WO2015194504A1 - Polyolefin microporous membrane, separator for cell, and cell - Google Patents

Abstract

　The present invention provides a polyolefin microporous film in which it is possible to prevent degradation in the air permeability of a separator due to high-pressure press-working during the procedure of manufacturing a cell, the polyolefin microporous film having exceptional compression resistance. In addition, using this polyolefin microporous film makes it possible to provide a cell having exceptional cycle characteristics. A polyolefin microporous film in which the rate of change in the air permeation resistance after 5 minutes of heat-compression at a temperature of 90°C and a pressure of 5.0 MPa is 50% or less, and the rate of change in the film thickness after 5 minutes of heat-compression at a temperature of 90°C and a pressure of 5.0 MPa is 10% or less in relation to the film thickness of the polyolefin microporous film before the heat-compression.

Description

Polyolefin microporous membrane, battery separator and battery

The present invention relates to a polyolefin microporous membrane, a battery separator and a battery.

Polyolefin microporous membranes are widely used as separators and filters. For example, separators for lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, separators for electric double layer capacitors, and reverse osmosis filtration membranes, ultrafiltration membranes for filters In addition, it is used for moisture-permeable and waterproof clothing, medical materials, etc. Especially, it uses suitably as a separator for lithium ion secondary batteries.

Lithium ion secondary batteries are widely used not only for small electronic devices such as notebook computers and mobile phones, but also for power tools such as electric tools and hybrid electric vehicles in recent years.

In a lithium ion secondary battery, the separator has a function of preventing a short circuit between the positive electrode and the negative electrode while maintaining ion permeability. However, due to the influence of the expansion / contraction of the electrode accompanying the charging / discharging of the battery, the separator is repeatedly loaded / released with force in the thickness direction, resulting in changes in deformation and permeability, resulting in a decrease in battery capacity (cycle characteristics). It has been pointed out that there is a risk of causing deterioration. Therefore, in order to maintain the cycle characteristics of the battery, it is required to suppress the deformation of the separator and the change in permeability due to compression.

Therefore, in recent years, the development of separators focusing on compression resistance has been promoted.
Patent Document 1 describes a microporous film having a content of ultrahigh molecular weight polyethylene having a mass average molecular weight of 1 × 10 ⁶ or more and having polyethylene as a main component of polyethylene of 100% by mass as a whole and 5% by mass or less. ing. The microporous film of Patent Document 1 has a porosity of 25 to 80%, and the rate of change in film thickness after heating and compression at a pressure of 2.2 MPa for 90 degrees and 5 minutes is the film thickness before compression. 20% or less as 100%, reaching air resistance after heat compression under the above conditions (Gurley value) is described to be less 700sec / 100cm ³ / 20μm.

In Patent Document 2, a polyolefin having a viscosity average molecular weight (Mv) of less than 300,000, a polyolefin having an Mv of 500,000 or more, and electrochemically inert particles larger than a film thickness are essential components, and the particles are formed on the film surface. A polyolefin microporous membrane characterized in that 0 <A / B × 100 <25 holds between the height A (μm) of the portion protruding from the film and the film thickness B (μm). . Particles protruding from both sides are selectively compressed, reducing the compression load on other parts of the hole structure, porosity of 39-43%, and cutting 20 samples into 50 mm x 50 mm samples It is described that after being stacked, the air permeability resistance was 190 to 430 sec when compressed by a press machine at 55 ° C. for 5 seconds so as to be 80% of the initial total thickness including protruding particles.

Patent Document 3 proposes a microporous membrane having heat resistance and flexibility by extruding an α-olefin and a propylene-based elastomer with a polyolefin resin, forming into a sheet shape, stretching, washing, and drying. . The porosity of the microporous film is 35 to 75%, and the rate of change in film thickness after heating and compression at 90 ° C. for 5 minutes under a pressure of 2.2 MPa by a press machine is the film thickness before compression. It is described that the ultimate air resistance (Gurley value) after heating and compression under the above conditions is 600 seconds / 100 ml / 20 μm or less.

JP 2008-81513 A JP 2007-262203 A JP 2013-57045 A

Although the compression resistance is improved by the techniques disclosed in Patent Documents 1 to 3 and deterioration of the cycle characteristics of the battery is suppressed, it is still not sufficient.
One cause of the deterioration of the cycle characteristics is the precipitation of lithium during the initial charging of the lithium ion secondary battery. When lithium is deposited, the cycle characteristics deteriorate due to a decrease in the lithium ion concentration in the electrolyte. And in order to suppress precipitation of lithium at the time of initial charge, it turned out that the air permeability resistance of a separator and the adhesiveness of a separator and an electrode are important. If the separator's air resistance is high, the flow of ions will be hindered, and if the adhesion is not sufficient, a gap will be created between the separator and the electrode due to expansion of the electrolyte or electrode, which promotes lithium deposition. It is. Therefore, in order to suppress the deterioration of the cycle characteristics, it is necessary to suppress an increase in the air permeability resistance of the separator and improve the adhesion between the separator and the electrode.

As a result of intensive investigations on the air permeability resistance of the separator and the adhesion between the separator and the electrode, the inventors of the present invention produced a battery element in the battery manufacturing process, sealed the battery element together with the electrolyte, However, it has been found that by performing hot pressing at a high pressure, the air permeability of the separator and the adhesion between the separator and the electrode are impaired, leading to deterioration of cycle characteristics. That is, the inventors have found that deterioration of cycle characteristics can be suppressed by a separator that can maintain air permeability resistance and adhesion with an electrode even by the pressure in the hot press, and the present invention has been achieved. The techniques of Patent Documents 1 to 3 only assume a pressure of about 2.2 MPa accompanying charging / discharging of the battery, and do not consider the pressure in the hot press (about 3 to 5 MPa).

