JP2022048518A - Polyolefin microporous film, and coating film and secondary battery including the same - Google Patents

Abstract

To provide a polyolefin microporous film that shows low thermal shrinkage at high temperature/in a short time even when made into a thinner, stronger film, and also has low shutdown temperature and excellent battery safety, and a separator including the same.SOLUTION: A polyolefin microporous film has a piercing strength of 0.4 N/μm or more, and a thermal shrinkage of 30% or less up to 15 minutes in a width (TD) direction with a fixed mechanical (MD) direction, measured in a 135°C atmosphere.SELECTED DRAWING: Figure 1

Description

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

Microporous membranes are used in various fields such as filters such as filtration membranes and dialysis membranes, and separators for batteries and electrolytic capacitors. Among these, the microporous polyolefin membrane containing polyolefin as a main component is excellent in chemical resistance, insulating property, mechanical strength, etc., and has shutdown characteristics, and is therefore widely used as a separator for a secondary battery in recent years.

On the other hand, a secondary battery, for example, a lithium ion secondary battery, has a high energy density and is therefore widely used as a battery used in personal computers, mobile phones, and the like. Lithium-ion secondary batteries are also expected as a power source for driving motors of electric vehicles and hybrid vehicles.

In recent years, with the increase in energy density of lithium ion secondary batteries, there is a demand for thinning of a polyolefin microporous membrane used as a separator. In particular, the capacity of batteries for smartphones is increasing, and the safety of batteries is of the utmost importance. There are various safety of batteries for smartphones, but HoTBox safety is the most important. Under these circumstances, in order to achieve HoTBox safety, various studies have been conducted to produce a polyolefin microporous membrane having sufficient insulation, mechanical strength, and shutdown characteristics while satisfying low thermal shrinkage. ing.

For example, Patent Document 1 is characterized in that the shrinkage rate of TD in a fixed state of MD is 8 to 16% at 150 ° C., and the puncture strength is 0.2 N / μm to 0.375 N / μm. The polyolefin microporous membrane described above is disclosed.

Patent Document 2 is characterized in that the puncture strength is 0.21 to 0.22 N / μm or more, the hole closing temperature is 137 ° C. or less, and the heat shrinkage rate at 135 ° C. is 40% or less in both the vertical and horizontal directions. A microporous polyolefin membrane is disclosed.

Patent Document 3 discloses a polyolefin separator characterized by having a puncture strength of 20 g / μm or more, a pore closing temperature of 135 ° C. or less, and a shrinkage rate of 30% or less at 135 ° C.

Japanese Unexamined Patent Publication No. 2004-323820 Japanese Patent Application Laid-Open No. 2003-203257 Japanese Unexamined Patent Publication No. 2002-376589

However, in the polyolefin microporous membranes disclosed in Patent Documents 1 to 3, the thermal shrinkage of TD is low, but the puncture strength is less than 0.4 N / μm, and the film has sufficient strength at a film thickness of 7 μm or less. I can't say that. Further, in order to secure higher thin film strength, it is necessary to increase the high molecular weight component or the draw ratio, which increases the heat shrinkage rate and the shutdown temperature required for battery safety. There was a problem that battery safety was impaired.

In view of the above circumstances, the present invention is a polyolefin microporous membrane having a low heat shrinkage rate at a high temperature and a short time, a lower shutdown temperature, and excellent battery safety even when the film is thin and has high strength. It is an object of the present invention to provide a separator and a secondary battery using the above.

As a result of diligent studies to solve the above problems, the present inventors have excellent hot box characteristics in a microporous membrane having high puncture strength and suppressed thermal shrinkage of TD in a state where MD is fixed. The present invention has been completed by clarifying that the above problems can be solved.

The microporous polyolefin membrane of the first aspect of the present invention has a puncture strength of 0.4 N / μm or more and is measured in an atmosphere of 135 ° C. for 15 minutes in the machine (MD) direction and the width (TD) direction. It is characterized in that the heat shrinkage rate up to is 30% or less.

Further, in the polyolefin microporous film, the heat shrinkage ratio of MD and TD for 1 hour in a free state measured in an atmosphere of 120 ° C./130 ° C./135 ° C. was 0.8 or more and 1.2 or less. May be good.

Further, the microporous polyolefin membrane may have a shutdown (SD) temperature of 135 ° C. or higher and lower than 140 ° C. Further, the film thickness of the microporous polyolefin membrane may be 3 μm or more and 7 μm or less.

Further, the polyolefin microporous membrane may be mainly composed of polyethylene resin. The battery separator according to the second aspect of the present invention includes the polyolefin microporous membrane.

