increased, which is not preferable. . In the case of the heating roll method, it is preferable to perform heat treatment at a melting point or lower, preferably 130 ° C. or lower. The heat treatment is preferably performed at a temperature, pressure and time such that the sheet before heat treatment shrinks about 2% to 20% in the thickness direction.

Next, a polyolefin-based nonwoven fabric forming a laminate with the porous film will be described. The non-woven fabric is a non-woven fabric containing polyethylene as an outer layer and a multi-layered fiber made of a synthetic resin having a melting point of 20 ° C. or higher, preferably 30 ° C. or higher as the outer layer of polyethylene. Examples of the resin that can be used as the core of the multilayer fiber include polypropylene,
Poly-4 methyl pentene-1, poly-3 methyl butene-
1 and other polyolefin resins, polyethylene terephthalate, polybutylene terephthalate, polyester resins such as polycarbonate, polyamide resins and the like.
The thickness of the core is preferably about 1/5 to 4/5 of the fiber diameter in order to maintain the adhesiveness of the outer layer and the shape retaining effect of the core, and the position of the core is the center of the fiber. Alternatively, it may be eccentric.

The fibers constituting the non-woven fabric are 0.5 denier or more.
A fiber having a thickness of about 15 denier, preferably about 1.5 to 5 denier is preferable. The basis weight of non-woven fabric is 5
30 g / m ^2, preferably about preferably about 8 to 20 g / m ^2. As the non-woven fabric, one produced by a spunbond method, a dry method, a wet method, or the like can be used, but a so-called wet method in which short fibers are dispersed in water is suitable without unevenness in thickness or unit weight. Used. It is preferable to mix the synthetic fibers made of polyethylene and other synthetic fibers with the above-mentioned multilayer fibers at the time of producing the nonwoven fabric. The mixing ratio is 5 to 60% by weight, preferably 20 to 20% by total amount with the multilayer fibers. It is 40% by weight. In order to improve the adhesiveness, the above-mentioned multi-layered fibers, polyethylene synthetic pulp and other synthetic fibers may be modified resins grafted with an unsaturated carboxylic acid such as maleic anhydride. Graft amount is 500p
It is preferably about pm to 30,000 ppm.

Although it is conceivable that the polyethylene porous membrane and the nonwoven fabric are laminated using an adhesive, they are usually laminated by low temperature hot melt adhesion (thermal lamination). The hot melt adhesion is usually performed by heating the non-woven fabric side and pressing it against the porous membrane. At this time, polyethylene of the non-woven fabric is melted and laminated with the porous membrane while maintaining the shape with the core of the fiber having a high melting point. As a result, a laminated body can be stably obtained without excessively crushing the pores of the porous film and maintaining the shape of the nonwoven fabric.

Next, an example of the method for producing the laminate of the present invention will be described. The manufacturing method of the laminate is, for example, a non-woven fabric preheated between the heating roll and the cooling roll and the porous film obtained as described above are fed so that the non-woven fabric is in contact with the heating roll side and heat lamination. Then, the obtained laminate is wound on a product winding roll.

Upon lamination, a heating roll or an embossing heat roll is used to uniformly heat laminate the porous membrane and the nonwoven fabric. If a heating roll having a smooth surface is used, the entire surface can be heat-sealed uniformly. In this case, since the surface of the non-woven fabric is melted by the heating roll, the fibers forming the non-woven fabric are fused to the surface of the porous membrane. Therefore, the air permeability of the porous film does not change significantly. When using an embossed heat roll, for example, form a heat-sealed portion with a dot-shaped, linear-shaped, or grid-shaped embossed heat roll. The laminate welding structure is such that the portions are scattered. With such a partial laminated structure, the degree of freedom for partial deformation of the porous membrane is secured, and even if stress is applied, the porous membrane stretches and a buffering force acts to prevent damage. The area of the thermal laminate (the area where the fibers forming the nonwoven fabric are fused to the surface of the porous membrane) is
The range is 1 to 50%, preferably 5 to 20% of the total area.

In the present invention, it is particularly important that the non-woven fabric is preheated for thermal lamination. That is, when preheating is performed using a hot roll, the pores of the porous film may be closed due to heat when the preheating is performed on the porous film. On the other hand, since the non-woven fabric has a structure in which fibers are entangled with each other, the above-described blockage of pores does not occur. Moreover, as described above, the non-woven fabric used in the present invention is a non-woven fabric containing polyethylene as an outer layer and a multi-layer fiber made of a synthetic resin whose core has a melting point of 30 ° C. or more higher than that of polyethylene.

