JP2022051238A - Polyolefin microporous film - Google Patents

Abstract

To provide a polyolefin microporous film that can reduce the short circuit rate of a battery when assembled into the battery as a separator.SOLUTION: In the inventive polyolefin microporous film, an average resistance voltage at 20 points measured in an area of 5 cm×10 cm is 0.125 kV/μm or more and 1.250 kV/μm or less, and the standard deviation is 0 or more and 0.0060 or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to a microporous polyolefin membrane and the like.

Polyolefin microporous membranes (hereinafter, may be simply abbreviated as "PO microporous membranes") are widely used for separating various substances, or as a selective permeation separation membrane, a separating material, and the like. Applications include, for example, microfiltration membranes; battery separators for lithium-ion batteries, fuel cells, etc .; condenser separators; functional membrane base materials for filling holes with functional materials to create new functions. And so on. Among them, the PO microporous film is preferably used as a separator for a lithium ion battery (LIB) widely used in mobile phones, smartphones, wearable devices, notebook personal computers (PCs), tablet PCs, digital cameras, and the like. ing.

Conventionally, the PO microporous membrane is required to have various performances because it is used as a separator for LIB. Specifically, it has mechanical strength against deformation or foreign matter due to external force (piercing strength), dimensional stability at temperatures below the melting point of PO (heat shrinkage), shutdown performance near the melting point, and rupture resistance at higher temperatures. Performance etc.

In recent years, devices equipped with LIBs such as smartphones and wearable devices have been remarkably miniaturized / thinned, while the capacity of batteries has been required to be further improved. Therefore, LIB makers are proceeding with increasing the potential of batteries and thinning separators. Further, as the separator becomes thinner, the distance between the electrodes becomes shorter, so that the separator is required to have higher withstand voltage characteristics. Further, in order to shorten the distance between the electrodes, a step of pressing the wound body may be performed at the time of manufacturing the battery, and the unevenness of the electrodes may cause the distance between the electrodes to be locally reduced. As a result, if the withstand voltage characteristics vary over the entire range of the separator, there is a high possibility that the part with the low withstand voltage of the separator and the part where the distance between the electrodes is locally small will be combined, resulting in a slight short circuit. easy. That is, even if it has a high withstand voltage characteristic on average, it is inferior as a withstand voltage characteristic as a battery. Therefore, there is a demand for a separator capable of suppressing a slight short circuit of a battery while narrowing the distance between electrodes and increasing the capacity by thinning the layer or pressing the electrodes.

For example, Patent Document 1 describes that in a method for producing a microporous polyolefin membrane, a separator having excellent withstand voltage characteristics can be produced by adjusting the melt mass flow rate (MFR) of the polyolefin solution.

Further, in Patent Document 2, a layer having an average pore diameter of less than 15 nm by a poromometer, a shutdown temperature of 140 ° C. or less, an air permeation resistance of 1000 sec / 100 cm ³ or less, and a polypropylene resin containing 80% by mass or more. It is described that a separator having excellent thermal insulation characteristics can be obtained by using a porous polyolefin film characterized by having at least one layer A having the above. Further, in Patent Document 2, in addition to the average value of withstand voltage performance, the maximum value and the minimum value are also described.

International Publication No. 2019/189522 Japanese Unexamined Patent Publication No. 2020-6979

However, as the thinning of the separator progresses further, a higher withstand voltage per thickness is required. In addition, when the thin film is thinned, it is easily affected by heating unevenness during stretching, and the withstand voltage varies in the plane. It tends to grow. Therefore, the separator as described in Patent Document 1 or 2 is insufficient from the viewpoint of reducing the short circuit rate of the battery, and there is room for improvement.

Therefore, in view of the above circumstances, it is an object of the present invention to provide a polyolefin microporous membrane capable of reducing the short circuit rate of a battery when incorporated into the battery as a separator.

As a result of diligent studies, the present inventors have found that the above problems can be solved by specifying the withstand voltage resistance of the polyolefin microporous membrane and suppressing the in-plane variation in the withstand voltage resistance. Was completed. Examples of embodiments of the present invention are listed below.
[1]
It is a polyolefin microporous film, and the average value of the withstand voltage values at 20 points measured in the region of 5 cm × 10 cm of the polyolefin microporous film is 0.125 kV / μm or more and 1.250 kV / μm or less, and is standard. A microporous polyolefin membrane with a deviation of 0 or more and 0.0060 or less.
[2]
Item 2. The polyolefin microporous film according to Item 1, wherein the polyolefin microporous film has a film thickness of 0.1 μm to 6.5 μm.
[3]
Item 2. The polyolefin microporous film according to Item 1 or 2, wherein the polyolefin microporous film has a basis weight equivalent strength of 80 gf / (g / m ² ) or more.
[4]
Item 2. The polyolefin microporous membrane according to any one of Items 1 to 3, wherein the polyolefin microporous membrane has a melting point (Tm) of 120.0 ° C to 135.0 ° C.
[5]
Item 2. The polyolefin microporous membrane according to any one of Items 1 to 4, wherein the polyolefin microporous membrane has a viscosity average molecular weight (Mv) of 600,000 to 2,000,000.
[6]
Item 2. The polyolefin microporous film according to any one of Items 1 to 5, wherein the average value of the smoothness of both surfaces of the polyolefin microporous film is 5,000 sec / 100 cm ³ to 1,000,000 sec / 100 cm ³ .
[7]
Item 2. The polyolefin microporous film according to any one of Items 1 to 6, wherein the heat shrinkage rate of the polyolefin microporous film at 120 ° C. is 20% or less for both MD and TD.

According to the present invention, it is possible to reduce the short-circuit rate of a battery provided with a microporous polyolefin membrane as a battery separator.

FIG. 1 is a schematic view of a measurement point of a withstand voltage value. FIG. 2 is a schematic diagram of an impact test.

Hereinafter, embodiments for carrying out the present invention (hereinafter, may be abbreviated as “the present embodiment”) will be described in detail, but the present invention is not limited thereto and does not deviate from the gist thereof. Various deformations are possible within the range. In this specification, the flow direction of the film during film formation is defined as MD, and the direction of intersection with MD at 90 ° in the film plane is defined as TD. Further, the inclusion of a specific component as a main component means that the content of the specific component is 50% by mass or more.

<Polyolefin microporous membrane>
The polyolefin microporous membrane (hereinafter, may be abbreviated as "PO microporous membrane") is a membrane containing polyolefin (PO) and having microporous.

<Withstand voltage of PO microporous membrane>
In the PO microporous membrane according to this embodiment, the average value of the withstand voltage values at 20 points measured in the polyolefin microporous membrane of 5 cm × 10 cm is 0.125 kV / μm or more and 1.250 kV / μm or less, and the standard deviation. Is 0 or more and 0.0060 or less.

The average value of the withstand voltage values at 20 points measured in the plane having a size of 5 cm × 10 cm of the PO microporous film is 0.125 kV / μm or more, so that PO can be stored in a battery having a relatively high potential. No short circuit or micro short circuit occurs even when charging / discharging is performed by incorporating a microporous film. Here, in the evaluation test of a micro short circuit, the electrodes come close to each other due to a press or the like, and the unevenness of the electrode material may affect the PO microporous film as a separator. It may occur, and it can be said that the larger the number of occurrences, the larger the voltage drop.

The average value of the withstand voltage values measured under the same conditions as above is preferably 0.130 kV / μm or more, preferably 0.135 kV / μm or more, from the viewpoint of suppressing a slight short circuit in battery charging / discharging. It is more preferably 0.140 kV / μm or more, 0.145 kV / μm or more, or 0.150 kV / μm or more. There is no preferable upper limit of the withstand voltage value, and the higher the value, the better. Is considered to be.

The average value of the withstand voltage values of 20 points measured in a plane having a size of 5 cm × 10 cm of the PO microporous film is, for example, the raw material of the PO microporous film in the manufacturing process of the PO microporous film described later. The above values are obtained by increasing the molecular weight, lowering the melting point of the polyolefin raw material (for example, using a copolymer as the polyolefin raw material), biaxial stretching at a relatively low temperature, and heat fixation (HS) at a relatively high temperature. It can be adjusted within the range.