The present invention provides a polyolefin microporous membrane that can prevent deterioration of the air permeability of the separator due to high-pressure press working in the battery manufacturing process and has excellent compression resistance. Moreover, if the polyolefin microporous film of the present invention is used, a battery having excellent cycle characteristics can be provided.

In order to solve the above problems, the battery separator of the present invention has the following configuration.
That is,
Polyolefin microporous membrane having a rate of change in air resistance after heating and compression at a temperature of 90 ° C. and a pressure of 5.0 MPa for 5 minutes of 50% or less, and at a temperature of 90 ° C. and a pressure of 5.0 MPa for 5 minutes. It is a polyolefin microporous film whose rate of change in thickness after heat compression is 10% or less with the film thickness of the polyolefin microporous film before heat compression as 100%.
In the polyolefin microporous membrane of the present invention, the content of ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 1 × 10 ⁶ or more is preferably 10 to 40% by mass with respect to 100% by mass of the total mass of polyethylene.
The polyolefin microporous membrane of the present invention preferably has a thickness of 16 μm or less.
The polyolefin microporous membrane of the present invention preferably has a porosity of 25 to 40%.
The polyolefin microporous membrane of the present invention preferably has an average pore size determined by a palm porometer of 0.05 μm or less and a bubble point (BP) pore size of 0.06 μm or less.
The polyolefin microporous membrane of the present invention is preferably a battery separator.
In order to solve the above problems, the battery of the present invention has the following configuration.
That is,
The battery uses a battery separator made of the polyolefin microporous membrane.

According to the present invention, the present invention provides a polyolefin microporous membrane excellent in compression resistance, which can prevent deterioration of the air permeability resistance of the separator due to high pressure pressing in the battery manufacturing process. Moreover, if the polyolefin microporous film of the present invention is used, a battery having excellent cycle characteristics can be provided.

Hereinafter, the polyolefin microporous membrane of the present invention will be described in detail.
[1] Polyolefin resin The polyolefin resin constituting the polyolefin microporous membrane of the present invention contains a polyethylene resin as a main component. The content of the polyethylene resin is preferably 70% by mass or more, more preferably 90% by mass or more, and further preferably 100% by mass, where the total mass of the polyolefin resin is 100% by mass. Therefore, in the polyolefin microporous membrane of the present invention, the polymer component is preferably made of polyethylene resin, and in that case, polypropylene is not included.

Examples of the polyolefin include a two-stage polymer obtained by polymerizing ethylene, propylene, 1-butene, 4-methylpentene-1, 1-hexene, or a copolymer and a blend thereof.

The polyethylene resin that is the main component of the polyolefin resin is a polyethylene having a weight average molecular weight (Mw) of less than 1 × 10 ⁶ (hereinafter referred to as “polyethylene (A)”) and an ultra-high Mw of 1 × 10 ⁶ or more. A polyethylene composition comprising a molecular weight polyethylene (hereinafter referred to as “polyethylene (B)”) is more preferred.

The polyethylene (A) may be any of high density polyethylene (HDPE), medium density polyethylene (MDPE), and low density polyethylene (LDPE), and two or more types having different Mw or density may be used. In particular, it is preferable to use high-density polyethylene as the polyethylene (A). The Mw of the polyethylene (A) is preferably 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , more preferably 5 × 10 ⁴ or more and less than 4 × 10 ⁵ .

The polyethylene (B) is ultra high molecular weight polyethylene (UHMWPE), and Mw is 1 × 10 ⁶ or more, and Mw is more preferably 1 × 10 ⁶ to 3 × 10 ⁶ . By making Mw of polyethylene (B) 3 × 10 ⁶ or less, melt extrusion can be facilitated.

The content of polyethylene (B) in the polyethylene resin is preferably 10% by mass or more and 40% by mass or less, more preferably 15% by mass or more and 30% by mass or less, based on 100% by mass of the total mass of the polyethylene resin. When the content of polyethylene (B) is within the above-mentioned preferable range, the average pore diameter of the entire film can be reduced under the same production conditions, and the pores are not easily crushed by compression. Moreover, a heat shrinkage rate can be restrained low as content of polyethylene (B) exists in the said preferable range.

The Mw of the polyolefin resin is preferably 1 × 10 ⁶ or less, more preferably 1 × 10 ⁵ to 1 × 10 ⁶ , still more preferably 2 × 10 ⁵ to 1 × 10 ⁶ . By making Mw of polyolefin resin into the said preferable range, it is easy to extrude with a twin-screw extruder, and it can prevent a fracture | rupture at the time of extending | stretching.

The ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (Mw / Mn (molecular weight distribution)) of polyethylene (A), polyethylene (B), and polyolefin resin is not limited, but preferably 5 to 300 for all. 5 to 100 is more preferable, and 5 to 25 is more preferable. When Mw / Mn is in the above preferred range, melt extrusion can be facilitated and the strength of the resulting polyolefin microporous membrane can be increased.

In order to improve the characteristics as a battery separator, the polyolefin resin may contain a polyolefin imparting a shutdown function. For example, LDPE or polyethylene wax can be added as the polyolefin imparting the shutdown function. The LDPE is preferably at least one selected from the group consisting of branched LDPE, linear LDPE (LLDPE), and an ethylene / α-olefin copolymer produced by a single site catalyst. However, the addition amount is preferably 20% by mass or less, based on 100% by mass of the total mass of the polyolefin resin. Strength fall can be prevented by making addition amount into the said preferable range.

If necessary, various additives such as an antioxidant and finely divided silicic acid (pore forming agent) may be added as long as the effects of the present invention are not impaired.

