JP2520310B2 - Method for manufacturing separator for lithium battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a lithium battery separator, and in particular, it has a low electric resistance, is thin, is excellent in mechanical strength, and has a shutdown phenomenon at a low temperature. The present invention relates to a method for producing a separator for a lithium battery, which is made of a microporous film that is easy to raise and handle.

[Problems to be Solved by Prior Art and Invention]

A lithium battery is a general term for batteries that use lithium for the negative electrode, and fluorinated graphite, manganese dioxide, and
Lithium fluoride graphite batteries using thionyl chloride, copper oxide, etc., lithium manganese dioxide batteries, lithium thionyl chloride batteries, lithium copper oxide batteries, etc. are generally used as primary batteries. Recently, a carbon lithium battery using a wood alloy for the negative electrode and activated carbon for the positive electrode has begun to be used as a secondary battery.

Since lithium metal is used in these lithium batteries, a protic electrolyte cannot be used, and lithium salts such as LiBF ₄ and LiClO ₄ are dissolved in an organic solvent such as γ-butyrolactone, polypropylene carbonate, and dimethoxyethane. The electrolyte is used as the electrolyte. Therefore, for the separator installed between the positive electrode and the negative electrode, an olefinic material such as polypropylene, which is insoluble in the above organic solvent, is processed into a microporous membrane or a nonwoven fabric and used.
For example, a cylindrical lithium battery for a camera generally has a 25μ
A m-thick polypropylene porous membrane is used.

By the way, a so-called aprotic electrolytic solution obtained by dissolving a lithium salt in an organic solvent as described above generally has a high internal resistance, and therefore it is necessary to suppress an increase in resistance due to the separator in order to cover this drawback. For that purpose, it is necessary to reduce the thickness of the membrane and improve the permeability of ions in the membrane, but in order to achieve this without lowering the mechanical strength of the membrane, the pore size of the pores It is desirable to increase the porosity without increasing it.

However, conventional polypropylene porous membranes have 30-60
Since it is produced by uniaxially stretching a polypropylene resin having a molecular weight of about 10,000, the thickness of about 25 μm is the limit of thinning. Further, the pores formed in the film have a slit shape along the stretching direction, and it is difficult to form a film having fine pores and high porosity. Further, since the polypropylene polymer is oriented in the uniaxial stretching direction, the film is likely to be torn along the stretching direction, which makes it difficult to handle the film.

In view of the above points, the film thickness is small, it has a good mechanical strength, it is possible to reduce the internal resistance of the battery,
Moreover, the development of a highly reliable separator is desired, and many proposals have been made so far.

For example, Japanese Examined Patent Publication No. 59-37292 has a porosity of a polyolefin resin having a weight average molecular weight of 150,000 or more and less than 300,000.
Disclosed is an alkaline storage battery separator having 50 to 80% and an average pore diameter of 0.05 to 0.5 μm. However, the thickness of the porous film that constitutes this separator is relatively thick at 0.05 to 10 mm,
Not suitable for small lithium battery separators.

By the way, as described above, for the lithium battery separator, a thin film having low electric resistance, excellent mechanical strength even if it is thin, and easy to handle in the production of the film is desirable, but in addition to this, the safety of the battery is improved. It is required that the film be made to be. For example, a cylindrical lithium battery for a camera employs a structure in which a sheet having a large electrode area is spirally wound so that a high output can be obtained. Therefore, when an electrode is short-circuited, a large current flows and heat is generated. If this condition continues, the internal temperature of the battery rises, and lithium melts at about 180 ° C, resulting in ignition. Therefore, suppress the temperature rise at the time of short circuit,
It is necessary to prevent accidents such as ignition. Before the ignition of lithium, the separator is melted to clog the holes and shut down the current. In a separator made of polypropylene, this shutdown phenomenon occurs at about 140 ° C, but in order to improve safety, it is desirable to use a separator that causes a current shaddown at a lower temperature.

Therefore, the object of the present invention is to reduce the electrical resistance and thin, excellent in mechanical strength, causes a shutdown phenomenon at low temperature, and also a method for producing a lithium battery separator comprising a microporous membrane that is easy to handle. Is to provide.