The secondary battery of the third aspect of the present invention includes the above-mentioned battery separator.

The polyolefin microporous membrane of the present invention is a polyolefin microporous membrane and a battery which are thin and have high strength, have a low heat shrinkage rate at high temperature and in a short time, have a lower shutdown temperature, and are excellent in battery safety. Separators and secondary batteries can be provided.

It is a schematic diagram which shows the test method of high temperature heat shrinkage in this invention.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the embodiments described below.

1. 1. Polyolefin Microporous Membrane As used herein, the polyolefin microporous membrane refers to a microporous membrane containing polyolefin as a main component, and for example, a microporous membrane containing 90% by mass or more of polyolefin with respect to the total amount of the microporous membrane. Hereinafter, the physical characteristics of the microporous polyolefin membrane of the present embodiment will be described.

(Puncture strength)
Puncture strength and maximum load L1 (N) when a microporous polyolefin membrane with a thickness of T1 (μm) is pierced at a speed of 2 mm / sec with a needle having a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm. ) Is measured and is a value obtained by the formula: L2 = L1 / T1 (value converted into a maximum load L2 per 1 μm of film thickness). The lower limit of the puncture strength per film thickness of the microporous polyolefin membrane of the present invention is 0.4 N / μm or more, preferably 0.42 N / μm or more, and more preferably 0.45 N / μm or more. When the puncture strength is 0.4 N / μm or more, when used as a battery separator, the separator is less likely to break due to winding or foreign matter, and short circuit is less likely to occur, so that the safety of the battery can be improved. .. If the puncture strength is less than 0.4 N / μm, when used as a battery separator, the separator may break the film due to winding or foreign matter, and a short circuit may easily occur.

(MD fixed TD heat shrinkage rate)
In the present specification, the MD (mechanical direction) means the direction in which the film is extruded when the polyolefin microporous film of the present invention is formed (the film forming direction), and the TD (width direction) is the film. Above, it means the direction orthogonal to the MD.

A major feature of the present invention is that the heat shrinkage of TD up to 15 minutes is less than 30%. The thermal shrinkage referred to here is the shrinkage rate of TD at 135 ° C. measured with both ends of the MD fixed. The shrinkage of the TD of the separator at 135 ° C. is preferably less than 30%, more preferably less than 25%. The separator is wound around the MD in the battery, and the MD of the separator is fixed and shrinkage is suppressed. On the other hand, the TD is not fixed and contraction is easier. Under the HoTBox test, if the heat shrinkage of the TD at the initial stage (15 minutes after reaching the test temperature) is large, the contact area between the electrodes is large, and the battery temperature rises due to the generation of a large amount of Joule heat, and there is a risk of heat generation and ignition. Increases. When the shrinkage rate of the TD of the separator at 135 ° C. is 30% or less, the positive area between the electrodes can be reduced, and the battery safety such as HoTBox characteristics is improved. This cannot be achieved even if attention is paid to the conventional long-time heat shrinkage of 1 hour or more.

(MD / TD heat shrinkage ratio in the free state)
The heat shrinkage ratio of MD and TD in the free state in which both ends of the MD and TD of the microporous polyolefin membrane of the present invention are not fixed is preferably 0.8 or more and 1.2% or less, more preferably 0.85 or more. It is 1.15 or less, and more preferably 0.9 or more and 1.1 or less. When used as a battery separator, the MD is fixed and the MD cannot shrink. However, when the heat shrinkage ratio of the MD / TD in the free state becomes high, the TD becomes due to the shrinkage stress of the MD. It shrinks easily and the shrinkage rate increases. When the shrinkage ratio of MD / TD in the free state is within the above range, it is not possible to prevent a short circuit due to the shrinkage of the separator even when exposed at a high temperature, and the risk of heat generation and ignition of the battery can be reduced.

(Shutdown temperature)
The shutdown temperature of the microporous polyolefin membrane is preferably less than 140 ° C, more preferably less than 138 ° C, still more preferably less than 136 ° C. When the shutdown temperature is within the above range, the battery can be shut down earlier when abnormal heat generation occurs, and abnormal heat generation can be prevented. The shutdown temperature in the above range can be achieved by setting the microporous polyolefin membrane in the above-mentioned pore diameter range.

(Film thickness)
The film thickness of the microporous polyolefin membrane is 3 μm or more and 7 μm or less, preferably 3 μm or more and 6 μm or less, and more preferably 3 μm or more and 5 μm or less. When the film thickness is within the above range, when the polyolefin microporous membrane is used as a battery separator, the battery capacity can be improved and sufficient insulating properties can be imparted.