A rubber winding roll is usually used as the heat roll. The heating method may be either indirect heating in which a heating medium is circulated inside a metal roll as a core material or direct heating in which a heater is provided inside the metal roll. The shape of the embossing on the surface of the roll when the embossing heat roll is used is not particularly limited, and any shape such as a dot shape, a linear shape, or a lattice shape can be adopted. The surface temperature of the heat roll is appropriately determined from the melting point of polyethylene used for the nonwoven fabric, but is usually 120 to 150 ° C, preferably 130 to
It is in the range of 145 ° C. The line speed is usually 10
-100 m / min, preferably 40-60 m / min. The thickness of the laminated body of the porous membrane and the non-woven fabric is 10 to 200 μ, preferably 5 because of its use as a separator.
It is about 0 to 150 μ.

The thus obtained laminated body is interposed between the positive electrode and the negative electrode as a separator of a lithium battery. In the case of an accident such as an external short circuit, the separator made of this laminated body automatically blocks the pores of the porous membrane due to heat and stops the passage of ions when the temperature inside the battery rises to about 135 ° C. Since the shape of the laminate is maintained by the nonwoven fabric, even if the temperature further rises, there is no possibility of causing an internal short circuit due to physical contact between the positive electrode and the negative electrode due to the membrane rupture. It has been confirmed that the separator composed of this laminate does not break even when heated to a temperature of about 200 ° C. That is, it can be said that the separator achieves a good balance between the self-closing property and the high temperature film shape maintaining property.

[0026]

EXAMPLES The present invention will now be described in detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. The test method in the examples is as follows. 1. Air permeability (unit: second / 100cc) JIS P81
17 2. Porosity (%) = pore volume / porous membrane volume × 100% 3. Breaking strength (unit: Kg / cm ² ) JIS K67
81 4. Bubble point (BP) JIS K3832 5. Permeability (unit: liter / hr · m ² · atm)
Using Amicon 8010 type cell, differential pressure 1kg / c
Measured at m ² temperature of 23 ° C. 6. Pore Size Measurement (Styrene Latex Rejection Rate) Dow's weight average particle diameters of 0.091 μm and 0.212 μm styrene latex particles are dispersed in water.
A 10-type cell was used to conduct a permeation test at a differential pressure of 1 kg / cm ^2, and the styrene latex concentration before and after the permeation test was measured with a UV meter, and the blocking rate was determined by the following formula.

[0027]

## EQU1 ## Rejection rate (%) = (concentration before permeation-concentration after permeation) / (concentration before permeation) × 100

Example 1 A dry blend of 20 parts by weight of a polyethylene powder having a viscosity average molecular weight of 2 × 10 ⁶ (melting point: 135 ° C.) and 80 parts by weight of granular stearyl alcohol was fed to an extruder for 24 hours.
While kneading at 0 ° C, it is continuously extruded from a T-die with a width of 550 mm and a die clearance of 0.2 mm to a thickness of 0.07.
A sheet of mm was obtained.

Stearyl alcohol was extracted from this sheet in an isopropyl alcohol bath at 60 ° C. to obtain a polyethylene porous membrane. The physical properties of this film are as follows: (a) film thickness 47 μm (b) air permeability 105 seconds / 100 cc (c) porosity 67% (d) breaking strength 170 Kg / cm ² (vertical direction), 1
20 Kg / cm ² (horizontal direction) (e) Bubble point 3.4 Kg / cm ² (f) Water permeability 400 liters / hr · m ² · atm (g) Styrene latex blocking rate (SR blocking rate) 98
% Or more.

This film was placed in a hot air circulation oven at 150 ° C. for 1 hour.
The air permeability of the sample heated for 1 minute cannot be measured (1200 seconds / 1
00cc or more). That is, it has a good self-closing property.
You can see that In addition, this membrane is placed at 175 ° C for 1 minute.
The film shape was maintained even after the heat treatment. Immediately
It has been found that it has good high-temperature film shape retention characteristics.
It Polyethylene porous membrane obtained above and polyethylene type
Non-woven fabric (outer layer polyethylene, melting point: 120 ℃, core material poly
Ester, melting point: 230 ° C) (Basis weight: 15 g / m ²,
Thermal lamination with a wet method) was performed.