Further, when the standard deviation of the withstand voltage values of the measured 20 points is within the range of 0 or more and 0.0060 or less in the plane having a dimension of 5 cm × 10 cm of the PO microporous film, the PO micropores are contained in the battery. A short circuit or a slight short circuit is unlikely to occur even when a porous film is incorporated and charging / discharging is performed. The standard deviation of the withstand voltage value measured under the same conditions as above is preferably 0 to 0.0058, more preferably 0 to 0.0055, and further, from the viewpoint of suppressing a slight short circuit in charging and discharging the battery. It is preferably in the range of 0 to 0.0050, more preferably 0 to 0.0047, 0 to 0.0044, 0 to 0.0042, or 0 to 0.0040. Here, since the variation in withstand voltage includes not only the variation in film thickness and permeability but also the variation in pore diameter and draw ratio, the standard deviation of withstand voltage and the standard deviation of film thickness and permeability are the film thickness. In the range of control of transparency and transparency, it is not sufficient as a requirement to reduce the short circuit rate, and it does not always match.

In the plane having a size of 5 cm × 10 cm of the PO microporous film, the standard deviation of the withstand voltage values at 20 points measured is, for example, polypropylene in the PO raw material in the manufacturing process of the PO microporous film described later. Reduction of PP) ratio (for example, PP ratio of 10% by mass or less), blending of high molecular weight polyethylene (PE) in PO raw material, adjustment of kneading conditions and simultaneous biaxial stretching conditions of resin raw material, when extruding raw material By suppressing fluctuations in, single-layer extrusion, simultaneous biaxial stretching, controlling preheating / stretching temperature (for example, balancing preheating temperature and stretching temperature), controlling preheating coefficient (preheating temperature x time x air volume), etc. Can be controlled within the numerical range of.

The method for measuring the withstand voltage resistance of the PO microporous membrane is as described later in Examples with reference to FIG. The size of the PO microporous membrane according to the present embodiment is not limited to 5 cm × 10 cm, and when the PO microporous membrane does not have a size of 5 cm × 10 cm, it is finally measured. The withstand voltage values of 20 points may be measured so that the total area of the area to be measured is 50 cm ² and the measurement points are separated from each other by 15 mm or more. For example, the PO microporous membrane is 2 cm × 25 cm. Withstand voltage values at 20 points can be measured so that the total area is 50 cm ² in terms of dimensions.

If desired, the PO microporous membrane can also be specified with molecular weight, film thickness, average pore size, heat shrinkage, porosity, air permeability, puncture strength, smoothness, melting point and the like as shown below. An inorganic coating layer or an adhesive layer may be formed on the surface thereof. The plurality of properties described in this embodiment can be utilized independently or can be arbitrarily combined. Unless otherwise specified, the method for measuring the physical properties of the PO microporous membrane will be described in detail in the section of Examples.

<Viscosity average molecular weight of PO microporous membrane>
The viscosity average molecular weight (Mv) of the PO microporous membrane is preferably 600,000 or more and 2,000,000 or less. When the Mv of the PO microporous membrane is in the range of 600,000 or more, a large amount of components having a relatively high molecular weight are contained, the pore diameter is small, and the pore diameter tends to be uniform, so that the withstand voltage resistance of the PO microporous membrane becomes high. It tends to improve. From such a viewpoint, the Mv of the PO microporous membrane is more preferably 700,000 or more, still more preferably 800,000 or more. On the other hand, adjusting the Mv of the PO microporous membrane to 2,000,000 or less is preferable from the viewpoint of lowering the fuse temperature and suppressing heat shrinkage, and the upper limit of the Mv of the PO microporous membrane is 1,800,000. The following is more preferable.

In the present specification, the viscosity average molecular weight of the PO microporous membrane is obtained by measuring the Mv of the PO microporous membrane itself. The viscosity average molecular weight of the PO microporous membrane can be adjusted within the above range, for example, by changing the composition ratio of the raw material polymers having different molecular weights.

<Heat shrinkage rate>
Increasing the withstand voltage value of the PO microporous membrane tends to worsen the heat shrinkage of the PO microporous membrane as a trade-off. Further, for example, in order to ensure safety in an oven test of a battery, the heat shrinkage rate of a PO microporous membrane used as a separator at a high temperature (for example, near the melting point of PO or near the melting point of PO microporous membrane). It is considered important to control.

From the viewpoint of balancing the withstand voltage and heat shrinkage of the PO microporous membrane and not deteriorating the safety in the oven test of the battery containing the PO microporous membrane, the temperature of the PO microporous membrane according to the present embodiment is 120 ° C. The heat shrinkage rate in MD and TD is preferably 20% or less, preferably 18% or less in MD and TD, more preferably 15% or less in MD and TD, and 12%. It is more preferably less than or equal to, and particularly preferably 10% or less. The lower limit of the heat shrinkage rate of the PO microporous membrane at 120 ° C. can be, for example, −5% or higher, −2% or higher, -1% or higher, or 0% or higher for both MD and TD.

As a means for controlling the heat shrinkage rate of the PO microporous membrane at 120 ° C. within the above numerical range, for example, in the production process of the PO microporous membrane, the stretching temperature of the stretching step is adjusted and the heat is fixed (HS). TD stretching and / or relaxation temperature adjustment and the like.

<Puncture strength>
The puncture strength when converted to the basis weight (g / m ² ) of the PO microporous membrane (hereinafter referred to as the basis weight equivalent puncture strength) is preferably 80 gf / (g / m ² ) or more. When the puncture strength of the PO microporous membrane is 80 gf / (g / m ² ) or more, for example, in the pressing process of the battery including the PO microporous membrane as a separator and the electrode, the PO microporous membrane may also be formed by the protrusions of the electrode. There is a tendency that holes are less likely to be formed in the pores, and microshort circuits are less likely to occur, and the average withstand voltage value of the PO microporous membrane tends to be improved. From these tendencies, the basis weight equivalent puncture strength of the PO microporous membrane is more preferably 85 gf / (g / m ² ) or more, and further preferably 90 gf / (g / m ² ) or more. The upper limit of the basis weight-equivalent puncture strength can be, for example, 140 gf / (g / m ² ) or less from the viewpoint of suppressing heat shrinkage.

The lower limit of the puncture strength (hereinafter, simply referred to as puncture strength) that is not converted into the basis weight of the PO microporous membrane is preferably 250 gf or more, more preferably 280 gf or more, and further preferably 300 gf or more. A puncture strength of 250 gf or more is preferable from the viewpoint of safety of the battery containing the PO microporous membrane. The upper limit of the puncture strength of the PO microporous membrane is preferably 600 gf or less, more preferably 500 gf or less, or 450 gf or less from the viewpoint of relaxation of orientation during heating of the membrane and the stretching step of the membrane.

As a means for controlling the piercing strength or the piercing strength of the PO microporous membrane within the above numerical range, for example, in the manufacturing process of the PO microporous membrane, Mv adjustment of the PO or PE raw material, and the stretching temperature of the stretching step Adjustment etc. can be mentioned.

<Hole diameter>
The pore size of the PO microporous membrane is preferably 0.050 μm or less. In the present specification, the pore size of the PO microporous membrane means the average pore size measured using a palm pollometer (Porous Materials, Inc.: CFP-1500AE) according to the half-dry method. Since the PO microporous membrane has a pore size of 0.050 μm or less, the average withstand voltage value tends to improve.

The pore size of the PO microporous membrane is more preferably 0.047 μm or less, still more preferably 0.044 μm or less, from the viewpoint of further improving the cycle performance of the battery in addition to the withstand voltage resistance of the PO microporous membrane. The lower limit of the pore size of the PO microporous membrane can exceed 0 μm as long as the microporous is present, and can be, for example, 0.10 μm or more.

As a means for controlling the pore size of the PO microporous film within the above numerical range, for example, in the production process of the PO microporous film, a stretching step is performed before the extraction step; the total stretching ratio of the resin molded product is adjusted. To: Adjusting the HS stretching temperature and the HS relaxation temperature, adjusting the HS stretching temperature <HS relaxation temperature, adjusting the HS stretching ratio, adjusting the HS relaxation ratio during the heat fixing (HS) step. Etc. can be used alone or in combination as appropriate.