(Air permeability resistance)
The air permeability resistance (Garley value) of the microporous polyolefin membrane is not particularly limited, but is, for example, 30 seconds / 100 cm ³ or more and 300 seconds / 100 cm ³ or less. Further, the upper limit of the air permeation resistance when used as a battery separator is preferably 250 seconds / 100 cm ³ or less, more preferably 200 seconds / 100 cm ³ or less, and further preferably 160 seconds / 100 cm ³ or less. be. When the air permeability resistance is within the above range, when used as a battery separator, the ion permeability is excellent, the impedance of the battery is lowered, and the battery output is improved. The air permeation resistance can be set in the above range by adjusting the stretching conditions and the like when producing the microporous polyolefin membrane.

(Porosity)
The porosity of the microporous polyolefin membrane is not particularly limited, but is, for example, 10% or more and 70% or less. When used as a battery separator, the porosity is preferably 20% or more and 60% or less, and more preferably 20% or more and 50% or less. When the porosity is within the above range, a high holding amount of the electrolytic solution and high ion permeability can be ensured, and the rate characteristics of the battery can be improved. The porosity is adjusted in the manufacturing process by the blending ratio of the constituent components of the polyolefin resin, the draw ratio, and the like.

(Composition of Polyolefin Microporous Membrane)
The polyolefin microporous membrane contains a polyolefin resin as a main component. The "main component" means, for example, containing 90% by mass or more of the polyolefin resin with respect to the total amount of the microporous polyolefin membrane. As the polyolefin resin, for example, polyethylene resin, polypropylene resin and the like can be used. For example, the polyethylene resin can be contained in an amount of 50% by mass or more based on the total amount of the microporous polyolefin membrane. The polyethylene resin is not particularly limited, and various polyethylene resins can be used. For example, high-density polyethylene, medium-density polyethylene, branched low-density polyethylene, linear low-density polyethylene and the like are used. The polyethylene resin may be a homopolymer of ethylene or a copolymer of ethylene and another α-olefin. Examples of the α-olefin include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like.

When the polyolefin microporous film contains high-density polyethylene (density: 0.920 g / m ³ or more and 0.970 g / m ³ or less), it is excellent in melt extrusion characteristics and uniform stretching processing characteristics. The weight average molecular weight (Mw) of the high-density polyethylene used as a raw material is, for example, about 1 × 10 ⁴ or more and less than 1 × ¹⁰⁶ . Mw is a value measured by gel permeation chromatography (GPC). The content of the high-density polyethylene is, for example, 30% by mass or more with respect to 100% by mass of the entire polyolefin resin. The upper limit of the content of high-density polyethylene is, for example, 70% by mass or less, and when it contains other components, it is, for example, 90% by mass or less.

Further, the polyolefin microporous membrane can contain ultra-high molecular weight polyethylene (UHMwPE). The ultra-high molecular weight polyethylene used as a raw material has a weight average molecular weight (Mw) of 1 × 106 or more (1 million or more), preferably 1 × 10 ⁶ or more and 8 × ^{10 6 or} less. When the Mw of the ultra-high molecular weight polyethylene is in the above range, the moldability is good. The ultra-high molecular weight polyethylene may be used alone or in combination of two or more, and for example, two or more types of ultra-high molecular weight polyethylene having different Mw may be mixed and used.

The ultra-high molecular weight polyethylene can be contained, for example, 2% by mass or more and 70% by mass or less with respect to 100% by mass of the entire polyolefin resin. For example, when the content of ultra-high molecular weight polyethylene is 10% by mass or more and 60% by mass or less, the Mw of the obtained polyolefin microporous membrane can be easily controlled within a specific range described later, and production such as extrusion kneading property can be easily performed. It tends to be excellent in sex. Further, when the ultra-high molecular weight polyethylene is contained, high mechanical strength can be obtained even when the microporous polyolefin membrane is thinned.

The polyolefin microporous membrane may contain a polypropylene resin. The type of the polypropylene resin is not particularly limited, and may be any one of a homopolymer of propylene, at least one of a copolymer of propylene and another α-olefin and a diolefin (propylene copolymer), or a mixture thereof. However, from the viewpoint of mechanical strength and miniaturization of through-hole diameter, it is preferable to use a polypropylene homopolymer. The content of the polypropylene resin with respect to the entire polyolefin resin is, for example, 0% by mass or more and 15% by mass or less, and preferably 2.5% by mass or more and 15% by mass or less from the viewpoint of heat resistance.

Further, the polyolefin microporous membrane can contain other resin components other than the polyethylene resin and the polypropylene resin, if necessary. As the other resin component, for example, a heat-resistant resin or the like can be used. Further, the polyolefin microporous film is an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent or a filler, a crystal nucleating agent, and a crystallization retarder as long as the effects of the present invention are not impaired. It may contain various additives such as.