[2] Method for Producing Polyolefin Microporous Membrane The method for producing a polyolefin microporous membrane of the present invention is as follows. (1) After adding a film-forming solvent to the polyolefin resin, melt-kneading to prepare a polyolefin resin solution. A step, (2) a step of extruding a polyolefin resin solution from a die lip and then cooling to form a gel-like molded product, (3) a step of stretching the gel-like molded product in at least a uniaxial direction (first stretching step), (4) a step of removing the film-forming solvent, (5) a step of drying the obtained film, (6) a step of stretching the dried film in at least a uniaxial direction (second stretching step), (7) A heat treatment step, and (8) a winding step. If necessary, any one of a heat setting treatment step, a heat roll treatment step, and a heat solvent treatment step may be provided before the film forming solvent removal step (4). Further, after the steps (1) to (7), a drying step, a heat treatment step, a crosslinking treatment step by ionizing radiation, a hydrophilization treatment step, a surface coating treatment step, and the like can be provided.

It is important that the method for producing a polyolefin microporous membrane of the present invention is a wet method that undergoes the following “preparation step of polyolefin resin solution” and “formation step of gel-like molded product”. A production method that does not go through the “preparation step of polyolefin resin solution” is called a dry method.
(1) Preparation Step of Polyolefin Resin Solution After adding a suitable film-forming solvent to the polyolefin resin, it is melt-kneaded to prepare a polyolefin resin solution. Since the melt-kneading method is known, detailed description thereof is omitted, but as the melt-kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. However, the polyolefin resin concentration of the polyolefin resin solution is preferably 25 to 50% by mass, more preferably 25 to 45% by mass, with the total mass of the polyolefin resin and the film-forming solvent being 100% by mass. It is. By making the ratio of the polyolefin resin within the above-mentioned preferable range, it is possible to prevent a decrease in productivity and a decrease in moldability of the gel-like molded product.

(2) Step of forming a gel-like molded product A polyolefin resin solution is extruded from a die through an extruder and cooled to form a gel-like molded product. The rate of cooling the polyolefin resin solution extruded from the die to 50 ° C. or less is preferably 180 ° C./min or more, more preferably 200 ° C./min or more, and further preferably 210 ° C./min or more. By setting the cooling rate within the above preferable range, the number of crystal nuclei is increased and the number of microcrystals is increased. As a result, the gel-like molded product is easy to orientate crystals when stretched, the fibril strength is improved, and the resulting microporous film is less likely to be crushed by increasing the strength against compression in the film thickness direction. Extrusion methods and gel-like molded product formation methods are known and will not be described here. For example, the methods disclosed in Japanese Patent Nos. 2132327 and 3347835 can be used.

(3) First stretching step The gel-like molded product is stretched in at least a uniaxial direction. The first stretching causes cleavage between the polyethylene crystal lamella layers, the polyethylene phase is refined, and a large number of fibrils are formed. The obtained fibrils form a three-dimensional network structure (a network structure that is irregularly connected three-dimensionally). Since the gel-like molded product contains a film-forming solvent, it can be stretched uniformly. The first stretching can be carried out at a predetermined magnification by heating the gel-like molded product and then using a normal tenter method, roll method, inflation method, rolling method, or a combination of these methods. The first stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be performed.

Although the draw ratio varies depending on the thickness of the gel-like molded product, it is preferably 2 times or more, more preferably 3 to 50 times in uniaxial stretching. In biaxial stretching, at least 3 times or more is preferable in any direction.

The first stretching temperature is preferably in the range of not less than the crystal dispersion temperature of the polyolefin resin to the crystal dispersion temperature + 30 ° C., more preferably in the range of the crystal dispersion temperature + 10 ° C. to the crystal dispersion temperature + 25 ° C., It is particularly preferable to set the temperature within the range of crystal dispersion temperature + 15 ° C. to crystal dispersion temperature + 20 ° C. By setting the stretching temperature within the above preferred range, deterioration of the orientation of molecular chains after stretching is prevented, and when the resin is sufficiently softened, film breakage due to stretching can be prevented and stretching at a high magnification can be performed. Here, the crystal dispersion temperature refers to a value obtained by measuring temperature characteristics of dynamic viscoelasticity based on ASTM D4065. When the polyolefin resin is polyethylene, the crystal dispersion temperature is generally 90 to 100 ° C. Therefore, the stretching temperature is usually preferably 90 to 130 ° C, more preferably 100 to 125 ° C, and further preferably 105 to 120 ° C.

¡Multi-stage stretching at different temperatures may be performed during the first stretching. In this case, the stretching is preferably performed at two different temperatures, the temperature of the subsequent stage being higher than the temperature of the previous stage. As a result, a highly porous microporous membrane having a large pore size and high permeability can be obtained without lowering strength or reducing physical properties in the width direction. Although not limited, it is preferable that the difference in the stretching temperature between the former stage and the latter stage is 5 ° C. or more. When the temperature of the film is increased from the former stage to the latter stage, (a) the temperature may be raised while continuing the stretching, or (b) the stretching is stopped while reaching the predetermined temperature while the temperature is raised, and the latter stage stretching is performed. Although it may be started, the former (a) is preferred. In any case, it is preferable to rapidly heat at the time of temperature rise. Specifically, heating is preferably performed at a temperature rising rate of 0.1 ° C./second or more, more preferably heating at a temperature rising rate of 1 to 5 ° C./second. Needless to say, the stretching temperature and the total stretching ratio of the former stage and the latter stage are within the above ranges, respectively.

Depending on the desired physical properties, the film may be stretched with a temperature distribution in the film thickness direction, whereby a polyolefin microporous film having further excellent mechanical strength can be obtained. As the method, for example, the method disclosed in Japanese Patent No. 3347854 can be used.

(4) Film forming solvent removal step A cleaning solvent is used to remove (wash) the film forming solvent. Since the polyolefin phase is phase-separated from the film-forming solvent, a porous film can be obtained by removing the film-forming solvent. The cleaning solvent and the method for removing the film-forming solvent using the same are well known and will not be described here. For example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(5) Membrane drying step The polyolefin microporous membrane obtained by removing the film-forming solvent is dried by a heat drying method, an air drying method or the like.