<Film thickness>
The upper limit of the film thickness of the PO microporous membrane is preferably 8.0 μm or less, more preferably 7.0 or less, and most preferably, from the viewpoint of improving the capacity of the battery containing the PO microporous membrane as a separator. It is preferably 6.5 μm or less. The lower limit of the film thickness of the PO microporous film is preferably 0.1 μm or more from the viewpoint of developing the strength of the PO microporous film and ensuring the withstand voltage.

As means for controlling the film thickness of the PO microporous membrane within the above numerical range, for example, in the method for producing the PO microporous membrane, the sheet thickness at the time of sheet processing of the melt-kneaded product, the draw ratio in the stretching step, and HS. Control of the stretch ratio or relaxation ratio of the process can be mentioned.

<Air permeability>
The air permeability of the PO microporous membrane is preferably 300 seconds or less, more preferably 290 seconds or less per 100 cm ³ of air, from the viewpoint of ensuring the membrane permeability and maintaining the output characteristics of the battery incorporated as a separator. , More preferably 270 seconds or less. The lower limit of the air permeability is preferably 1 second or longer, more preferably 55 seconds or longer, still more preferably 70 seconds or longer, or 80 per 100 cm ³ of air from the viewpoint of balancing the film thickness, porosity, and pore diameter. More than a second. The air permeability of the PO microporous membrane can be controlled within the above numerical range by adjusting, for example, the HS stretch ratio, the HS relaxation ratio, the HS relaxation temperature, etc. in the production process of the PO microporous membrane. ..

<Porosity>
The porosity of the PO microporous membrane is preferably 25% or more, more preferably 28% or more. A porosity of 25% or more is suitable from the viewpoint of permeability of the PO microporous membrane. The upper limit of the porosity is preferably 60% or less, more preferably 55% or less. A porosity of 60% or less is suitable from the viewpoint of improving the average value of the withstand voltage values at 20 points of the PO microporous membrane described above. The porosity of the PO microporous membrane can be controlled within the above numerical range by the same method as the control of the air permeability.

<Melting point>
The melting point (Tm) of the PO microporous membrane is preferably 120.0 ° C to 138.0 ° C. Although not bound by the theory, the relatively low melting point of 138.0 ° C. or lower of the PO microporous membrane means that the crystals in the PO microporous membrane are small and the proportion of amorphous parts is large. Therefore, it is considered that the withstand voltage resistance of the PO microporous film is improved by increasing the number of interfaces between the crystalline portion and the amorphous portion. In addition, if the melting point of the membrane is too low, the fuse temperature will be low, and the environmental temperature of the battery will be around 100 ° C, and the melting point of the PO microporous membrane will be set to prevent the membrane structure from changing when exposed to high temperatures. 120.0 ° C. or higher is preferable. Further, the temperature is preferably 140.0 ° C. or lower from the viewpoint of quickly blocking the ion permeability when the insulation is destroyed due to the growth of dendrites and the battery temperature rises.

The upper limit of Tm of the PO microporous membrane is preferably 138.0 ° C. or lower, more preferably 137. It is 0 ° C. or lower, more preferably 135.0 ° C. or lower, and most preferably 134.0 ° C. or lower. The lower limit of Tm of the PO microporous membrane is preferably 120.0 ° C. or higher, more preferably 125.0 ° C. or higher, from the viewpoint of the strength development of the PO microporous membrane and the balance between Tm and the heat shrinkage rate. , More preferably 128.0 ° C. or higher.

The Tm of the PO microporous membrane can be controlled within the above numerical range by, for example, selecting the melting point of the PO raw material or the PE raw material used for producing the PO microporous membrane.

<Smoothness>
The average value of the smoothness on both sides of the PO microporous membrane is 5,000 sec / 100 cm ³ from the viewpoint of reducing the unevenness of the PO microporous membrane and suppressing the variation in the withstand voltage in the surface of the PO microporous membrane. The above is preferable, more preferably 8,000 sec / 100 cm ³ or more, further preferably 15,000 sec / 100 cm ³ or more, and particularly preferably 30,000 sec / 100 cm ³ or more. The upper limit is not particularly limited, but is generally 1,000,000 sec / 100 cm ³ or less due to the presence of minute irregularities derived from the fibril of the PO microporous membrane.

As a means for controlling the average value of the smoothness of both sides of the PO microporous film within the above numerical range, for example, in the manufacturing process of the PO microporous film, the ratio of PP in the PO raw material is suppressed (for example, 10% by mass). (PP ratio below, etc.), high molecular weight PE blend in PO raw material, adjustment of resin kneading conditions, suppression of fluctuation in extrusion process or single-layer extrusion, molded body in wet state (when called "raw material") (There is also), adjustment of cooling conditions, simultaneous biaxial stretching, control of stretching temperature in the stretching step (for example, balancing the preheating temperature and stretching temperature), etc. can be used alone or in combination as appropriate.

<Ingredients of PO microporous membrane>
The PO microporous membrane according to this embodiment is formed from a resin composition containing a polyolefin resin. If desired, the resin composition may further contain inorganic particles, a resin other than the polyolefin, and the like. The total content (PC) of all the resins contained in the PO resin composition is preferably 34% by mass or less, preferably 32% by mass or less, from the viewpoint of the strength and film thickness of the PO microporous film. More preferably, it is more preferably 30% by mass or less.

The polyolefin resin used in the present embodiment is not particularly limited, and for example, monomers such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene are polymerized. Examples thereof include the obtained polymers (for example, homopolymers, copolymers, multistage polymers, etc.). These polymers may be used alone or in combination of two or more. From the viewpoint of lowering the melting point of the PO raw material and improving the withstand voltage average value of the obtained PO microporous membrane, it is preferable to use a copolymer.

The total ratio of the PE raw materials in the PO raw materials is preferably 50% by mass to 100% by mass, more preferably 80% by mass to 100% by mass, from the viewpoint of exhibiting fusability.

The amount of the polyolefin resin contained in the PO microporous film is preferably 60% by mass or more, preferably 70% by mass or more, and preferably 80% by mass or more, based on the mass of the PO microporous film. It is preferably 90% by mass or more, or 100% by mass.

The PO microporous membrane preferably contains 55% by mass or more, preferably 65% by mass or more, of a polyethylene (PE) raw material having an Mv of 700,000 or more and 2,000,000 or less based on the mass of the PO microporous membrane. It is more preferable, and it is further preferable to contain 75% by mass or more. When a PE raw material having an Mv of 700,000 or more and 2 million or less is contained in the PO microporous membrane in an amount of 55% by mass or more, the crystals are highly oriented during stretching, whereby the PO microporous membrane tends to have high strength and low heat shrinkage. There is.

Further, from the viewpoint of suppressing the variation in withstand voltage in the plane of the PO microporous membrane, the PO raw material used for producing the PO microporous membrane is a blend of PE and PP, or PE and Mv30 having an Mv of 500,000 or more. It is preferably a blend of PE of 10,000 or less.

Examples of the polyolefin resin include low-density polyethylene (density 0.910 g / cm ³ or more and less than 0.930 g / cm ³ ) and linear low-density polyethylene (density 0.910 g / cm ³ or more and 0.940 g / cm ³ ). Less than), medium density polyethylene (density 0.930 g / cm ³ or more and less than 0.942 g / cm ³ ), high density polyethylene (density 0.942 g / cm ³ or more), ultra high molecular weight polyethylene (density 0.910 g / cm ³ ) (More than 0.970 g / cm less than ³ ), isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber and the like can be mentioned. These can be used alone or in combination of two or more. Above all, it is preferable to use polyethylene alone, polypropylene alone, or a mixture of polyethylene and polypropylene alone from the viewpoint of obtaining a uniform film.

The polyolefin resin is medium-density polyethylene (PE) having a density of 0.930 g / cm ³ or more and less than 0.942 g / cm ³ from the viewpoint of safety of a battery having a PO microporous film as a separator. Is preferable, and for example, PE other than high-density polyethylene (HDPE) may be used. Furthermore, from the viewpoint that the PO microporous film improves the safety of the battery even if it is a thin film, medium density polyethylene with Mv of less than 1 million, and Mv of 1 million or more and 2 million or less and a density of 0.930 g / cm ³ or more and 0.942 g / At least one selected from ultra-high molecular weight polyethylene of less than cm ³ is preferably contained in an amount of 50% by mass or more, more preferably 60% by mass or more, and more preferably 70% by mass or more based on the mass of the PO microporous film. Is most preferable.