[Means for solving the problem]

As a result of earnest research in view of the above object, the present inventor has produced a microporous membrane using a resin composed of ultra-high-molecular polyethylene or substantially polyethylene containing it in a predetermined amount or more, and an organic electrolyte solution in the pores. , The electric resistance is small,
The present invention has been completed based on the finding that the separator can be made thin, has excellent mechanical strength, and operates in a shutdown phenomenon at a low temperature.

That is, the first method of the present invention comprises a polyethylene composition containing 1% by weight or more of ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more, and having a weight average molecular weight / number average molecular weight of 10 to 300. Made of microporous membrane, thickness is 0.1-2
5μm, porosity 40 ~ 95%, average through hole diameter 0.001 ~ 0.1
A method for producing a lithium battery separator having a rupture strength of 0.5 kg or more and a width of 10 mm and a polyethylene composition in a non-volatile solvent composed of an aliphatic hydrocarbon, a cyclic hydrocarbon or a mineral oil fraction. It is characterized in that it is heated and dissolved to form a uniform solution, the solution is extruded from a die to form a gel-like sheet, and after the nonvolatile solvent is removed, it is stretched at least uniaxially at least twice.

In addition, the second method of the present invention has a weight average molecular weight of 7 × 10 7.
Made of a microporous membrane consisting of ⁵ or more ultra-high molecular weight polyethylene, thickness 0.1-25μm, porosity 40-95%, average through-pore diameter 0.001-0.1μm, and breaking strength of 10mm width 0.5kg
A method for producing a lithium battery separator as described above, wherein the ultrahigh molecular weight polyethylene is heated and dissolved in a non-volatile solvent consisting of an aliphatic hydrocarbon, a cyclic hydrocarbon or a mineral oil fraction to form a uniform solution, and The solution is extruded from a die to form a gel-like sheet, and after removing the non-volatile solvent, it is stretched at least twice in the uniaxial direction.

 The present invention will be described in detail below.

In the present invention, the thin film used for the lithium battery separator is substantially made of polyethylene. Polyethylene which can be used in the present invention includes (a) ultra high molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more, and (b) 1 weight of ultra high molecular weight polyethylene having a polymerization average molecular weight of 7 × 10 ⁵ or more. %, And the balance is polyethylene having a relatively low molecular weight. The polyethylene used in both cases is a crystalline linear polyethylene composed of a homopolymer of ethylene or a copolymer of ethylene and 10 mol% or less of α-olefin.

When the above (a) ultrahigh molecular weight polyethylene is used alone, its weight average molecular weight is 7 × 10 ⁵ or more, preferably 1 × 10 ⁶ or more. If the weight average molecular weight is not 7 × 10 ⁵ or more, the maximum draw ratio in film formation is reduced, and a high strength and thin microporous film cannot be obtained. On the other hand, the upper limit of the molecular weight is not particularly limited, but those having a weight average molecular weight of more than 15 × 10 ⁶ are not preferable because the moldability is poor.

When the polyethylene composition (b) is used, a composition containing ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more, preferably 1 × 10 ⁶ or more in a proportion of 1% by weight or more is used. If the ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more is not contained in an amount of 1% by weight or more, the maximum draw ratio in film formation is lowered, and a high strength and thin microporous film cannot be obtained. On the other hand, the upper limit of the amount of ultra-high molecular weight polyethylene is not particularly limited, but one having a weight average molecular weight of more than 15 × 10 ⁶ is not preferable because the moldability is poor.

The polyethylene composition contains 1% by weight or more of ultrahigh molecular weight polyethylene, and has a weight average molecular weight / number average molecular weight of 10 to 300, preferably 12 to 250. If the weight average molecular weight / number average molecular weight is less than 10, the average molecular chain length is large and the entanglement density of the molecular chains becomes high during dissolution, making it difficult to prepare a high-concentration solution. On the other hand, when it exceeds 300, the low molecular weight component is broken during stretching, and the strength of the entire film is lowered.