(6) Second stretching step The dried film is stretched again in at least a uniaxial direction. The second stretching can be performed by a tenter method or the like, similar to the first stretching, while heating the film. The second stretching may be uniaxial stretching or biaxial stretching.

The temperature of the second stretching is preferably in the range of not less than the crystal dispersion temperature of the polyolefin resin constituting the microporous membrane to the crystal dispersion temperature + 40 ° C. or less, and the crystal dispersion temperature + 10 ° C. or more to the crystal dispersion temperature + 40 ° C. More preferably, it is within the following range. By setting the temperature of the second stretching within the above preferable range, it is possible to prevent a decrease in air permeability and occurrence of variations in physical properties in the sheet width direction when stretched in the lateral direction (width direction: TD direction). it can. By setting the temperature of the second stretching within the above preferable range, it is possible to suppress the occurrence of variations in the air permeability resistance particularly in the stretched sheet width direction. In addition, by setting the temperature of the second stretching within the above-mentioned preferable range, the polyolefin resin can be sufficiently softened, and the film can be prevented from being broken in the stretching and can be stretched uniformly. When the polyolefin resin consists of polyethylene only, the stretching temperature is usually in the range of 90 to 140 ° C, preferably in the range of 100 to 140 ° C.

The magnification in the uniaxial direction of the second stretching is preferably 1.0 to 1.8 times. For example, in the case of uniaxial stretching, the length is 1.0 to 1.8 times in the longitudinal direction (machine direction: MD direction) or TD direction. In the case of biaxial stretching, 1.0 to 1.8 times each in the MD direction and the TD direction. In the case of biaxial stretching, each stretching ratio in the MD direction and TD direction may be different in each direction as long as it is 1.0 to 1.8 times, but is preferably the same. By setting the magnification within the above preferable range, it is possible to prevent a decrease in permeability, electrolyte solution absorbability, and compression resistance. Moreover, it can prevent that a fibril becomes thin too much and a heat-resistant shrinkage fall by making a magnification into the said preferable range. The magnification of the second stretching is more preferably 1.2 to 1.6 times.

The second stretching speed is preferably 3% / second or more in the direction of the stretching axis. For example, in the case of uniaxial stretching, it is 3% / second or more in the MD direction or TD direction. In the case of biaxial stretching, it is 3% / second or more in the MD direction and TD direction, respectively. The stretching speed (% / second) in the stretching axis direction is the ratio of the length stretched per second with the length in the stretching axis direction before re-stretching being 100% in the region where the film (sheet) is re-stretched. Represents. By setting the stretching speed within the above preferable range, a decrease in the permeability is prevented, and a variation in physical properties in the sheet width direction when the film is stretched in the TD direction is prevented. By setting the stretching speed within the above preferable range, it is possible to prevent the occurrence of variation in the air resistance particularly in the width direction of the stretched sheet. The speed of the second stretching is preferably 5% / second or more, and more preferably 10% / second or more. In the case of biaxial stretching, each stretching speed in the MD direction and TD direction may be different from each other in the MD direction and the TD direction as long as it is 3% / second or more, but is preferably the same. Although there is no restriction | limiting in particular in the upper limit of the speed | rate of 2nd extending | stretching, It is preferable that it is 50% / second or less from a viewpoint of fracture | rupture prevention.

(7) Heat treatment process The film | membrane after 2nd extending | stretching is heat-processed. As the heat treatment method, heat setting treatment and / or heat relaxation treatment may be used. In particular, the crystal of the film is stabilized by the heat setting treatment. By performing the heat setting treatment, it is possible to maintain a network composed of fibrils formed by the second stretching, to produce a microporous membrane having a large pore diameter and excellent strength. The heat setting treatment is performed within a temperature range from the crystal dispersion temperature to the melting point of the polyolefin resin constituting the microporous membrane. The heat setting treatment is performed by a tenter method, a roll method or a rolling method. Further, the thermal relaxation treatment may be performed by a tenter method, a roll method or a compression method, or may be performed using a belt conveyor or a floating roll. The thermal relaxation treatment is preferably performed in at least one direction in a range where the relaxation rate is 20% or less, more preferably in a range where the relaxation rate is 10% or less.

The heat setting treatment temperature and the heat relaxation temperature are preferably within the range of the second stretching temperature ± 5 ° C., which stabilizes the physical properties. This temperature is more preferably within the range of the temperature of the second stretching ± 3 ° C. As the thermal relaxation treatment method, for example, the method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(8) Winding process The polyolefin microporous film after film formation is wound around a cylindrical core to form a wound film roll, which is then heat-treated. The temperature of the heat treatment is preferably 50 to 70 ° C. In the present invention, the core for winding the film is cylindrical, and the material thereof is not particularly limited, and examples thereof include paper, plastic, and a combination thereof. Examples of the winding method include a method of winding a polyolefin microporous film around the core by applying a tension with a winding motor. The winding tension at the time of winding the polyolefin microporous film around the core is preferably 5 to 15N, more preferably 7 to 15N. When winding at a winding tension of 15 N or more, distortion due to stretching in the roll state tends to remain after winding, and thermal shrinkage after unwinding increases. When the winding tension is 1 N or less, winding deviation and winding shape are deteriorated, which causes a wrinkle defect. Furthermore, by heat-treating the film roll at 60 ° C., the strain in the three-dimensional structure hardly remains due to the shrinkage, and the resulting polyolefin microporous film has a small shrinkage during hot pressing, and the air resistance after heat compression The rate of change is reduced.