Polyolefin resins not only ensure safety in high temperature regions (160 ° C or higher) where it is difficult to ensure safety with PE, but also improve the average in-plane withstand voltage resistance of PO microporous membranes. It preferably contains polypropylene (PP). As PP, a homopolymer of propylene is preferable. Further, from the viewpoint of suppressing the variation in withstand voltage in the plane, it is more preferable that the polyolefin resin contains polyethylene as a main component and polypropylene. Therefore, when PP is contained as the PO raw material, the ratio of the PP raw material in the PO raw material is preferably more than 0% by mass and 10% by mass or less.

When PE and PP are used in combination, as polyethylene, medium-density polyethylene with a viscosity average molecular weight of less than 1 million and a viscosity average molecular weight of 1 million or more and 2 million or less and a density of 0.930 g / cm ³ or more and 0.942 g / cm ³ It is preferable to use at least one selected from less than ultra-high molecular weight polyethylene from the viewpoint of balancing strength and permeability and maintaining an appropriate fuse temperature. Further, it is more preferable not to contain ultra-high molecular weight polyethylene having a viscosity average molecular weight of 2 million or more from the viewpoint of maintaining an appropriate fuse temperature.

The viscosity average molecular weight of the polyolefin resin (measured according to the measurement method in the examples described later) is preferably 50,000 or more, more preferably 100,000 or more, and the upper limit thereof is preferably 3 million or less. , More preferably 2 million or less or 1 million or less. Setting the viscosity average molecular weight (Mv) to 50,000 or more is from the viewpoint of maintaining a high melt tension during melt molding to ensure good moldability, or by imparting sufficient entanglement to the resin to provide a microporous film. It is preferable from the viewpoint of increasing the strength of the. On the other hand, setting the Mv to 3 million or less is preferable from the viewpoint of realizing uniform melt-kneading and improving the formability of the sheet, particularly the thickness formability.

From the viewpoint of improving the withstand voltage average value of the obtained PO microporous membrane, the melting point (Tm) of the PO resin used as the PO raw material is preferably 165 ° C. or lower, preferably 150 ° C. or lower. More preferably, it is 138 ° C. or lower. From the same viewpoint, the Tm of PE used as a raw material is preferably 137.5 ° C or lower, more preferably 136.5 ° C or lower, and the Tm of PP used as a raw material is 163. It is preferably 0.0 ° C. or lower. The content of the raw material having a Tm of 150 ° C. or higher is preferably 10% by mass or less with respect to the total weight of the PO raw materials.

If necessary, the resin composition may contain inorganic particles, phenol-based or phosphorus-based or sulfur-based antioxidants; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorbers, light stabilizers, and charges. Various known additives such as an inhibitor, an antifogging agent, and a coloring pigment may be mixed.

[Method for manufacturing microporous polyolefin membrane]
The method for producing the PO microporous membrane according to the present embodiment is not particularly limited, but for example,
A mixing step (a) of mixing a resin composition containing a polyolefin resin and, if desired, various additives, and
The extrusion step (b) in which the mixture obtained in the step (a) is melt-kneaded and extruded, and
The sheet molding step (c) of molding the extruded product obtained in the step (b) into a sheet shape, and
The primary stretching step (d) of stretching the sheet-shaped molded product obtained in the step (c) at least once in the uniaxial direction,
The extraction step (e) of extracting the pore-forming material from the primary stretched film obtained in the step (d), and the extraction step (e).
Examples thereof include a method including a heat fixing step (f) in which the extraction film obtained in the step (e) is heat-fixed (HS) at a predetermined temperature.

According to the above method for producing a PO microporous film, the PO microporous rate can be reduced when used as a separator for a lithium ion secondary battery and other electrochemical devices. A membrane can be provided. The method for producing the PO microporous membrane of the present embodiment is not limited to the above-mentioned production method, and various modifications can be made without departing from the gist thereof. Each process will be described below.

[Mixing step (a)]
The mixing step (a) is a step of mixing the resin composition containing the polyolefin resin and, if desired, various additives. In the mixing step (a), other components may be mixed with the resin composition, if necessary. The resin composition described in the above item <components contained in the PO microporous membrane> can be subjected to the mixing step (a). Above all, from the viewpoint of improving the withstand voltage average value of the obtained PO microporous film and controlling the melting point (Tm) or viscosity average molecular weight (Mv) of the obtained PO microporous film, Mv is 50,000. It is preferable to use a PO raw material of 3 million or less, or a resin composition containing a PO copolymer having a Tm of 165 ° C or less. Further, from the viewpoint of controlling the standard deviation of the withstand voltage value of the obtained PO microporous film and controlling the puncture strength of the PO microporous film, PE and PP of 10% by mass or less are used as PO raw materials. , Or a resin composition containing a blend of PE of Mv 500,000 or more and PE of Mv 300,000 or less is preferable.

The pore-forming material may be arbitrary as long as it is distinguished from the material of the PO resin and the inorganic particles, and may be, for example, a plasticizer. As the plasticizer, a non-volatile solvent capable of forming a uniform solution above the melting point of the PO resin, for example, hydrocarbons such as liquid paraffin (LP) and paraffin wax; esters such as dioctyl phthalate and dibutyl phthalate; oleyl Higher alcohols such as alcohol and stearyl alcohol may be used.

The content of the plasticizer in the resin composition is preferably 66% by mass to 90% by mass, more preferably 68% by mass to 88% by mass, and further preferably 70% by mass to 80% by mass. By adjusting the content of the plasticizer to 66 or more by mass, the melt viscosity of the resin composition is lowered and the melt fracture is suppressed, so that the film-forming property at the time of extrusion tends to be improved. On the other hand, by adjusting the content of the plasticizer to 90% by mass or less, it may be possible to suppress the elongation of the original fabric during the film forming process.

(Any additive)
In the step (a), the resin composition containing PO may contain any additive. The additive is not particularly limited, and is, for example, a polymer other than a polyolefin resin; an antioxidant such as a phenol-based compound, a phosphorus-based compound, and a sulfur-based compound; metal soaps such as calcium stearate and zinc stearate; ultraviolet absorption. Agents; light stabilizers; antistatic agents; antifogging agents; coloring pigments and the like. The total amount of these additives added is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and further preferably 5 parts by mass or less with respect to 100 parts by mass of the polyolefin resin.

The mixing method in the step (a) is not particularly limited, and examples thereof include a method of premixing a part or all of the raw materials using a Henschel mixer, a ribbon blender, a tumbler blender, or the like, if necessary. Among them, a method of mixing using a Henschel mixer is preferable.

[Extrusion step (b)]
The extrusion step (b) is a step of melt-kneading and extruding the resin composition obtained in the step (a). In the extrusion step (b), other components may be mixed with the resin composition, if necessary.

The method of melt-kneading in the step (b) is not particularly limited, but for example, all the raw materials including the mixture mixed in the step (a) are used in a screw extruder such as a single-screw extruder or a twin-screw extruder; a kneader; Examples thereof include a method of melting and kneading with a mixer or the like. Among them, melt kneading is preferably performed by using a screw with a twin-screw extruder. The melt-kneading conditions are appropriately set from the viewpoint of controlling the standard deviation of the withstand voltage value in a specific area region of the obtained PO microporous membrane or the average value of the smoothness of both sides of the PO microporous membrane. Is preferable. Further, when performing melt-kneading, it is preferable to add the plasticizer in two or more times, and further, when the additive is added in a plurality of times, the first addition amount is the total addition amount. It is preferable to adjust the content to 90% by weight or less from the viewpoint of suppressing aggregation of the contained components and uniformly dispersing them. This is preferable from the viewpoint of suppressing heat generation by shutting down in a large area and improving the safety of the battery.