The weight average molecular weight / number average molecular weight is used as a measure of the molecular weight distribution, and the larger the ratio of the molecular weights, the wider the width of the molecular weight distribution. That is, in a composition composed of polyethylenes having different weight average molecular weights, the larger the ratio of the molecular weights of the composition, the larger the difference in the weight average molecular weight of the polyethylene to be blended, and the smaller the ratio, the smaller the difference in the weight average molecular weight. There is.

It is preferable to set the weight average molecular weight / number average molecular weight of the polyethylene composition to 10 to 300, which is larger than the weight average molecular weight / number average molecular weight of the ultrahigh molecular weight polyethylene itself (usually about 6). As a result, the molecular weight distribution shows a spread toward the lower molecular weight side, so that a high-concentration polyethylene solution can be prepared.

The polyethylene composition as described above includes, for example, ultra high molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more,
It can be obtained by mixing an appropriate amount of polyethylene having a weight average molecular weight of less than 7 × 10 ^{5 so} that the weight average molecular weight / number average molecular weight falls within the above range, but it may be a homopolymer obtained by multistage polymerization.

The content of this ultra high molecular weight polyethylene is 1% by weight or more, based on 100% by weight of the total polyethylene composition. When the content of the ultra high molecular weight polyethylene is less than 1% by weight, the entanglement of the molecular chains of the ultra high molecular weight polyethylene, which contributes to the improvement of the stretchability, is hardly formed, and a high-strength microporous membrane cannot be obtained. On the other hand, the upper limit is not particularly limited, but if it exceeds 90% by weight, it becomes difficult to achieve the desired high concentration of the polyethylene solution.

The polyethylene other than the ultrahigh molecular weight polyethylene in the polyethylene composition has a weight average molecular weight of less than 7 × 10 ⁵ , and the lower limit of the molecular weight is preferably 1 × 10 ⁴ or more. The use of polyethylene having a weight average molecular weight of less than 1 × 10 ⁴ is not preferable because breakage easily occurs during stretching and the desired microporous membrane cannot be obtained. Particularly, it is preferable to add polyethylene having a weight average molecular weight of 1 × 10 ⁵ or more and less than 7 × 10 ⁵ to the ultrahigh molecular weight polyethylene.

Examples of such polyethylene include those of the same kind as the above-mentioned ultra-high molecular weight polyethylene, and high density polyethylene is particularly preferable.

In the present invention, an antioxidant, an ultraviolet absorber, a lubricant, an anti-antibacterial agent, if necessary, both in the case of (a) using only ultrahigh molecular weight polyethylene and in the case of using (b) polyethylene composition. Blocking agent,
Various additives such as pigments, dyes and inorganic fillers can be added within a range that does not impair the object of the present invention.

Next, a method for producing a polyethylene microporous membrane will be described.

(A) Microporous Membrane Made of Ultra High Molecular Weight Polyethylene An ultra high molecular weight polyethylene solution is prepared by heating and dissolving ultra high molecular weight polyethylene in a solvent. The solvent is not particularly limited as long as it can sufficiently dissolve ultra high molecular weight polyethylene, and examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, decalin, and paraffin oil, or a boiling point. Can use mineral oil fractions corresponding to these. Among these, a non-volatile solvent such as paraffin oil having a stable amount of solvent contained in the gel-like sheet is preferable.

The heating dissolution is performed with stirring at a temperature at which the ultrahigh molecular weight polyethylene is completely dissolved in the solvent. The temperature varies depending on the polymer and solvent used, but is preferably in the range of 140 to 250 ° C. The concentration of the ultra high molecular weight polyethylene solution is 1 to 15% by weight, preferably 2 to 8% by weight. If the concentration is less than 1% by weight, not only is the amount of solvent used large and it is not economical, but also when forming into a sheet, swell and neck-in are large at the die outlet, making it difficult to form the sheet. On the other hand, if the concentration exceeds 15% by weight, it becomes difficult to prepare a uniform solution. In addition, at the time of heating and dissolving, it is preferable to add an antioxidant to prevent oxidation of polyethylene.