Although not limited, it is preferable to adopt an in-line method in which the first stretching, the solvent removal for film formation, the drying process, the second stretching and the heat treatment are continuously performed on a series of lines. However, if necessary, an off-line system in which the film after the drying treatment is once wound to form a film roll and the second stretching and heat treatment are performed while rewinding may be adopted.

(9) Other steps Before removing (cleaning) the film-forming solvent from the gel-like molded product subjected to the first stretching, any one of a heat setting treatment step, a heat roll treatment step and a heat solvent treatment step is provided. May be. Moreover, you may provide the process which heat-sets with respect to the film | membrane after a washing | cleaning or a 2nd extending | stretching process.
(I) Heat setting treatment The method of heat-setting the stretched gel-like molded product before and / or after washing and the film in the second stretching step may be the same as described above.

(Ii) Hot roll treatment process You may perform the process (hot roll process) which makes a hot roll contact at least one surface of the stretched gel-like molded object before washing | cleaning. As the hot roll treatment, for example, a method described in JP-A-2007-106992 can be used. When the method described in Japanese Patent Application Laid-Open No. 2007-106992 is used, the stretched gel-like molded product is brought into contact with a heated roll adjusted to a crystal dispersion temperature of the polyolefin resin + 10 ° C. or higher and lower than the melting point of the polyolefin resin. The contact time between the heating roll and the stretched gel-like molded product is preferably 0.5 seconds to 1 minute. As a hot roll process, you may make it contact in the state which hold | maintained heating oil on the roll surface. The heating roll may be either a smooth roll or a roll having a function of sucking a stretched gel-like molded product toward the roll, or a concavo-convex roll having irregularities on the contact surface (outer peripheral surface) with the stretched gel-like molded product.

(Iii) Thermal solvent treatment process You may perform the process which makes the extending | stretching gel-shaped molding before washing | cleaning contact a thermal solvent. As the hot solvent treatment method, for example, the method disclosed in WO2000 / 20493 can be used.

[3] Physical Properties of Polyolefin Microporous Membrane The polyolefin microporous membrane according to a preferred embodiment of the present invention has the following physical properties.
(1) Film thickness (μm)
The film thickness of the polyolefin microporous membrane is preferably 3 to 16 μm, more preferably 5 to 12 μm, and even more preferably 6 to 10 μm because of the recent progress of high density and high capacity batteries.

(2) Average pore diameter (average flow pore diameter) and bubble point (BP) pore diameter (nm)
The polyolefin microporous membrane preferably has an average pore size determined by a palm porometer of 0.05 μm or less. The bubble point (BP) pore diameter is preferably 0.06 μm or less. By making the pore diameter of the entire membrane small, the pores are not easily crushed, and the change in film thickness and air resistance is reduced.

(3) Air permeability resistance (sec / 100 cm ³ )
The air permeability resistance (Gurley value) is preferably 300 sec / 100 cm ³ or less. If it is 300 sec / 100 cm ³ or less, it has good permeability when used in a battery.

(4) Porosity (%)
The porosity is preferably 25 to 80%. When the porosity is 25% or more, good air resistance can be obtained. When the porosity is 80% or less, the strength when the microporous membrane is used as a battery separator is sufficient, and a short circuit can be suppressed. A porosity of 25 to 40% is preferable because the pores of the separator are not easily crushed during compression.

(5) Puncture strength (mN)
The puncture strength is 1,300 mN or more. If the puncture strength is less than 1,300 mN, a short circuit between the electrodes may occur when the microporous membrane is incorporated in a battery as a battery separator.

(6) Tensile strength at break (MPa)
The tensile strength at break is preferably 80 MPa or more in both the MD direction and the TD direction. This eliminates the worry of rupture. The tensile strength at break in the MD direction is preferably 110 MPa or more, more preferably 140 MPa. The tensile strength at break in the TD direction is preferably 120 MPa or more, and more preferably 170 MPa. When the tensile strength at break is in the above-mentioned preferable range, even when hot pressing is performed at a high pressure in the battery production process, the film is hardly broken and the pores are not easily collapsed.

(7) Tensile elongation at break (%)
The tensile elongation at break is 60% or more in both the MD direction and the TD direction. This eliminates the worry of rupture.

(8) Thermal shrinkage (%) after exposure for 8 hours at 105 ° C
The thermal shrinkage after exposure for 8 hours at a temperature of 105 ° C. is 15% or less in both the MD and TD directions. When the thermal shrinkage rate exceeds 15%, when the microporous membrane is used as a lithium battery separator, the end of the separator shrinks during heat generation, and the possibility of short circuit between the electrodes increases. The thermal shrinkage rate is preferably 8% or less in both the MD direction and the TD direction. The thermal contraction rate is more preferably 4% or less in both the MD direction and the TD direction.

(9) Change rate of film thickness after heat compression (%)
The rate of change in thickness after heating and compression at 90 ° C. for 5 minutes under a pressure of 5.0 MPa is preferably 10% or less, more preferably 5% or less, even more preferably 3% or less, with the film thickness before compression being 100%. It is. When the film thickness change rate is 10% or less, when a microporous film is used as a battery separator, lithium deposition can be prevented and a battery having good cycle characteristics can be obtained.

(10) Permeability change rate after heat compression (%)
The rate of change in air resistance after heating and compression at 90 ° C. for 5 minutes under a pressure of 5.0 MPa (the rate of change in the Gurley value (sec / 100 cm ³ ) before and after heating and compression) is preferably 50% or less, more preferably It is 40% or less, more preferably 35% or less. When the air permeability resistance change rate is 50% or less, when used as a battery separator, the cycle characteristics of the target battery can be obtained even through a hot press process at a high pressure during battery manufacture.

The film thickness change rate after heat compression and the air resistance change rate (%) after heat compression are easily affected by crystal orientation, film pore structure, heat shrinkage rate, and the like. Therefore, it can be controlled by the composition of the polyolefin resin, the cooling rate after extruding the polyolefin resin solution from the die lip, the heat treatment of the wound body, and the like.