When the pore forming agent is used in the step (b), the temperature of the melt-kneaded portion is preferably less than 200 ° C. from the viewpoint of uniformly kneading the resin composition. The lower limit of the temperature of the melt-kneaded portion is equal to or higher than the melting point of PO from the viewpoint of uniformly dissolving the PO resin in the plasticizer.

In the present embodiment, there is no particular limitation during kneading, but after mixing the raw material PO with an antioxidant at a predetermined concentration, the surroundings of the mixture are replaced with a nitrogen atmosphere, and the mixture is melted in a state where the nitrogen atmosphere is maintained. It is preferable to knead.

In the step (b), the kneaded product obtained through the above kneading is extruded by an extruder such as a T-type die or an annular die. At this time, single-layer extrusion or laminated extrusion can be performed, and from the viewpoint of controlling the standard deviation of the withstand voltage value of the obtained PO microporous membrane or the average value of the smoothness of both sides, single-layer extrusion is used. preferable. The conditions for extrusion can be varied as much as possible, for example, by adopting known methods and from the viewpoint of controlling the standard deviation of the withstand voltage value of the obtained PO microporous membrane or the average value of the smoothness of both sides. It is preferable to set the conditions in which the above is suppressed. Further, it is preferable to control the (die) lip clearance and the like from the viewpoint of the film thickness of the obtained PO microporous membrane.

[Sheet molding step (c)]
The sheet forming step (c) is a step of forming the extruded product obtained in the extrusion step (b) into a sheet shape. The sheet-shaped molded product obtained by the sheet molding step (c) may be a single layer or may be laminated. The sheet forming method is not particularly limited, and examples thereof include a method of solidifying the extruded product by compression cooling.

The compression cooling method is not particularly limited, and examples thereof include a method of bringing the extruder into direct contact with a cooling medium such as cold air or cooling water; and a method of bringing the extruder into contact with a metal roll cooled with a refrigerant, a press machine, or the like. Be done. Among these, a method in which the extruded product is brought into contact with a metal roll cooled with a refrigerant, a press machine, or the like is preferable because the film thickness can be easily controlled.

After the melt kneading in the step (b), the set temperature in the step of forming the melt into a sheet is preferably set to a temperature higher than the set temperature of the extruder. The upper limit of the set temperature is preferably 300 ° C. or lower, more preferably 260 ° C. or lower, from the viewpoint of thermal deterioration of the polyolefin resin. For example, when continuously producing a sheet-shaped molded product from an extruder, the process of forming into a sheet after the melt-kneading step, that is, the path from the extruder outlet to the T-die and the set temperature of the T-die are extruded. When the temperature is set higher than the set temperature of the process, the melt can be formed into a sheet without separating the resin composition and the pore-forming material, which is preferable. Further, it is preferable to control the cast clearance and the like from the viewpoint of the film thickness of the obtained PO microporous membrane.

The thickness of the obtained sheet-shaped molded product is, for example, 600 μm or more and 2000 μm or less depending on the thickness after stretching in the step (d) or (f), the film thickness of the finally produced PO microporous film, and the like. It is preferably present, and more preferably 1000 μm to 1500 μm.

[Primary stretching step (d)]
The primary stretching step (d) is a step of stretching the sheet-shaped molded product obtained in the sheet molding step (c) at least once and at least in the uniaxial direction. This stretching step (stretching step performed before the next extraction step (e)) is referred to as "primary stretching", and the film obtained by the primary stretching is referred to as "primary stretching film". In the primary stretching, the sheet-shaped molded product can be stretched in at least one direction, and may be performed in both MD and TD directions, or may be performed in only one of MD or TD.

From the viewpoint of controlling the standard deviation of the withstand voltage value in a specific area region of the obtained PO microporous film and the average value of the smoothness of both sides, the sheet-shaped molded product is prior to the primary stretching of the sheet-shaped molded product. It is preferable to perform the preheating of the above, and it is more preferable that the preheating temperature is equal to or higher than the primary stretching temperature. Specifically, the preheating temperature may be in the range of 116 ° C. to 121 ° C. as long as it is equal to or higher than the primary stretching temperature. From the same viewpoint, when preheating is performed, the preheating coefficient is preferably 100,000 ° C.m or higher, more preferably 128,000 ° C.m or higher, and 128,000 ° C.m to m. It is more preferably 200,000 ° C.m.

The stretching method of the primary stretching is not particularly limited, but for example, uniaxial stretching by a roll stretching machine; TD uniaxial stretching by a tenter; sequential biaxial stretching by a roll stretching machine and a tenter, or a combination of a plurality of tenters; simultaneous biaxial tenter. Alternatively, simultaneous biaxial stretching by inflation molding and the like can be mentioned. Above all, simultaneous biaxial stretching is preferable from the viewpoint of controlling the standard deviation of the withstand voltage value in a specific area of the obtained PO microporous film and the average value of the smoothness of both sides.

The draw ratio of MD and / or TD of the primary stretching is preferably 5 times or more, and more preferably 6 times or more. When the draw ratio of the MD and / or TD of the primary stretching is 5 times or more, the film thickness of the obtained PO microporous film can be controlled and the strength tends to be further improved. The draw ratio of MD and / or TD for primary stretching is preferably 9 times or less, and more preferably 8 times or less. When the draw ratio of MD and / or TD of the primary stretch is 9 times or less, the breakage during stretching tends to be further suppressed. When biaxial stretching is performed, sequential stretching or simultaneous biaxial stretching may be performed, but the stretching ratio in each axial direction is preferably 5 times or more and 9 times or less, and more preferably 6 times or more and 8 times or less. , Or 6 times or more and 7 times or less. Further, from the viewpoint of suppressing the variation in the withstand voltage value in the plane, the MD / TD stretching ratio is equal to 1.5 to 0.7 as compared with the highly anisotropic manufacturing condition in which stretching is excessive in one direction. Anisotropy stretching conditions are preferred.

The primary stretching temperature can be selected with reference to the raw material resin composition and concentration contained in the PO resin composition. The lower the primary stretching temperature, the more preferable, from the viewpoint of controlling the average value of the withstand voltage values in a specific area of the obtained PO microporous membrane. The stretching temperature of MD and / or TD is preferably in the range of 100 ° C. to 135 ° C. from the viewpoint of withstand voltage resistance, puncture strength, and heat shrinkage rate at 120 ° C. of the obtained PO microporous membrane. More preferably 112 ° C. to 135 ° C., even more preferably 115 ° C. to 130 ° C., even more preferably 116 ° C. to 122 ° C., 118 ° C. to 122 ° C., 119 ° C. to 124 ° C. or lower, or 116 ° C. to 118 ° C. Within. The stretching temperature for both MD and TD is preferably 100 ° C. or higher, more preferably 115 ° C. or higher, and 135 ° C. or lower from the viewpoint of increasing the film strength, from the viewpoint of fracture suppression and heat shrinkage. Is preferable.

[Extraction step (e)]
The extraction step (e) is a step of extracting a pore-forming material from the primary stretching film obtained in the primary stretching step (d) to obtain an extraction film. Examples of the method for removing the pore-forming material include a method in which a primary stretching film is immersed in an extraction solvent to extract the pore-forming material and sufficiently dried. The method for extracting the pore-forming material may be either a batch method or a continuous method. Further, the residual amount of the pore-forming material, particularly the plasticizer, in the porous membrane is preferably less than 1% by mass.

As the extraction solvent used when extracting the pore-forming material, a solvent that is poor with respect to the polyolefin resin, is a good solvent with respect to the pore-forming material or the plasticizer, and has a boiling point lower than the melting point of the polyolefin resin is used. Is preferable. Such an extraction solvent is not particularly limited, but for example, hydrocarbons such as n-hexane and cyclohexane; halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; hydrofluoroether and hydrofluorocarbon. Non-chlorine hydrocarbon solvents such as; alcohols such as ethanol and isopropanol; ethers such as diethyl ether and tetrahydrofuran; ketones such as acetone and methyl ethyl ketone can be mentioned. These extraction solvents may be recovered and reused by an operation such as distillation.

[Heat fixing step (f)]
The heat fixing step (f) is a step of heat fixing the extraction film obtained in the extraction step (e) at a predetermined temperature. The heat treatment method at this time is not particularly limited, and examples thereof include a heat fixing method in which stretching and relaxation operations are performed using a tenter or a roll stretching machine.