(Weight average molecular weight of resin component: Mw)
The weight average molecular weight (Mw) of the resin component (resin component after molding as a film) constituting the polyolefin microporous film is, for example, ³ × 105 or more and less than 2 × ¹⁰⁶ . When the Mw of the resin component constituting the polyolefin microporous film is in this range, the moldability, mechanical strength and the like are excellent. Then, in the process of manufacturing the microporous polyolefin membrane, even if it is stretched at a relatively high magnification, local stress concentration does not occur, and a uniform and fine pore structure can be formed. The Mw of the microporous polyolefin membrane can be within the above range by appropriately adjusting the blending ratio of the constituent components of the polyolefin resin and the conditions of melt kneading.

(Manufacturing method of microporous polyolefin membrane)
The method for producing the microporous polyolefin membrane is not particularly limited as long as the microporous polyolefin membrane having the above characteristics can be obtained, and a known method for producing the microporous polyolefin membrane can be used. Examples of the method for producing a polyolefin microporous film include a dry film forming method and a wet film forming method. As the method for producing the microporous polyolefin membrane of the present embodiment, a wet membrane forming method is preferable from the viewpoint of ease of controlling the structure and physical properties of the membrane. As the wet film forming method, for example, the methods described in Japanese Patent No. 2132327, Japanese Patent No. 3347835, International Publication No. 2006/137540 and the like can be used.

A method for producing a microporous polyolefin membrane (wet film forming method) will be described. The following description is an example of a manufacturing method, and is not limited to this method.

First, a polyolefin resin and a film-forming solvent are melt-kneaded to prepare a resin solution. 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. Since the melt-kneading method is known, the description thereof will be omitted.

As the polyolefin resin, the same ones as described above can be used. In addition, the resin solution contains antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, blocking inhibitors and fillers, crystal nucleating agents, crystallization retarders, etc., as long as the effects of the present invention are not impaired. It may contain various additives of. As the crystal nucleating agent, the same as described above can be used.

Next, the resin solution prepared above is fed from the extruder to the die, extruded into a sheet shape, and the obtained extruded molded product is cooled to form a gel-like sheet. A plurality of polyolefin resin compositions having the same or different compositions may be fed from a plurality of extruders to one die, laminated in layers there, and extruded into a sheet. As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

The gel sheet is then stretched at least uniaxially. Stretching of the gel-like sheet (first stretching) is also referred to as wet stretching. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferable. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the wet stretching, the final area stretching ratio (plane magnification) is, for example, preferably 3 times or more, more preferably 4 times or more and 30 times or less in the case of uniaxial stretching. Further, in the case of biaxial stretching, 9 times or more is preferable, 16 times or more is more preferable, and 25 times or more is further preferable. The upper limit is preferably 100 times or less, more preferably 64 times or less. Further, both MD (mechanical direction: longitudinal direction) and TD (width direction: lateral direction) are preferably 3 times or more, and the draw ratios in MD and TD may be the same or different from each other. When the draw ratio is 5 times or more, improvement in puncture strength can be expected. The stretching ratio in this step refers to the stretching ratio of the gel-like sheet immediately before being subjected to the next step with reference to the gel-like sheet immediately before this step. Further, the TD direction is a direction orthogonal to the MD direction when the microporous membrane is viewed in a plane.

The stretching temperature is preferably in the range of the crystal dispersion temperature (TcD) to TcD + 30 ° C. of the polyolefin resin, and more preferably in the range of the crystal dispersion temperature (TcD) + 5 ° C. to the crystal dispersion temperature (TcD) + 28 ° C. It is preferably in the range of TcD + 10 ° C. to TcD + 26 ° C., particularly preferably. When the stretching temperature is within the above range, film breakage due to polyolefin resin stretching is suppressed, and high-magnification stretching is possible. Here, the crystal dispersion temperature (TcD) means a value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. Polyethylene and polyethylene compositions other than the above-mentioned ultra-high molecular weight polyethylene and ultra-high molecular weight polyethylene have a crystal dispersion temperature of about 90 to 100 ° C. The stretching temperature can be, for example, 90 ° C. or higher and 130 ° C. or lower.