[4] Physical Properties of Cell Using Polyolefin Microporous Membrane An electrochemical cell including an electrolyte in which a separator using a polyolefin microporous membrane according to a preferred embodiment of the present invention is disposed between an anode and a cathode is as follows. It has the physical properties of
(1) Impedance change rate (%)
The change rate of the impedance of the cell measured by the measurement method described later is preferably 7% or less. When the rate of change in impedance is within the above preferable range, deterioration of the cycle characteristics of the battery can be suppressed.

(2) Cell thickness change rate (%)
The cell thickness change rate measured by the measurement method described later is preferably 15% or less. When the rate of change in cell thickness is 15% or less, the separator and the electrode are sufficiently adhered by hot pressing, and lithium is unlikely to precipitate during initial charging.

[5] Battery The separator made of the polyolefin microporous membrane of the present invention is not particularly limited in the type of battery using this, but is particularly suitable for lithium secondary battery applications. A well-known electrode and electrolyte may be used for the lithium secondary battery using the separator made of the microporous membrane of the present invention. The structure of the lithium secondary battery using the separator made of the microporous membrane of the present invention may also be a known one.

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.
The physical properties of the polyolefin microporous membrane were measured by the following methods.
(1) Average pore diameter (average flow pore diameter) and bubble point (BP) pore diameter (nm)
The average pore diameter (average flow pore diameter) and bubble point (BP) pore diameter (nm) of the polyolefin microporous membrane were measured as follows.
Using a palm porometer (trade name, model: CFP-1500A) manufactured by PMI, measurement was performed in the order of Dry-up and Wet-up. In the wet-up, pressure was applied to a polyolefin microporous film sufficiently soaked with Galwick (trade name) having a known surface tension, and the pore diameter converted from the pressure at which air began to penetrate was defined as the maximum pore diameter. For the average flow diameter, the hole diameter was converted from the pressure at the point where the curve showing a half of the pressure / flow curve in the Dry-up measurement and the curve of the Wet-up measurement intersect. The following formula was used for conversion of pressure and pore diameter.
d = C · γ / P (where d (μm) is the pore diameter of the microporous membrane, γ (dynes / cm) is the surface tension of the liquid, P (Pa) is the pressure, and C is the pressure constant (2860). is there.)

(2) Air permeability resistance (sec / 100 cm ³ )
The air resistance (Gurley value) was measured according to JIS P8117 against microporous membrane having an average thickness _{T AV.}

(3) Porosity (%)
The porosity is calculated from the mass w ₁ of the microporous membrane and the mass w ₂ of the same size non-porous membrane made of the same polyethylene composition as the microporous membrane, and the porosity (%) = (w ₂ −w ₁ ) / W ₂ × 100.

(4) Puncture strength (mN)
The puncture strength was measured by measuring the maximum load value when a polyolefin microporous membrane was pierced at a speed of 2 mm / sec using a needle having a diameter of 1 mm (0.5 mmR).

(5) Tensile strength at break (kPa)
The tensile strength at break was measured by ASTM D882 using a strip-shaped test piece having a width of 10 mm.

(6) Tensile elongation at break (%)
The tensile elongation at break was determined by taking three strips of a 10 mm wide strip from the central portion in the width direction of the polyolefin microporous membrane, measuring by ASTM D882, and calculating the average value.

(7) Thermal shrinkage after exposure for 8 hours at 105 ° C (%)
The thermal shrinkage was determined by measuring the shrinkage in the MD and TD directions three times each when the microporous membrane was exposed at 105 ° C. for 8 hours, and calculating the average value.

(8) Film thickness change rate after heat compression (%)
The film thickness was measured with a contact thickness meter (manufactured by Mitutoyo Corporation).
The polyolefin microporous membrane is sandwiched between a pair of press plates having a high smooth surface, and this is heated and compressed at 90 ° C. for 5 minutes under a pressure of 5.0 MPa by a press. The value obtained by subtracting the film thickness after compression (b (μm)) from the film thickness before compression (a (μm)) and dividing by (a (μm)) is expressed as a percentage ((ab) ÷ a X100) is defined as the film thickness change rate (%). The film thickness was obtained by taking three points from the central portion in the width direction of the polyolefin microporous film, measuring it, and calculating the average value.

(9) Permeability change rate after heat compression (sec / 100 cm ³ )
The polyolefin microporous membrane is heated and compressed under the same conditions as in (8) above, and the air permeability resistance (α (sec / 100 cm ³ )) before heat compression is heated and compressed (β (sec / sec). 100 cm ³⁾⁾ subtracted from, the (alpha (sec / 100 cm ³⁾⁾ in that represents a value obtained by dividing a percentage ((β-α) ÷ α × 100) the air resistance change ratio (%). The air resistance was obtained by taking three points from the central portion in the width direction of the polyolefin microporous membrane, measuring it, and calculating an average value.