The stretching operation in the heat fixing step (f) is an operation of stretching the PO microporous membrane in at least one direction of MD and TD, and may be performed in both directions of MD and TD, or one of MD and TD. You may just go. From the viewpoint of providing a PO microporous film capable of reducing the short-circuit rate or the micro-short-circuit rate of the battery including the separator and the electrode, it is preferable to heat-fix to at least the TD.

The draw ratios of MD and / or TD in the heat fixing step (f) are preferably 1.50 times or more and 2.30 times or less, and more preferably 1.70 times or more and 2.20 times or less, respectively. The draw ratio of MD and / or TD in the step (f) is preferably 1.50 times or more from the viewpoint of suppressing variation in the obtained withstand voltage value or from the viewpoint of controlling the film thickness thinly, and from the viewpoint of suppressing fracture, 2 .30 times or less is preferable.

The MD and / or TD stretching temperature at the time of heat fixation is preferably higher, from the viewpoint of controlling the average value of the withstand voltage values in a specific area region of the obtained PO microporous film, and the withstand voltage of the PO microporous film. It is preferably 128 ° C. or higher, more preferably 129 ° C. or higher from the viewpoint of battery output characteristics, for controlling the average value and the heat shrinkage rate at 120 ° C., and the safety of the battery. The temperature is particularly preferably 130 ° C. or higher, and most preferably 132 ° C. or higher from the viewpoint of safety in the oven test.

The relaxation operation in the heat fixing step (f) is an operation of reducing the PO microporous membrane in at least one direction of MD and TD, and may be performed in both directions of MD and TD, or MD or TD. You may go to only one of them. The relaxation ratio in the heat fixing step (f) is preferably 0.95 times or less, more preferably 0.90 times or less, and further preferably 0.85 times or less. When the relaxation ratio in the step (f) is 0.95 or less, the film thickness of the obtained PO microporous film tends to be easily controlled and heat shrinkage tends to be suppressed. Further, the relaxation ratio is preferably 0.70 times or more, more preferably 0.75 times or more, from the viewpoint of increasing the relaxation temperature. Here, the "relaxation ratio" is a value obtained by dividing the size of the film after the relaxation operation by the size of the film before the relaxation operation, and when both MD and TD are relaxed, the relaxation ratio and TD of MD are used. It is the value multiplied by the relaxation factor of.
Relaxation magnification = (dimension of membrane after relaxation operation (m)) / (dimension of membrane before relaxation operation (m))

From the viewpoint of the pore size of the obtained PO microporous membrane, the relaxation temperature in this relaxation operation is preferably equal to or higher than the stretching temperature in the heat fixing step (f). Further, when the temperature in the relaxation operation is within the above range, not only the residual stress due to the drawing step can be removed, but also the orientation of the molecular chain can be firmly fixed, so that the PO microporous membrane can be used. It is preferable from the viewpoint of preventing a decrease in ion permeability near the melting point and improving the performance of a battery provided with a PO microporous membrane as a separator.

As the specific relaxation temperature at the time of heat fixation, the average value of the withstand voltage values in a specific area region of the PO microporous membrane is controlled, or the heat shrinkage rate of the PO microporous membrane at 120 ° C. is controlled. From the viewpoint of the above, 128 ° C. or higher is preferable, 131 ° C. or higher is more preferable from the viewpoint of the cycle characteristics of the battery provided with the PO microporous membrane as a separator, and 133 ° C. or higher is further preferable from the viewpoint of battery safety. The upper limit of the relaxation temperature is preferably 140 ° C. or lower, more preferably 135 ° C. or lower, from the viewpoint of the permeability of the PO microporous membrane.

The order of the steps (a) to (f) can be arbitrarily changed as long as the effects of the present invention are not impaired. After the above steps (a) to (f), the total draw ratio of the PO microporous membrane is preferably in the range of 50 times or more and 100 times or less, more preferably 60 times or more and 90 times or less, and further preferably. Is in the range of 70 times or more and 80 times or less. Here, the "total draw ratio" is a value obtained by multiplying the draw ratio of MD and / or TD in the primary draw step (d) by the draw ratio and / or the relaxation ratio in the heat fixing step.

[Other processes]
The method for producing a PO microporous membrane of the present embodiment can include steps other than the above steps (a) to (f). The other steps are not particularly limited, but for example, in addition to the above heat fixing step, as a step for obtaining a PO microporous film which is a laminated body, a plurality of PO microporous films which are a single layer are laminated. A laminating process can be mentioned. Further, in the method for producing a PO microporous membrane of the present embodiment, the surface of the PO microporous membrane is subjected to surface treatment such as electron beam irradiation, plasma irradiation, application of a surfactant, and chemical modification. It may include a step. Further, the material of the inorganic particles may be applied to one side or both sides of the PO microporous membrane to obtain a PO microporous membrane provided with an inorganic material layer.

<Formation of inorganic coating layer>
From the viewpoint of safety, dimensional stability, heat resistance, etc., an inorganic coating layer can be provided on the surface of the PO microporous membrane. The inorganic coating layer is a layer containing an inorganic component such as inorganic particles, and may optionally contain a binder resin that binds the inorganic particles to each other, a dispersant that disperses the inorganic particles in a solvent, and the like.

Materials for the inorganic particles contained in the inorganic coating layer include, for example, oxide-based ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, itria, zinc oxide, and iron oxide; silicon nitride, titanium nitride, and iron oxide. Nitride-based ceramics such as boron nitride; silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, barium sulfate, aluminum hydroxide, aluminum hydroxide oxide, potassium titanate, talc, kaolinite, dekite, nacrite, halosite, pyrophyllium. Ceramics such as light, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, kaolin, and silica sand; and glass fiber. The inorganic particles may be used alone or in combination of two or more.
Examples of the binder resin include conjugated diene-based polymers, acrylic-based polymers, polyvinyl alcohol-based resins, and fluororesins. Further, the binder resin can be in the form of latex and can contain water or an aqueous solvent.
The dispersant is one that adsorbs to the surface of the inorganic particles in the slurry and stabilizes the inorganic particles by electrostatic repulsion or the like, and is, for example, a polycarboxylate, a sulfonate, a polyoxyether, a surfactant, or the like. ..

The inorganic coating layer can be formed, for example, by applying and drying the slurry of the contained components described above on the surface of the PO microporous film.

<Formation of adhesive layer>
An adhesive layer containing a thermoplastic resin is provided on the surface of the PO microporous film in order to prevent deformation or swelling due to gas generation of the laminated battery, which has been increasingly used for in-vehicle batteries in recent years in order to increase the energy density. be able to. The thermoplastic resin contained in the adhesive layer is not particularly limited, and for example, a polyolefin such as polyethylene or polypropylene; a fluororesin such as polyvinylidene fluoride or polytetrafluoroethylene; a vinylidene fluoride-hexafluoropropylene copolymer, and the like. Fluoro-containing rubber such as vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene tetrafluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer; styrene-butadiene copolymer and its hydride, Acrylonitrile-butadiene copolymer and its hydride, acrylonitrile-butadiene-styrene copolymer and its hydride, (meth) acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer weight Combined rubbers such as ethylene propylene rubber, polyvinyl alcohol and polyvinyl acetate; cellulose derivatives such as ethyl cellulose, methyl cellulose, hydroxyethyl cellulose and carboxymethyl cellulose; polyphenylene ether, polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide, polyamideimide, Examples thereof include resins having a melting point and / or a glass transition temperature of 180 ° C. or higher such as polyamide and polyester.

Further, after the heat fixing step (f), the laminating step, or the surface treatment step, the master roll wound with the PO microporous film is subjected to an aging treatment under predetermined temperature conditions, and then the master roll is subjected to an aging treatment. You can also perform a rewind operation. This tends to make it easier to obtain a PO microporous film having higher thermal stability than the PO microporous film before rewinding. In the above case, the temperature at which the master roll is aged is not particularly limited, but is preferably 35 ° C. or higher, more preferably 45 ° C. or higher, and further preferably 60 ° C. or higher. Further, from the viewpoint of maintaining the permeability of the PO microporous membrane, the temperature at the time of aging the master roll is preferably 120 ° C. or lower. The time required for the aging treatment is not particularly limited, but is preferably 24 hours or more because the above effects are likely to be exhibited.