Next, the film-forming solvent is removed from the stretched gel-like sheet to form a microporous film. To remove the solvent, cleaning is performed using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is removed, it is composed of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the above is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are known, the description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

Next, the microporous film from which the film-forming solvent has been removed is dried by a heat-drying method or an air-drying method. The drying temperature is preferably not less than the crystal dispersion temperature (TcD) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than TcD. Drying is preferably carried out with the microporous membrane film as 100% by mass (dry weight) until the residual cleaning solvent is 5% by mass or less, and more preferably 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the polyolefin microporous film is maintained when the subsequent microporous film film stretching step and heat treatment step are performed, and the deterioration of permeability is suppressed.

Next, the dried microporous membrane is stretched at least in the uniaxial direction at a predetermined area stretching ratio. Stretching of the film after drying (second stretching) is also referred to as dry stretching. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used, but sequential stretching is preferred. In the case of sequential stretching, it is preferable to stretch in the MD direction and then continuously in the TD direction.

The surface magnification (area stretching ratio) of the dry-type stretching is preferably 1.2 times or more, and more preferably 1.2 times or more and 9.0 times or less. By setting the surface magnification within the above range, the puncture strength and the like can be easily controlled within a desired range. In the case of uniaxial stretching, for example, it is 1.2 times or more, preferably 1.2 times or more and 3.0 times or less in the MD direction or the TD direction. In the case of biaxial stretching, the stretching ratios may be 1.0 times or more and 3.0 times or less in the MD direction and TD direction, respectively, and the stretching ratios in the MD direction and the TD direction may be the same or different, but in the MD direction and the TD direction. It is preferable that the draw ratios of the above are substantially the same. In the dry-type stretching, it is preferable that the stretching is performed in the MD direction by more than 1 times and 3 times or less (second stretching), and then continuously stretched in the TD direction by more than 1 times and 3 times or less (third stretching). The draw ratio in this step is the microporous film (film) immediately before this step.
Refers to the draw ratio of the microporous membrane immediately before being subjected to the next step.

The stretching temperature in this step is not particularly limited, but is usually 90 to 135 ° C, more preferably 95 to 133 ° C.

Further, when the second stretching is roll-stretched, it is preferably multi-stage stretching. In the case of high-fold stretching, the stretching point is not determined due to the occurrence of slip on the roll, and stretching unevenness is likely to occur. However, by increasing the number of stretching steps, the stretching unevenness can be reduced. In particular, when the draw ratio is 1.5 or more, it is preferable to stretch in 4 steps or more, and more preferably in 5 steps or more.

Further, the microporous membrane after drying may be heat-treated. The heat treatment method is realized by three steps of heat stretching treatment, heat relaxation treatment and heat fixing treatment. The heat stretching treatment is a heat treatment in which the film is stretched while being heated to a predetermined magnification of TD. As the heat relaxation treatment, the heat relaxation treatment is a treatment of heat-shrinking the film to TD during heating. The heat fixing process is a heat treatment in which the film is heated while keeping the TD dimensions unchanged. In the present invention, by setting only the temperature of the heat relaxation zone to 135 ° C., including the adjustment of the air permeation resistance, it is possible to suppress short-time shrinkage at 135 ° C.

Further, a cross-linking treatment and a hydrophilization treatment can also be performed. For example, the microporous membrane is crosslinked by irradiating it with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 MraD is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the microporous membrane. Further, the hydrophilization treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge or the like. The monomer graft is preferably performed after the cross-linking treatment.

(Coating film)
A laminated polyolefin porous film (coating film) may be formed by laminating a porous layer other than the polyolefin resin on at least one surface of the polyolefin microporous film. The other porous layer is not particularly limited, and for example, an inorganic particle layer containing a binder and inorganic particles may be laminated by coating.

The thickness of the porous layer can be set in the range of 1 to 5 μm, preferably 1 to 4 μm, and more preferably 1 to 3 μm. When the thickness of the porous layer has such a thickness, a sufficient effect of forming the porous layer (effect of improving insulation and strength, etc.) can be obtained, product variation can be suppressed, productivity can be improved, and an electrode can be obtained. Adhesiveness to the product can be ensured. When the thickness of the porous layer is 5 μm or less, the bulk due to winding and stacking can be suppressed, which is suitable for increasing the capacity of the battery. Further, it is possible to prevent the curl from becoming large and contribute to the improvement of productivity in the battery assembly process.

The binder component constituting the inorganic particle layer is not particularly limited, and known components can be used, for example, acrylic resin, polyvinylidene fluoride resin, polyamideimide resin, polyamide resin, aromatic polyamide resin, polyimide resin and the like. Can be used.

The inorganic particles constituting the inorganic particle layer are not particularly limited, and known materials can be used. For example, titania, alumina, boehmite, barium sulfate, magnesium oxide, magnesium hydroxide, magnesium carbonate, silicon and the like are used. be able to.