That is, a non-woven fabric was brought into contact with a hot roll whose surface temperature was set at 145 ° C., and a polyethylene porous membrane was brought into contact with a cooling roll, and both rolls were pressed at a linear pressure of 10 kg / cm to obtain a line speed of 50 m / min. Lamination was performed. The obtained laminate (length: about 1000 m) was wound by a product winding roll. The thickness of the laminate was 65 μm on average. The area of the heat-sealed portion was 10% of the whole.

The presence of pinholes and the water pressure resistance of the obtained laminate were measured (in accordance with JIS L 1092). As a result, no pinholes were found and the water pressure resistance was 3 Kg / cm ² or more. there were. The tensile strength of the obtained laminate was 3,100 g / cm, which was 2.5 times that of the porous membrane alone. The electric conductivity of the porous membrane is 2.
It was the same as 3 ms / cms. It was successfully used as a separator for lithium batteries. The electric conductivity is 6 cm between electrodes using a platinum plate of 2 cm ² as electrodes.
And the electrolyte was a 1: 1 solution of propylene carbonate and diethoxy carbonate, and the electrolyte was 1 LiPF ₆ .
Used at a concentration of mole / l.

Example 2 The polyethylene porous film having a thickness of 47 μm obtained in Example 1 was heated with a heating pinch roll having a surface temperature of 120 ° C. for 30 minutes.
It was heat-treated for 2 seconds to form a 33 μm film, and a porous film was obtained.
This film was heat laminated with a polyethylene non-woven fabric (outer layer polyethylene, melting point: 135 ° C.) (core polypropylene, melting point: 175 ° C.) (unit weight: 15 g / m ² ) in the same manner as in Example 1.

With respect to the obtained laminate, the presence or absence of pinholes and the water pressure resistance were measured (according to JIS L 1092). As a result, no pinholes were observed and the water pressure resistance was 2.5 Kg / cm ^2. That was all. The obtained laminated body was allowed to stand at 130 ° C. to 5 ° C. at each temperature for 5 minutes, the change in air permeability was measured, and the presence or absence of film rupture due to heat was confirmed. As a result, the air permeability at 140 ° C. was 10,000 seconds or more by the Gurley method, and no film rupture was observed up to 200 ° C. It was successfully used as a separator for lithium batteries.

Comparative Example 1 The porous membrane obtained in Example 1 was used alone. This membrane was treated in the same manner as in Example 2 at 5 ° C increments from 130 ° C at 5 ° C for each temperature.
After leaving for a minute, the change in air permeability was measured, and the presence or absence of film rupture due to heat was confirmed. As a result, the air permeability at 140 ° C was 10,000 seconds or more in the Gurley method, but the membrane ruptured at 200 ° C.

Example 3 The non-woven fabric used in Example 1 was replaced with 20% maleic anhydride.
Nonwoven fabric using modified polyethylene grafted with 00 ppm as outer layer polyethylene (outer layer polyethylene, melting point:
A separator was prepared in the same manner as in Example 1 except that 120 ° C., polypropylene core material, melting point: 175 ° C. (unit weight: 15 g / m ² , wet method) was used, and the laminating temperature was 140 ° C. As a separator of a lithium battery, almost the same results as in Example 1 were obtained.

EFFECTS OF THE INVENTION According to the present invention, it is possible to prepare a film composed of a porous laminate which is excellent in low-temperature occluding property and retains the film shape even at high temperatures. This membrane can provide a battery separator with excellent safety.

Claims (4)

[Claims] 1. Viscosity average molecular weight (Mv) 500,000
It is composed of a laminate in which a porous membrane made of the above-mentioned ultra high molecular weight polyethylene and a non-woven fabric are laminated, the non-woven fabric has a polyethylene outer layer and a melting point of 20 ° C. higher than that of the polyethylene.
A separator for a battery, which is a non-woven fabric containing multi-layered fibers made of high resin. 2. A porous membrane made of ultra high molecular weight polyethylene (a) having a thickness of 10 to 50 μm, and (b) having an air permeability of 20 to 3
000 seconds / 100 cc, (c) Porosity 25-80%,
(D) Strength at break is 100 kg / c in both longitudinal and transverse directions
m ² or more, (e) bubble point (BP value) 2 to 5 kg
/ Cm ² , (f) Water permeability 100 liters / hr · m ² · a
The battery separator according to claim 1, wherein the separator is a porous film that blocks 50% or more of styrene latex particles having a particle size of tm or more and (g) 0.091 μm. 3. The battery separator according to claim 1, wherein the lamination is thermal lamination. 4. The battery separator according to claim 1, wherein the core of the fibers forming the nonwoven fabric is polypropylene.