<Separator for electrochemical devices>
The microporous polyolefin film according to this embodiment can be used as a separator for an electrochemical device such as a lithium ion secondary battery. By incorporating the polyolefin microporous film into the lithium ion secondary battery, thermal runaway of the lithium ion secondary battery can be suppressed.

<Electrochemical device>
An electrochemical device in which a wound body or a laminated body formed by winding or stacking a plurality of PO microporous membranes according to the present embodiment is also an aspect of the present invention. Examples of the electrochemical device include a non-aqueous electrolyte battery, a non-aqueous electrolyte battery, a non-aqueous lithium ion secondary battery, a non-aqueous gel secondary battery, a non-aqueous solid secondary battery, a lithium ion capacitor, an electric double layer capacitor, and the like. Can be mentioned.

The non-aqueous electrolyte battery according to the present embodiment includes the separator for a non-aqueous electrolyte battery including the above-mentioned PO microporous film, a positive electrode plate, a negative electrode plate, and a non-aqueous electrolyte solution (a non-aqueous solvent and a metal dissolved therein). Contains salt.). Specifically, for example, a positive electrode plate containing a transition metal oxide capable of occluding and releasing lithium ions and the like and a negative electrode plate capable of occluding and releasing lithium ions and the like are interposed via a separator for a non-aqueous electrolyte battery. It is wound or laminated so as to face each other, holds a non-aqueous electrolytic solution, and is housed in a container.

The positive electrode plate will be described below. As the positive electrode active material, for example, a lithium composite metal oxide such as lithium nickelate, lithium manganate or lithium cobalt oxide, a lithium composite metal phosphate such as lithium iron phosphate, or the like can be used. The positive electrode active material is kneaded with a conductive agent and a binder, applied as a positive electrode paste to a positive electrode current collector such as aluminum foil, dried, rolled to a predetermined thickness, and then cut to a predetermined size to form a positive electrode plate. Here, as the conductive agent, a metal powder stable under a positive electrode potential, for example, carbon black such as acetylene black or a graphite material can be used. Further, as the binder, a material stable under a positive electrode potential, for example, polyvinylidene fluoride, modified acrylic rubber, polytetrafluoroethylene, or the like can be used.

The negative electrode plate will be described below. As the negative electrode active material, a material capable of occluding lithium can be used. Specifically, for example, at least one selected from the group consisting of graphite, silicide, titanium alloy materials and the like can be used. Further, as the negative electrode active material of the non-aqueous electrolyte secondary battery, for example, a metal, a metal fiber, a carbon material, an oxide, a nitride, a silicon compound, a tin compound, various alloy materials and the like can be used. In particular, a simple substance of silicon (Si) or tin (Sn), an alloy, a compound, a silicon compound such as a solid solution, or a tin compound is preferable because the capacity density of the battery tends to increase.

Examples of the carbon material include various natural graphite, coke, developing carbon, carbon fiber, spherical carbon, various artificial graphite, amorphous carbon and the like.

As the negative electrode active material, one of the above materials may be used alone, or two or more of them may be used in combination. The negative electrode active material is kneaded with a binder, applied as a negative electrode paste to a negative electrode current collector such as copper foil, dried, rolled to a predetermined thickness, and then cut to a predetermined size to form a negative electrode plate. Here, as the binder, a material stable under the negative electrode potential, for example, PVDF or a styrene-butadiene rubber copolymer or the like can be used.

The non-aqueous electrolyte solution will be described below. The non-aqueous electrolyte solution generally contains a non-aqueous solvent and a metal salt such as a lithium salt, a sodium salt, and a calcium salt dissolved therein. As the non-aqueous solvent, a cyclic carbonate ester, a chain carbonate ester, a cyclic carboxylic acid ester and the like are used. Examples of the lithium salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF ₆ , and lower aliphatic carboxylics. Examples thereof include lithium acid, LiCl, LiBr, LiI, borates, imide salts and the like.

Unless otherwise specified, the above-mentioned measurement methods for various parameters are measured according to the measurement methods in the examples described later.

Next, the present embodiment will be described more specifically with reference to Examples and Comparative Examples, but the present embodiment is not limited to the following Examples as long as the gist of the present embodiment is not exceeded. The physical properties in the examples were measured by the following methods. Unless otherwise specified, measurements were taken in an environment of room temperature of 23 ° C. and humidity of 40%.

<Film thickness (μm)>
The film thickness of the PO microporous membrane or the porous layer was measured at room temperature of 23 ± 2 ° C. using a microthickening instrument manufactured by Toyo Seiki Co., Ltd., KBM ™.

<Preheating coefficient (° C ・ m)>
As used herein, "preheating" refers to a step in the stretching step of a molded product (raw fabric) in a wet state from the time when the raw fabric enters the stretching machine furnace to the start of stretching. Therefore, the preheating coefficient (° C.m) can be obtained from the following equation: Preheating coefficient (° C.m) = Preheating temperature (° C) x Preheating time (sec) x Wind speed (m / sec)
Here, the preheating temperature refers to the temperature inside the stretching machine furnace in the preheating step, the preheating time refers to the time from entering the stretching machine furnace to the start of stretching, and the wind speed is the preheating. It refers to the wind speed at the nozzle outlet during the process, and was measured using a thermal anemometer (Anemomaster anemometer MODEL6162 manufactured by Nippon Kanomax Co., Ltd.).

<Porosity (%)>
A 10 cm × 10 cm square sample was cut from a microporous membrane, its volume (cm ³ ) and mass (g) were determined, and the pore ratio was calculated from them and their density (g / cm ³ ) using the following equation.
Porosity (%) = (volume-mass / density) / volume x 100

<Air permeability (sec / 100cm ³ )>
The air permeability resistance according to JIS P-8117 was defined as the air permeability. The air permeability of the multilayer porous membrane and the air permeability of the PO microporous membrane are measured in accordance with JIS P-8117, and a Garley type air permeability meter manufactured by Toyo Seiki Co., Ltd., GB2 (trademark) is used. The air permeability of the PO microporous membrane was measured in an atmosphere of a temperature of 23 ° C. and a humidity of 40%, and the air permeability was determined.

<Smoothness (sec / 100 cm ³ )>
In accordance with ISO 8791-5: 2020, the smoothness of the polyolefin microporous membrane is measured in an atmosphere with a temperature of 23 ° C and a humidity of 40% using an air permeability smoothness meter EYO-5 manufactured by Asahi Seiko Co., Ltd. did.

<Puncture strength (gf) and basis weight conversion puncture strength (gf / (g / m ² ))>
A microporous membrane was fixed with a sample holder having an opening diameter of 11.3 mm using a handy compression tester KES-G5 ™ manufactured by Kato Tech. Next, a puncture test was performed on the central portion of the fixed microporous membrane in an atmosphere of a needle tip with a radius of curvature of 0.5 mm, a puncture speed of 2 mm / sec, a temperature of 23 ° C., and a humidity of 40%. The raw piercing strength (gf) was obtained as the maximum piercing load, and the value converted into the basis weight (gf / (g / m ² )) was also calculated.

<Viscosity average molecular weight (Mv)>
Based on ASTM-D4020, the ultimate viscosity [η] (dl / g) at 135 ° C. in a decalin solvent was determined.
For polyethylene or PO microporous membrane, the viscosity average molecular weight (Mv) was calculated by the following formula.
[Η] = 6.77 × 10 ^-4 Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ^-4 Mv ^0.80

<Melting point Tm (° C)>
The melting points of the polyolefin microporous film and the polyolefin raw material were determined using a differential scanning calorimetry (DSC) measuring device "DSC-60" (manufactured by Shimadzu Corporation). After raising the temperature from room temperature to 200 ° C at a rate of 10 ° C / min (first temperature raising process), lowering the temperature to 30 ° C at 10 ° C / min (first temperature lowering process), and then again to 200 ° C at 10 ° C / min. The temperature of the minimum point of the heat absorption peak in the second temperature rise process when the temperature was raised at the above rate was taken as the melting point of the polyolefin raw material. The melting point of the polyolefin raw material was taken by rounding off the first decimal place of the obtained value.