(Lithium-ion secondary battery)
The polyolefin microporous film and coating film according to this embodiment can be suitably used as a separator for a lithium ion secondary battery. By using the polyolefin microporous film and the coating film according to the present embodiment as the separator, it is possible to provide a lithium ion secondary battery having excellent battery characteristics.

As an example of a lithium ion secondary battery to which the polyolefin microporous film and the coating film according to the present embodiment are applied, an electrolytic solution containing an electrolyte is impregnated into a battery element in which a negative electrode and a positive electrode are arranged so as to face each other via a separator. , Those having a structure in which these are enclosed in an exterior material can be mentioned.

Examples of the negative electrode include a negative electrode mixture composed of a negative electrode active material, a conductive auxiliary agent, and a binder, which is formed on a current collector. As the negative electrode active material, a material capable of doping and dedoping lithium ions is used. Specific examples thereof include carbon materials such as graphite and carbon, silicon oxides, silicon alloys, tin alloys, lithium metals, lithium alloys and the like. As the conductive auxiliary agent, a carbon material such as acetylene black or ketjen black is used. As the binder, styrene-butadiene rubber, polyvinylidene fluoride, polyimide and the like are used. Copper foil, stainless steel foil, nickel foil and the like are used as the current collector.

Examples of the positive electrode include a positive electrode mixture composed of a positive electrode active material, a binder and, if necessary, a conductive auxiliary agent, which is formed on a current collector. Examples of the positive electrode active material include a lithium composite oxide containing at least one transition metal such as MN, Fe, Co, and Ni. Specific examples thereof include lithium nickel oxide, lithium cobalt oxide, and lithium manganate. As the conductive auxiliary agent, a carbon material such as acetylene black or ketjen black is used. As the binder, polyvinylidene fluoride or the like is used. Aluminum foil, stainless steel foil, etc. are used as the current collector.

As the electrolytic solution, for example, a solution obtained by dissolving a lithium salt in a non-aqueous solvent can be used. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiN (SO ₂ CF ₃ ) ₂ . Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, γ-butyrolactone and the like, and usually, two or more of these are mixed together with various additives such as vinylene carbonate. Is used. Further, an ionic liquid (normal temperature molten salt) such as an imidazolium cation system can also be used.

Examples of the exterior material include metal cans and aluminum laminated packs. Examples of the shape of the battery include a coin type, a cylindrical type, a square type, and a laminated type.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these examples.

(Measurement method and evaluation method)
The microporous polyolefin membranes of Examples and Comparative Examples were measured and evaluated by the following methods.

(Puncture strength)
The maximum load L1 (N) when a microporous film having a thickness of T1 (μm) was pierced at a speed of 2 mm / sec was measured with a needle having a spherical tip (radius of curvature R: 0.5 mm) and a diameter of 1 mm. .. Further, the measured value L1 of the maximum load is a value obtained by the formula: L2 = (L1 × 5) / T1 (value converted into the maximum load L2 per 1 μm film thickness).
<Heat shrinkage of MD-fixed TD>
A sample of MD50 mm × TD50 mm is cut from a microporous membrane, and as shown in FIG. 1, TD is placed on a cardboard frame having an outer frame of 80 mm in width × 75 mm in length and an inner frame of 50 mm in width × 35 mm in length. Fix the MD with heat-resistant tape according to the width of the inner frame. Place horizontally in an oven at 135 ° C and leave for 15 minutes. Then, it is air-cooled and the shortest length of the TD width (mm) is measured.
MD shrinkage rate (%) = (1-shortest TD width (mm) / 50) × 100.

(MD / TD heat shrinkage ratio in the free state)
The heat shrinkage rate of MD (heat shrinkage rate MD) and the heat shrinkage rate of TD (heat shrinkage rate TD) after the microporous membrane was placed in an environment of 120 ° C., 130 ° C. and 135 ° C. for 1 hour were as follows. Was measured.
(1) The size of the test piece of the polyolefin microporous membrane at room temperature (25 ° C.) is measured for both MD and TD.
(2) The test piece of the polyolefin microporous membrane is equilibrated at temperatures of 120 ° C., 130 ° C. and 135 ° C. for 1 hour without applying a load.
(3) The size of the microporous polyolefin membrane is measured for both MD and TD.
(4) For the heat shrinkage to MD and TD, the measurement result (3) was divided by the measurement result (1), the obtained value was subtracted from 1, and the value was expressed as a percentage (%).
(5) The MD / TD shrinkage ratio is the value of the shrinkage rate of MD / the shrinkage rate of TD obtained in (4).