Next, this ultra high molecular weight polyethylene solution is extruded from a die and molded. At this time, a gelled material is obtained by extruding a heated solution of ultrahigh molecular weight polyethylene while quenching, or by extruding from a heating die and then rapidly quenching. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelling temperature or lower. When the cooling rate is slow, the degree of crystallinity increases and it is difficult to form a gel-like material suitable for stretching. As a cooling method, a method of directly contacting cold air, cooling water, or other cooling medium with a die or a solution, a method of contacting with a roll cooled with a refrigerant, or the like can be used. The solution extruded from the die may be taken up at a take-up ratio of 1 to 10, preferably 1 to 5 before or during cooling. The take-back ratio is
When it is 10 or more, neck-in becomes large, and stretching is particularly likely to occur, which is not preferable.

A sheet die having a rectangular die shape is usually used as the die, but a double cylindrical hollow die, an inflation die or the like can also be used. When a sheet die is used, the die gap is usually 0.1 to 10 mm. The extrusion speed depends on the concentration of the solution, the temperature of the refrigerant, and the length of the cooling portion, but is usually 2-3 cm / min to 2-3 m / min.

Next, desolvation treatment is performed. At this time, the polyethylene contained in the gel-like molded product is 10 to 90% by weight, preferably 10 to 90% by weight.
It is necessary to perform desolvation treatment so that the amount becomes 60% by weight.

Examples of the desolvation treatment method include a method of immersing the gel-like sheet in an easily volatile solvent to extract the solvent and drying, a method of compressing, a method of heating, or a method using a combination thereof. Among these, extraction of the solvent with a volatile solvent that can adjust the amount of solvent without significantly changing the structure of the gel-like sheet is preferable, but in order to shorten the extraction time, a method in combination with a compression method is effective. Target. When the amount of the solvent contained in the gel-like sheet is less than 10% by weight, the network of the gel-like sheet is reduced and the densification proceeds too much, so that high-stretching cannot be performed and an ultrathin and high-strength film cannot be obtained. On the other hand, if the amount of the solvent exceeds 90% by weight, there is a problem in handling such that a large amount of the solvent exudes during stretching. The amount of solvent in the gel-like sheet can be adjusted by the amount of the easily volatile extraction solvent contacting the gel-like sheet, the pressure during compression, the heating temperature, and the like.

Further, in the solvent preparation treatment of the gel-like sheet with the volatile solvent, the gel-like sheet shrinks or bends in the three axial directions as the volatile solvent replaced in the gel-like sheet evaporates. Therefore, to prevent this, and to obtain a smooth and biaxial (longitudinal, lateral) direction with a small shrinkage that enables uniform and high-stretching, the gel-like sheet is selectively shrunk in the thickness direction. Preferably. The shrinkage is 50% or more in the thickness direction, preferably 70% or more, and in the biaxial direction
It is preferably 20% or less. As a method of selectively shrinking the gel-like sheet in the thickness direction, for example, a gel-like sheet is adhered to a smooth support, or a volatile solvent is used in a state of being gripped from two axial directions or sandwiched by a porous plate. There is a method of evaporating.

Note that, as the volatile solvent used for extracting these solvents, pentane, hexane, heptane, and other hydrocarbons,
Chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, diethyl ether,
Examples include ethers such as dioxane, and alcohols such as methanol, ethanol and propanol. These solvents are appropriately selected depending on the solvent used for dissolving the ultra high molecular weight polyethylene, and are used alone or as a mixture.

Next, this gel-like molded product is stretched. The stretching is carried out by heating the gel-like molded product and using a usual tenter method, roll method, inflation method, rolling method or a combination of these methods at a predetermined magnification. Biaxial stretching is preferred, and either longitudinal / transverse simultaneous stretching or sequential stretching may be employed, but especially simultaneous 2
Axial stretching is preferred.

The stretching temperature is not higher than the melting point of ultra-high molecular weight polyethylene + 10 ° C, preferably in the range from the crystal dispersion temperature to below the crystal melting point. That is, 90 to 140 ° C., more preferably 100 to 13
It is in the range of 0 ° C. If the stretching temperature is higher than the melting point + 10 ° C, the molecular chain cannot be oriented by stretching because the resin melts. On the other hand, if the stretching temperature is lower than the crystal dispersion temperature, the softening of the resin is insufficient, the film is easily broken during stretching, and high-stretching cannot be performed.