The physical properties of the cell using the polyolefin microporous membrane were measured by the following method.
(1) Cell impedance change rate (%)
A cell was sandwiched between a pair of press plates having a high smooth surface, and this was heated and compressed at 90 ° C. for 5 minutes under a pressure of 3.0 MPa and 5.0 MPa, respectively, and then an impedance measurement device (manufactured by Solartron, SI1250, SI1287). Subtract the impedance value (B) at high pressure (5.0 MPa) from the impedance value (A) at normal press pressure (3.0 MPa), and divide by (A) as the impedance change rate (%). To do.
Impedance change rate (%) = {(A) − (B)} / (A) × 100
(2) Cell thickness change rate (%)
A cell is sandwiched between a pair of press plates having a high smooth surface, and this is heated and compressed at 90 ° C. for 5 minutes under a pressure of 3.0 MPa and 5.0 MPa, respectively, and then charged under the following conditions to charge the cell thickness. Measured before and after charging. The cell thickness was measured at the center of the cell with a contact thickness meter (manufactured by Mitutoyo Corporation). The cell thickness change rate (%) is obtained by subtracting the cell thickness (b) after compression from the cell thickness (a) before compression and dividing the result by (a).
Cell thickness change rate (%) = {(a) − (b)} / (a) × 100

Example 1
Polyethylene (melting point: 135 ° C.) consisting of 18% by mass of UHMWPE (Mw / Mn: 8) having an Mw of 2.0 × 10 ⁶ and 82% by mass of HDPE (Mw / Mn: 6) having an Mw of 3.0 × 10 ⁵ Crystal dispersion temperature: 100 ° C., Mw / Mn: 10.0), tetrakis [methylene-3- (3,5-ditertiarybutyl-4-hydroxyphenyl) -propionate] methane as an antioxidant and polyethylene 100 0.2 parts by mass per mass part was dry blended to prepare a polyethylene composition. 30 parts by mass of the obtained polyethylene composition was charged into a twin screw extruder, 70 parts by mass of liquid paraffin [50 cSt (40 ° C.)] was supplied from the side feeder of this twin screw extruder, and conditions of 210 ° C. and 300 rpm Was melt-kneaded to prepare a polyolefin solution. This polyolefin solution was extruded from a T-die provided in a twin-screw extruder and taken up at a cooling rate of 210 ° C./min with a cooling roll adjusted to 30 ° C. to form a gel-like molded product. The obtained gel-like molded product was simultaneously biaxially stretched at 115 ° C. by 5 times in both the longitudinal direction and the width direction (surface magnification: 25 times) with a tenter stretching apparatus [first stretching], and directly in the longitudinal direction with a tenter stretching apparatus. And it fixed so that there may be no dimensional change in both directions of the width direction, and heat-fixed at the temperature of 110 degreeC. Next, the stretched gel-like molded product was immersed in a methylene chloride bath to remove liquid paraffin, and the microporous membrane obtained by washing was air-dried. Subsequently, the obtained microporous membrane was re-stretched 1.36 times in the width direction at 130 ° C. by a tenter stretching apparatus [second stretching], and then relaxed at a relaxation rate of 3% in the width direction. It fixed to the tenter stretching apparatus and heat-set at a temperature of 130 ° C. so that there was no dimensional change in both the longitudinal direction and the width direction. Next, after the polyolefin microporous membrane was cooled to room temperature, it was wound with a winding roll with a winding tension of 7 N to produce a polyolefin microporous membrane with a thickness of 7.1 μm.
In order to confirm the effect of the separator in the battery, the physical properties described above were measured using an electrochemical cell including an anode, a cathode, a separator, and an electrolyte as follows. As the cathode, a 40 mm × 40 mm sheet including a LiCoO ₂ layer having a unit area mass of 13.4 mg / cm ² and a density of 3.55 g / cm ³ on an aluminum substrate having a thickness of 15 μm was used. As the anode, a 45 mm × 45 mm sheet containing natural graphite having a unit area mass of 5.5 mg / cm ² and a density of 1.65 g / cm ³ on a 10 μm thick copper film substrate was used. The cell was assembled after the anode and cathode were dried in a 120 ° C. vacuum oven. The separator is a polyolefin microporous membrane produced in this example having a length of 50 mm and a width of 60 mm. After the separator was dried in a vacuum oven at 50 ° C., the cell was assembled, and LiPF ₆ was put in a mixture of ethylene carbonate and methyl ethyl carbonate (ethylene carbonate / methyl ethyl carbonate = 4/6, V / V (volume ratio)). An electrolyte was prepared by dissolving to form a 1M solution. A cell was made by stacking the anode, separator and cathode between aluminum laminates, impregnating the separator with electrolyte and then vacuum sealing.

Example 2
The first stretching temperature is 117.0 ° C., the second stretching ratio is 1.41 times, the relaxation rate in relaxation after the second stretching is set to 7%, and the winding tension is set to 9N. A polyolefin microporous membrane having a thickness of 9.4 μm was produced in the same manner as in Example 1 except that. Using this polyolefin microporous membrane, a cell was also produced in the same manner as in Example 1.