(Shutdown temperature)
A φ45 mm composite membrane sample is set in a heater block, and the air permeability resistance is measured with the Oken type air permeability resistance meter EGO-1T (manufactured by Asahi Seiko Co., Ltd.) while raising the temperature at 5 ° C / min. The temperature at which the degree reached 100,000 seconds / 100 ml was defined as the shutdown temperature.

(Film thickness)
The film thickness at 5 points within the range of 95 mm × 95 mm of the polyolefin microporous film was measured with a contact thickness meter (Lightmatic manufactured by Mitutoyo Co., Ltd.), and the average value was obtained.

(Air permeability resistance: Garley value)
For a polyolefin microporous membrane with a film thickness of T1 (μm), the air permeability resistance P1 (Sec / Sec /) measured with an air permeability meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P-8117. 100 cm ³ ) was measured.

(Porosity)
A 95 mm square sample was cut from a porous polyolefin film, its volume (cm ³ ) and weight (g) were determined, and it was calculated from them and the polymer density (g / cm ³ ) using the following formula. The above measurement was performed at three different randomly selected points in the same film, and the average value of the porosity of the three points was calculated.
Porosity = [(volume-weight / polymer density) / volume] x 100
(Weight average molecular weight (Mw)
The weight average molecular weight (Mw) of the obtained polyolefin microporous membrane was determined by the gel permeation chromatography (GPC) method under the following conditions.
-Measuring device: GPC-150C manufactured by WaterS CorporaTioN
-Column: Showa Denko Corporation ShoDex UT806M
・ Column temperature: 135 ℃
-Solvent (mobile phase): o-dichlorobenzene-Solvent flow velocity: 1.0 ml / min-Sample concentration: 0.1 wT% (dissolution condition: 135 ° C / 1h)
-Injection amount: 500 μl
-Detector: WaTerS CorporaTioN differential refractometer (RI detector)
-Calibration curve: Prepared using a polyethylene conversion constant (0.468) from a calibration curve obtained using a monodisperse polystyrene standard sample.

(HoTBox characterization)
Hereinafter, a method for manufacturing a secondary battery for evaluating HoTBox characteristics will be described.

(Preparation of positive electrode)
Lithium cobalt composite oxide LiCoO ₂ as the positive electrode active material, acetylene black as the conductive material, and polyvinylidene fluoride (PVDF) as the binder are mixed at a mass ratio of 93.5: 4.0: 2.5, and the solvent N is used. -A slurry was prepared by mixing and dispersing in methylpyrrolidone (NMP). This slurry was applied to both sides of a 12 μm-thick aluminum foil serving as a positive electrode current collector, dried, and then rolled by a roll press machine. The rolled product was slit to a width of 30 mm to obtain a positive electrode.

(Manufacturing of negative electrode)
Artificial graphite as the negative electrode active material, carboxymethyl cellulose as the binder, and styrene-butadiene copolymer latex were mixed and dispersed in purified water so as to have a mass ratio of 98: 1: 1 to prepare a slurry. This slurry was applied to both sides of a copper foil having a thickness of 10 μm as a negative electrode current collector, dried, and then rolled by a roll press machine. The rolled product was slit to a width of 33 mm to form a negative electrode.

(Non-water electrolyte)
LiPF ₆ as a solute was dissolved in a mixed solvent of ethylene carbonate: ethylmethyl carbonate: dimethyl carbonate = 3: 5: 2 (volume ratio) so as to have a concentration of 1.15 mol / liter. Further, 0.5% by mass of vinylene carbonate was added to 100% by mass of the non-aqueous electrolytic solution to prepare a non-aqueous electrolytic solution.

(Battery production)
After laminating the above positive electrode, the microporous polyolefin membrane of the present embodiment, and the above negative electrode, a flat wound electrode body (height 2.2 mm × width 36 mm × depth 29 mm) was produced. A tab with a sealant was welded to each electrode of this flat wound electrode body to form a positive electrode lead and a negative electrode lead. The flat wound electrode body is sandwiched between aluminum laminated films, sealed with some openings left, and dried in a vacuum oven at 80 ° C. for 6 hours. After drying, 0.7 ml of electrolytic solution is immediately poured. It was liquid, sealed with a vacuum sealer, and press-molded at 80 ° C. and 1 MPa for 1 hour. Subsequently, charging and discharging were carried out. The charging / discharging condition was a current value of 300 mA, and after constant current charging to a battery voltage of 4.2 V, constant voltage charging was performed at a battery voltage of 4.2 V until the battery voltage reached 15 mA. After a 10-minute pause, a constant current discharge was performed at a current value of 300 mA to a battery voltage of 3.0 V, and the 10-minute pause was performed. The above charging and discharging were carried out for 3 cycles to prepare a test secondary battery having a battery capacity of 300 mAh.