The stretching ratio varies depending on the thickness of the raw fabric, but is at least 2 times or more in the uniaxial direction, preferably 3 to 20 times, and the surface magnification is 10 times or more, preferably 20 to 400 times. Area magnification is 1
If it is less than 0 times, the stretching is insufficient and a highly elastic and high-strength microporous membrane cannot be obtained. On the other hand, if the areal magnification exceeds 400 times, there are restrictions on the stretching device, stretching operation, and the like.

The obtained stretch-molded product is washed with a solvent to remove the residual solvent. As the cleaning solvent, the above volatile solvent can be used. These solvents are appropriately selected according to the solvent used for dissolving the ultra high molecular weight polyethylene, and are used alone or as a mixture. The washing method can be performed by a method of immersing in a solvent for extraction, a method of showering the solvent, a method of a combination thereof, or the like.

The washing as described above is performed until the residual solvent in the stretch-molded product is less than 1% by weight. Thereafter, the washing solvent is dried, and the washing solvent can be dried by a method such as heat drying or air drying. It is desirable that the dried stretch molded product is heat-set at a temperature in the range of the crystal dispersion temperature to the melting point.

(B) Microporous Membrane Made of Polyethylene Composition A composition of ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more and relatively low molecular weight polyethylene is dissolved by heating in a solvent. As the solvent to be used, the same solvent as used in the production of the microporous membrane (a) made of the ultrahigh molecular weight polyethylene (a) can be used. In the case of a polyethylene composition, a solution having a relatively high concentration can be easily prepared, so the concentration of the solution is adjusted to 10 to 50% by weight, preferably 10 to 40% by weight. If the concentration is less than 10% by weight,
Not only is the amount of solvent used large and uneconomical, but at the time of forming into a sheet, the swell and neck-in are large at the die outlet, which makes it difficult to form the sheet. On the other hand, if the concentration exceeds 50% by weight, it becomes difficult to prepare a uniform solution. In addition, in heating and melting, it is preferable to add an antioxidant in order to prevent the oxidation of the polyolefin.

Next, the heated solution of this polyolefin composition is extruded from a die to form a gel. This molding can be performed under the same conditions as the molding conditions for the gel-like molded product in the production of the microporous membrane (a) made of the ultrahigh molecular weight polyolefin described above.

In this method, since a polyethylene solution having a high concentration of 10 to 50% by weight is used in advance, the step of removing the solvent of the gel-like molded article can be omitted, and then the gel-like molded article is stretched. The stretching method and conditions can be the same as the stretching method and conditions in the production of the microporous membrane consisting of (a) ultrahigh molecular weight polyethylene only.

Finally, the stretched molded product obtained is washed in the same manner as in (a) to obtain the desired microporous membrane.

Thus, when using a polyethylene composition,
The step of removing the solvent of the gel-like molded product is omitted, which is economically preferable.

According to the method detailed above, the thickness is 0.1 to 25 μm, the porosity is 40 to 95%, the average through hole diameter is 0.001 to 0.1 μm, 10 mm.
A microporous membrane having a width breaking strength of 0.5 kg or more can be obtained.

The thickness of the polyethylene microporous membrane is 0.1 to 25 μm, preferably 0.5 to 20 μm. If the thickness is less than 0.1 μm, the mechanical strength of the film is small and it is difficult to put it to practical use. on the other hand
If it exceeds 25 μm, the effective resistance in the aprotic electrolyte solution increases, which is not preferable. In addition, the film thickness is weight efficiency (Wh
/ kg) or area efficiency (Wh / l), which is one of the important factors for increasing the energy density of the battery. By setting the above film thickness range, it is advantageous in increasing the capacity of the battery. Become.

The porosity of the microporous membrane is 40 to 95%, preferably 40 to 90%. When the porosity is less than 40, the filling amount of the organic electrolyte into the pores is small, and the increase in capacity is restricted, which is not preferable in terms of practicality. On the other hand, if it exceeds 95%, the mechanical strength of the film becomes small and it is difficult to put it to practical use.