Example 3
The same as in Example 1 except that the temperature of the first stretching was 112.0 ° C., the magnification of the second stretching was 1.34 times, and the relaxation rate in the relaxation after the second stretching was set to 2%. Thus, a polyolefin microporous membrane having a thickness of 5.3 μm was produced. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Example 4
Polyethylene (melting point: 135 ° C.) comprising 30% by mass of UHMWPE (Mw / Mn: 8) having an Mw of 2.0 × 10 ⁶ and 70% by mass of HDPE (Mw / Mn: 6) having an Mw of 3.0 × 10 ⁵ Crystal dispersion temperature: 100 ° C., Mw / Mn: 10.0), the cooling rate in the cooling roll is set to 200 ° C./min, the temperature of the first stretching is set to 118.5 ° C., and the second stretching is performed. A polyolefin having a thickness of 11.7 μm was made in the same manner as in Example 1 except that the magnification of 1.40 was set, the relaxation rate in the relaxation after the second stretching was set to 14%, and the winding tension was set to 9N. A microporous membrane was produced. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Example 5
Polyethylene consisting of 40% by mass of UHMWPE (Mw / Mn: 8) with Mw of 2.0 × 10 ⁶ and 60% by mass of HDPE (Mw / Mn: 6) with Mw of 3.0 × 10 ⁵ (melting point: 135 ° C. , Crystal dispersion temperature: 100 ° C., Mw / Mn: 10.0), 25 parts by mass of the obtained polyethylene composition is supplied to a twin screw extruder, the temperature of the first stretching is 110 ° C., and the second Polyolefin microporous having a thickness of 3.0 μm as in Example 1 except that the stretching ratio of 1.60 times, the stretching temperature was 127 ° C., and the relaxation rate in the relaxation after the second stretching was set to 9%. A membrane was produced. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Example 6
Polyethylene consisting of 40% by mass of UHMWPE (Mw / Mn: 8) with Mw of 2.0 × 10 ⁶ and 60% by mass of HDPE (Mw / Mn: 6) with Mw of 3.0 × 10 ⁵ (melting point: 135 ° C. Crystal dispersion temperature: 100 ° C., Mw / Mn: 10.0), 25 parts by mass of the obtained polyethylene composition is supplied to a twin-screw extruder, and the first draw ratio is 7 in both the longitudinal direction and the width direction. The same as Example 1 except that the second draw ratio was 1.60 times, the draw temperature was 127 ° C., and the relaxation rate in the relaxation after the second draw was set to 6%. Thus, a polyolefin microporous membrane having a thickness of 3.0 μm was produced. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Example 7
Polyethylene (melting point: 135 ° C.) comprising 30% by mass of UHMWPE (Mw / Mn: 8) having an Mw of 2.0 × 10 ⁶ and 70% by mass of HDPE (Mw / Mn: 6) having an Mw of 3.0 × 10 ⁵ And 28.5 parts by mass of the obtained polyethylene composition are fed to a twin-screw extruder using a crystal dispersion temperature of 100 ° C. and Mw / Mn of 10.0). Polyolefin microporous having a thickness of 3.0 μm as in Example 1 except that the stretching ratio of 1.60 times, the stretching temperature was 127 ° C., and the relaxation rate in the relaxation after the second stretching was set to 9%. A membrane was produced.

Example 8
Thickness 3. As in Example 1, except that the first stretching temperature was 110 ° C., the second stretching ratio was 1.60 times, and the relaxation rate in relaxation after the second stretching was set to 9%. A 0 μm polyolefin microporous membrane was produced. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Comparative Example 1
Polyethylene consisting of 2% by mass of UHMWPE (Mw / Mn: 8) with an Mw of 2.0 × 10 ⁶ and 98% by mass of HDPE (Mw / Mn: 6) with an Mw of 3.0 × 10 ⁵ (melting point: 135 ° C. Crystal dispersion temperature: 100 ° C., Mw / Mn: 10.0), and a polyolefin solution was prepared with 40 parts by mass of the obtained polyethylene composition and 60 parts by mass of liquid paraffin. This polyolefin solution was extruded, the temperature of the first stretching was 119.5 ° C., the magnification of the second stretching was 1.4 times, and after the second stretching, it was not relaxed and wound with a winding tension of 9 N A polyolefin microporous membrane having a thickness of 9.0 μm was produced in the same manner as in Example 1 except that. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Comparative Example 2
A polyolefin microporous membrane having a thickness of 7.0 μm was produced in the same manner as in Example 1 except that the cooling rate was set to 160 ° C./min and winding was performed with a winding tension of 16 N. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Comparative Example 3
Polyethylene consisting of 40% by mass of UHMWPE (Mw / Mn: 8) with Mw of 2.0 × 10 ⁶ and 60% by mass of HDPE (Mw / Mn: 6) with Mw of 3.0 × 10 ⁵ (melting point: 135 ° C. Crystal dispersion temperature: 100 ° C., Mw / Mn: 10.0), and a polyolefin solution was prepared with 23 parts by mass of the obtained polyethylene composition and 77 parts by mass of liquid paraffin. This polyolefin solution was extruded, the first stretching temperature was 117.0 ° C., the second stretching temperature was stretched to 1.6 times at 128 ° C., and then relaxed by 12% in the width direction. A polyolefin microporous membrane having a thickness of 11.8 μm was produced in the same manner as in Example 1 except that the film was wound up. A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Comparative Example 4
A polyolefin solution is prepared with 25 parts by mass of a polyethylene composition and 75 parts by mass of liquid paraffin, the polyolefin solution is extruded, cooled at a cooling rate of 160 ° C./min, the temperature of the first stretching is set to 118.0 ° C., and the second A polyolefin microporous membrane having a thickness of 12.0 μm was produced in the same manner as in Example 1 except that the stretching temperature was stretched to 1.4 times at 126 ° C. and no relaxation was performed after the second stretching. . A cell was produced in the same manner as in Example 1 using this polyolefin microporous membrane.

Tables 1 to 4 show the production conditions of Examples 1 to 8 and Comparative Examples 1 to 4, the obtained polyolefin microporous membranes, and the physical properties of the cells using the polyolefin microporous membranes.

Claims (8)

1. The rate of change in air resistance after heat compression for 5 minutes at a temperature of 90 ° C. and a pressure of 5.0 MPa is 50% or less, and the rate of change in film thickness after heat compression for 5 minutes at a temperature of 90 ° C. and a pressure of 5.0 MPa A polyolefin microporous membrane having a thickness of 10% or less when the thickness of the polyolefin microporous membrane before heat compression is 100%.
2. The polyolefin microporous membrane according to claim 1, wherein the tensile strength in the MD direction is 110 MPa or more and the tensile strength in the TD direction is 120 MPa.
3. 3. The polyolefin microporous membrane according to claim 1, wherein the content of the ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 1 × 10 ⁶ or more is 10 to 40% by mass with respect to 100% by mass of the total mass of polyethylene.
4. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the film thickness is 16 µm or less.
5. The polyolefin microporous membrane according to any one of claims 1 to 4, which has a porosity of 25 to 40%.
6. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein an average pore size determined by a palm porometer is 0.05 µm or less and a bubble point (BP) pore size is 0.06 µm or less.
7. A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 6.
8. A battery using the battery separator according to claim 7.