(HoTBox test)
The assembled test secondary batteries were put into an oven one by one, heated from room temperature to 135 ° C. at 5 ° C./miN, and then left at 135 ° C. for 1 hour. This is repeated with 10 batteries, and the number of batteries that have reached a high temperature within 1 hour after reaching 135 ° C is × (impossible), and 1 to 4 batteries are △ (good) high temperature. Those that have not reached are marked as ○ (excellent).

<Example>
(Example 1)
40 parts by mass of ultra-high molecular weight polyethylene resin (UHMwPE) with a weight average molecular weight of 2.5 x ¹⁰⁶ and a melting point of 136 ° C, a weight average molecular weight of ^3.5 x 105, a melting point of 135 ° C and a weight average molecular weight / number. Biaxial extrusion of a mixture with 60 parts by mass of linear high-density polyethylene resin (HDPE) having an average molecular weight of 4.05 and an unsaturated terminal group weight of 0.14 / (1.0 × 10 ⁴ carbon atoms). It was put into the machine, and liquid paraffin (film-forming solvent, thermoplastic) was injected by a pump from the side feeder of the twin-screw extruder. The injection amount of the liquid paraffin was adjusted so that the total amount of the polyethylene resin mixture was 25% by mass when the total amount of the polyethylene resin mixture and the liquid paraffin was 100% by mass. Liquid paraffin was injected into a twin-screw extruder and then dissolved and kneaded to obtain a mixed solution of a polyethylene resin mixture and liquid paraffin (solvent for film formation). A mixed solution of the obtained polyethylene resin mixture and liquid paraffin (solvent for film formation) was put into a twin-screw extruder, and melt extrusion was performed at a temperature of 210 ° C. The stainless steel fiber was sintered and compressed with a filter having an average opening of 20 μm, and then extruded into a sheet from a T-shaped die and cooled with a cooling roll having a temperature of 20 ° C. to obtain a gel-like sheet. The gel sheet was simultaneously biaxially stretched with a tenter at a stretching ratio of 5 times in both TD (width direction) and MD (longitudinal direction) at 116 ° C., and then immersed in methylene chloride at 25 ° C. to remove liquid paraffin and at room temperature. A microporous sheet (dry sheet) was obtained by drying with the air blown from. The obtained microporous sheet was stretched 1.2 times to MD at 113 ° C. by a roll method using a roll method using a peripheral speed difference of the roll. Subsequently, after stretching in the TD (width direction) at 132 ° C. (maximum magnification 1.78 times), the TD was subjected to a relaxation treatment at 135 ° C. with a relaxation rate of 9.0%, and heated at 130 ° C. Immobilization treatment was carried out to obtain a polyolefin microporous film.

(Examples 2 to 4, Comparative Examples 1 to 3)
A microporous polyolefin membrane was produced in the same manner as in Example 1 except for the film forming conditions shown in Table 1. Table 1 shows the evaluation results and the like of the obtained polyolefin microporous membrane.

(evaluation)
The microporous polyolefin membranes of Examples 1 to 3 had a puncture strength of 0.4 N / μm or more and were measured in an atmosphere of 135 ° C. for up to 15 minutes in the width (TD) direction with a fixed machine (MD) direction. It was shown that the heat shrinkage rate was 30% or less, the hot box characteristics were good, and the battery safety was excellent.

The microporous polyolefin membrane of the present invention has excellent hot box characteristics when used as a battery separator. Therefore, it can be suitably used as a separator for a secondary battery that requires battery safety.

1: Polyolefin microporous film 2: Thick paper frame 3: Heat resistant tape

Claims (7)

It is characterized by a puncture strength of 0.4 N / μm or more and a heat shrinkage rate of 30% or less up to 15 minutes in the width (TD) direction in which the machine (MD) direction is fixed, as measured in an atmosphere of 135 ° C. Polyolefin microporous film. Claim 1 is characterized in that the heat shrinkage ratio in the MD and TD directions in the free state for 1 hour measured in an atmosphere of 120 ° C./130 ° C./135 ° C. is 0.8 or more and 1.2 or less. The polyolefin microporous membrane of the description. The microporous polyolefin membrane according to claim 1 or 2, wherein the shutdown (SD) temperature is 135 ° C. or higher and lower than 140 ° C. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the film thickness is 3 μm or more and 7 μm or less. The microporous polyolefin membrane according to any one of claims 1 to 4, wherein the main component is a polyethylene resin. A battery separator using the microporous polyolefin membrane according to any one of claims 1 to 5. A secondary battery comprising the battery separator according to claim 6.

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