The average through hole diameter is 0.001 to 0.1 μm, preferably 0.005 to
Within the range of 0.05 μm. If the average through hole diameter is less than 0.001 μm, it becomes physically difficult to fill the organic electrolyte solution into the pores, and the passage of ions is hindered. on the other hand
When it exceeds 0.1 μm, it becomes difficult to prevent the diffusion of the active material and the reaction product. The maximum pore size may be 200% or less, preferably 130% or less, of the average through-hole size. This is particularly important in secondary batteries, in order to prevent the minute diffusion of free lithium that accompanies occlusion and release of lithium ions at the negative electrode, and to improve the charge / discharge cycle efficiency.

Further, the breaking strength must be 0.5 kg or more in a 10 mm width in any direction of the film.
Further, by setting the ratio of the maximum load and the minimum load depending on the direction not to exceed 5, it is possible to obtain a separator which has high strength and is hard to tear.

A lithium battery using the separator manufactured by the method of the present invention is composed of the separator made of the polyethylene microporous membrane described above, an aprotic electrolytic solution, a negative electrode made of lithium, and a positive electrode.

First, the aprotic electrolytic solution is filled in the pores of the separator, but the filling can be easily performed by dropping, impregnating, coating or spraying. This is 0.001 microporous membrane
This is because the aprotic electrolytic solution having a contact angle of 90 ° or less with respect to the microporous membrane is easily taken into the pores by the capillary condensation action because it has an average through-pore diameter of 0.1 μm.

Examples of the aprotic electrolytic solution include propylene carbonate, dimethyl sulfoxide, 3-methyl-1,3-oxazolidin-2-one, sulfolane, 1,2-dimethoxyethane, 2-methyltetrahydrofuran, etc., or a multi-component system. A solution obtained by dissolving a lithium salt such as LiBF ₄ or LiClO _{4 in} an organic solvent can be used. Especially propylene carbonate, 1,2-dimethoxyethane, LiB
F _4, a combination of dimethyl sulfoxide, 1,2-dimethoxyethane, combinations of LiBF _6, propylene carbonate, 1,2-dimethoxyethane, combinations of LiClO _4, the electrical conductivity at room temperature is 10 ^-3 to 10 ^- It is known to be ² s / cm, which is preferable.

The effective resistance value of the separator made of polyethylene microporous membrane filled with aprotic electrolyte is 5 Ω · cm ² or less,
It is preferably 2.0 Ω · cm ² or less. When the effective resistance value exceeds 5 Ω · cm ² , it is difficult to increase the capacity of the battery.

By using the separator thus obtained, a lithium battery having excellent reliability and safety can be obtained.

The lithium battery using the separator produced by the method of the present invention is a so-called non-aqueous electrolyte type battery using the above-mentioned aprotic electrolyte as an electrolyte, and its structure is basically the same type of lithium. It is similar to a battery. The separator manufactured by the method of the present invention described in detail above is installed between the positive and negative electrodes.

In the case of a primary battery, lithium can be used for the negative electrode and silver chromate, fluorocarbon, manganese dioxide, etc. can be used for the positive electrode. In the case of a secondary battery, lithium, aluminum, or fusible metal (Pb, C
(alloy containing d, In) or carbon with lithium occluded, and the positive electrode has a layered structure of metal chalcogenides such as TiS ₂ , MoS ₂ and NbSe _3, and CoO ₂ with tunnel-like holes. C
A metal oxide such as r ₂ O ₅ , V ₂ O ₅ (· P ₂ O ₅ ), MnO ₂ (· LiO ₂ ) or a conjugated polymer compound such as polyacetylene or polyaniline can be used.

〔Example〕

Examples of the present invention will be shown below. The test method in the examples is as follows.

(1) Weight average molecular weight and molecular weight distribution: GPC device manufactured by Waters Co., Ltd., and GMH manufactured by Tosoh Co., Ltd. was used as a column.
-6, O-dichlorobenzene is used as the solvent, and the temperature is 135
Measured by gel permeation chromatography (GPC) method at ℃ and flow rate of 1.0 ml / min.

(2) Thickness: Measured with a micrometer.

(3) Porosity: Measured with a mercury porosimeter.

(4) Average through-hole diameter: A microporous membrane was installed in a flat membrane module, hydrophilized with distilled water / ethanol mixture (50/50 volume ratio), and thoroughly washed with distilled water, then 380 mmH
0.05% by weight pullulan under differential pressure of g (Showa Denko KK)
The concentration of pullulan contained in the filtrate when the aqueous solution prepared in (1) was circulated was determined from the differential refractive index measurement. Then, the pore diameter was converted from the molecular weight of pullulan having a rejection of 50% calculated by the following equation, using the Flory theory as described later.

Retention rate of pullulan = {1- (pullulan concentration in filtrate / pullulan concentration in undiluted solution)} x 100 The chain polymer in a solution state is a spherical thread, and its diameter d is at both ends of the molecular chain. It can be considered that there is an approximate relationship of [d / 2] ² = <γ ² > ... (1) with respect to the root mean square distance <γ ² > of.

Fl on viscosity and molecular chain broadening in polymer solutions.
According to the theory of ory, [η] M = 2.0 × 10 ²¹ <γ ² > ^3/2 (2) holds regardless of the repair of the polymer. Therefore, according to equations (1) and (2), The diameter d of the chain polymer can be calculated from the measured value of the intrinsic viscosity [η] and the molecular weight M at which the rejection is 50%. This d was defined as the average through-hole diameter of the polyethylene microporous membrane.

(5) Maximum pore size: In the measurement according to the above (4), the pore size was similarly converted from the value of the molecular weight of pullulan having a rejection rate of 90% to obtain the maximum pore size.

(6) Tensile breaking strength: The breaking strength of a strip test piece having a width of 10 mm was measured according to ASTM D882.

(7) Effective resistance: Propylene carbonate, 1,2-
Prepare an electrolytic solution in which 1 mol / l of LiClO ₄ was dissolved in a 1: 1 mixed solvent of dimethoxyethane, and using this electrolytic solution and a stainless steel electrode, place a separator between the positive and negative electrodes and place an argon atmosphere. It was calculated from the complex impedance plot measured at 25 ° C in the middle.

(8) Current shutdown temperature: A battery is constructed by setting the electrolyte prepared in (7) above and the separator between lithium (negative electrode) and manganese dioxide (positive electrode).
The battery temperature when short-circuited was measured with a thermocouple.

Example 1 A raw material resin prepared by mixing 2 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight (w) of 2.5 × 10 ⁶ and 13 parts by weight of polyethylene of 2.4 × 10 ⁵ and liquid paraffin (64 cst / 40).
85 ° C.) to prepare a solution of polyethylene composition. Next, 100 parts by weight of a solution of this polyethylene composition was added to 2,5-di-t-butyl-p-cresol ("BH
T ", manufactured by Sumitomo Chemical Co., Ltd., 0.125 parts by weight, and tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxylphenyl) -propionate] methane (" Irganox 1010 ", Ciba Geigy) 0.25 parts by weight) was added as an antioxidant. This mixed solution was filled in an autoclave equipped with a stirrer and stirred at 200 ° C. for 90 minutes to obtain a uniform solution.

This solution was extruded from a T-die with an extruder having a diameter of 45 mm, and a gel-like sheet was formed while being taken up by a cooling roll.

The obtained sheet was set in a biaxial stretching machine and the temperature was set to 115.
Simultaneous biaxial stretching was carried out at 7.degree. The stretched film obtained was washed with methylene chloride to remove residual liquid paraffin by extrusion, and then dried to obtain a polyethylene microporous film. The characteristics are shown in Table 1.

Also, in the obtained polyethylene microporous membrane, in a 1: 1 mixed solvent of propylene carbonate and 1,2-dimethoxyethane
An electrolytic solution in which LiClO ₄ was dissolved to be 1 mol / l was dropped, and its characteristics as a lithium battery separator were measured. The results are also shown in Table 1.

Example 2 Polyethylene 4.0 having a weight average molecular weight (w) of 2.5 × 10 ⁶
To 100 parts by weight of a mixed solution of 9 parts by weight of liquid paraffin (74 cst / 40 ° C.) and 2,6-di-t-butyl-p-cresol (“BHT”, manufactured by Sumitomo Chemical Co., Ltd.) 0.125 parts Parts by weight and tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane ("Irganox 1010", manufactured by Ciba Geigy) 0.25
Parts by weight of antioxidant were added. This mixed solution was filled in an autoclave equipped with a stirrer and stirred at 200 ° C. for 90 minutes to obtain a uniform solution.

This solution was extruded with a diameter of 45 mm and the tip of the die lip was cooled with water.
m), and a gel-like sheet having a thickness of 5 mm was extruded. The temperature of the sheet immediately after being extruded from the T die was 60 ° C., and the sheet was sufficiently gelled.

This gel-like sheet is 60 times in 4.5 ml of 250 ml of methylene chloride.
After soaking for a minute, hot air was blown while being gripped with a clip to evaporate and dry methylene chloride to prepare a raw material having a liquid paraffin content of 84.0% by weight.

Set the obtained raw sheet on the biaxial stretching machine,
Simultaneous biaxial stretching was carried out at 125 ° C. and a stretching speed of 30 cm / min at 7 × 7 times. The stretched film obtained was washed with methylene chloride to extract and remove residual liquid paraffin, and then dried to obtain an ultrahigh molecular weight polyethylene microporous film. The characteristics are shown in Table 1.

Further, using the obtained polyethylene microporous membrane, the characteristics as a lithium battery separator were measured in the same manner as in Example 1. The results are also shown in Table 1.

Comparative Example 1 Using a polypropylene porous membrane (Celgard 2400 manufactured by Celanese Co., Ltd.) as a separator substrate, a lithium battery separator was obtained in the same manner as in Example 1. The characteristics are shown in Table 1.

As is clear from the above, the separator according to the present invention has a low effective resistance and a high breaking strength. Also, the shutdown temperature of the battery according to the present invention is low.

〔The invention's effect〕

As described above in detail, the separator according to the present invention has a small effective resistance and excellent mechanical strength. In addition, the battery shutdown temperature is also low, improving battery safety.

Such a separator is particularly suitable as a separator for a lithium battery having a small size and light weight and a high energy density.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kenkichi Okamoto 1-3-1, Nishitsurugaoka, Oi-cho, Iruma-gun, Saitama Tonen Co., Ltd. Research Institute (56) Reference JP-A-60-255107 (JP, A) JP-A-2-94356 (JP, A)

Claims (2)

(57) [Claims] 1. A microporous membrane comprising a polyethylene composition containing 1% by weight or more of ultrahigh molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more, and having a weight average molecular weight / number average molecular weight of 10 to 300, Thickness is 0.1-25 μm, porosity is 40-95
%, The average through-hole diameter is 0.001 to 0.1 μm, and the breaking strength of 10 mm width is 0.5 kg or more, the method for producing a lithium battery separator, wherein the polyethylene composition is an aliphatic hydrocarbon, a cyclic hydrocarbon or a mineral oil. It is possible to heat-dissolve a non-volatile solvent consisting of a fraction to form a uniform solution, extrude the solution through a die to form a gel-like sheet, and after removing the non-volatile solvent, stretch at least uniaxially by a factor of 2 or more. A method for producing a lithium battery separator, which is characterized. 2. A microporous film made of ultra-high molecular weight polyethylene having a weight average molecular weight of 7 × 10 ⁵ or more and a thickness of 0.1 to 25 μm.
m, porosity 40-95%, average through-hole diameter 0.001-0.1μ
A method for producing a lithium battery separator having a breaking strength of 0.5 kg or more in m and 10 mm width, wherein the ultra high molecular weight polyethylene is in a non-volatile solvent composed of an aliphatic hydrocarbon, a cyclic hydrocarbon or a mineral oil fraction. A lithium battery separator, characterized in that it is heated and dissolved into a uniform solution, extruded from a die into a gel-like sheet, and after removing the non-volatile solvent, it is stretched at least twice in at least one axial direction. Manufacturing method.