<Heat shrinkage rate (%) at 120 ° C>
The separator was cut into 100 mm in the MD direction and 100 mm in the TD direction, and allowed to stand in an oven at 120 ° C. for 1 hour. At this time, the sample was sandwiched between two sheets of paper so that the warm air did not directly hit the sample. After taking out the sample from the oven and cooling it, the length (mm) was measured, and the heat shrinkage rate was calculated by the following formula. The measurement was performed in the MD direction and the TD direction, respectively.
Heat shrinkage rate (%) = {(100-length after heating) / 100} x 100

<Hole diameter (μm)>
According to the half-dry method, the average pore size d (μm) of the PO microporous membrane was measured using a palm pollometer (Porous Materials, Inc.: CFP-1500AE). Perfluoropolyester (trade name "Galwick", surface tension 15.6 dyn / cm) manufactured by the same company was used for the immersion liquid. For the dry curve and the wet curve, the applied pressure and the amount of air permeation were measured, and from the pressure P (Pa) where the half curve of the obtained dry curve and the wet curve intersect, the average pore diameter d was calculated by the following equation. (Μm) was determined and used as the pore diameter.
d = 2860 × γ / P

<Withstand voltage value (kV / μm)>
The film sample to be measured and the aluminum foil were cut out to a size of 100 mm × 50 mm. Place the sample on the aluminum foil, place an aluminum plate with a diameter of 5 mm on the sample, and use a withstand voltage tester (device name: TOS9201) under a load of 45 gf on the electrode with a diameter of 5 mm to set the current type. AC (60Hz), start voltage is 0kV, boosting speed is 0.1kV / s, voltage is applied to the electrodes and the voltage is gradually increased, and the voltage when a current of 0.2mA or more flows is measured. did. This measurement was measured at at least 20 different points in the plane of the same film sample. At this time, as shown in FIG. 1, the measurement points were set and measured at a place separated from the measurement points by 15 mm or more without any in-plane bias. The voltage value at which a current of 0.2 mA or more flowed was converted into the withstand voltage value of the film sample per 1 μm of the film thickness, and the average value and standard deviation of the measured values at 20 points were calculated. The measurement was performed in a dry room having a room temperature of 18 ° C. and a dew point of −40 ° C.

<Battery evaluation>
Batteries were manufactured by the following procedures a-1 to a-5.

a-1. Preparation of positive electrode Nickel, manganese, cobalt composite oxide (NMC) (Ni: Mn: Co = 1: 1: 1 (element ratio), density 4.70 g / cm ³ ) was used as the positive electrode active material in an amount of 90.4% by mass. As a conductive auxiliary material, 1.6% by mass of graphite powder (KS6) (density 2.26 g / cm ³ , number average particle diameter 6.5 μm) and acetylene black powder (AB) (density 1.95 g / cm ³ , number average) Particle size 48 nm) was mixed at a ratio of 3.8% by mass, and polyvinylidene fluoride (PVDF) (density 1.75 g / cm ³ ) as a binder was mixed at a ratio of 4.2% by mass, and these were mixed with N-methylpyrrolidone (NMP). A slurry was prepared by dispersing the particles in the mixture. This slurry is applied to one side of a 20 μm-thick aluminum foil that serves as a positive electrode current collector using a die coater, dried at 130 ° C. for 3 minutes, and then compression-molded using a roll press to obtain a positive electrode. Made. The amount of the positive electrode active material applied at this time was 109 g / m ² .

a-2. Preparation of negative electrode As negative electrode active material, graphite powder A (density 2.23 g / cm ³ , number average particle diameter 12.7 μm) was added to 87.6% by mass and graphite powder B (density 2.27 g / cm ³ , number average particle diameter). 9.7% by mass (6.5 μm), 1.4% by mass (solid content equivalent) of ammonium salt of carboxymethyl cellulose (solid content concentration 1.83% by mass aqueous solution) and 1.7% by mass (diene rubber-based latex) as a binder. A slurry was prepared by dispersing (in terms of solid content) (solid content concentration 40% by mass aqueous solution) in purified water. This slurry was applied to one side of a copper foil having a thickness of 12 μm as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression-molded with a roll press to prepare a negative electrode. The amount of the negative electrode active material applied at this time was 52 g / m ² .

a-3. Preparation of non-aqueous electrolyte solution A non-aqueous electrolyte solution is prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethylmethyl carbonate = 1: 2 (volume ratio) so as to have a concentration of 1.0 mol / L. Prepared.

a-4. Formation of Adhesive Layer An adhesive layer was formed on the PO microporous membranes obtained in Examples and Comparative Examples by the following procedure.
In a reaction vessel equipped with a stirrer, a reflux condenser, a dropping tank and a thermometer, 64 parts by mass of water and 0.25 parts of Perex SS-L (sodium alkyldiphenyl ether disulfonate solid content 45% by mass manufactured by Kao Corporation) were added. I put it in. Further, while maintaining the temperature of the reaction vessel at 80 ° C., 0.15 parts by mass of ammonium persulfate (2% by mass aqueous solution) was added to the reaction vessel.

Five minutes after the addition of ammonium persulfate (2% by mass aqueous solution), the emulsion prepared as follows was added dropwise from the dropping tank to the reaction vessel over 150 minutes.
Preparation of emulsion:
Methyl methacrylate (MMA) 24 parts by mass, butyl acrylate (BA) 34 parts by mass, acrylic acid (AA) 1.5 parts by mass, n-dodecyl mercaptan (nDDM) 0.1 parts by mass, Perex SS-L1.5 An emulsion was prepared by mixing parts by mass, 0.15 parts by mass of ammonium persulfate, and 69 parts by mass of water with a homomixer at 6000 rpm for 5 minutes.

After completion under the emulsified droplets, the temperature of the reaction vessel was maintained at 80 ° C. for 60 minutes, and then cooled to room temperature. Next, a 25% by mass aqueous ammonia solution was added to the reaction vessel to adjust the pH to 8.0, and water was further added to adjust the solid content to 40% by mass, and the acrylic emulsion as an adhesive coating liquid was used. Got

7.5 parts by mass of the obtained emulsion was uniformly dispersed in 92.5 parts by mass of water to prepare a coating liquid, which was applied to the surface of the PO microporous membrane using a gravure coater. The coating was dried at 60 ° C. to remove water. Further, the other side was similarly coated with the coating liquid and dried to obtain a separator for an electrochemical cell having an adhesive layer.

a-5. Battery production Using the positive electrode, negative electrode, and non-aqueous electrolytic solution obtained in the above a-1 to a-3, and the separator obtained in the above a-4, the current value is 1A (0.3C), and the termination battery is used. A laminated secondary battery having a size of 100 mm × 60 mm and a capacity of 3000 mAh charged with a constant current constant voltage (CCCV) for 3 hours under the condition of a voltage of 4.2 V was produced.

a-6. Capacity measurement (mAh)
The laminated secondary battery assembled as described above was charged with a constant current constant voltage (CCCV) for 6 hours under the conditions of a current value of 1500 mA (0.5 C) and a cutoff battery voltage of 4.2 V. At this time, the current value immediately before the end of charging was almost 0. Then, the battery was left for one week (aging) in an atmosphere of 25 ° C.

Next, the battery is charged with a constant current constant voltage (CCCV) for 3 hours under the conditions of a current value of 3000 mA (1.0 C) and a termination battery voltage of 4.2 V, and discharged to a battery voltage of 3.0 V at a constant current value (CC) of 3000 mA. I did a cycle of doing. The discharge capacity at this time was defined as the initial discharge capacity X. A battery having an initial discharge capacity X of 3000 ± 10 mAh or less was used for battery evaluation.

b. Output test (25 ° C)
For the laminated secondary battery assembled as described above and selected for evaluation, 1C discharge capacity and 5C discharge capacity up to a discharge end voltage of 3V were measured in a constant temperature state at an atmosphere of 25 ° C, and 5C capacity / The 1C capacity was used as the output characteristic value. The output characteristic values were evaluated according to the following criteria.
A: The output characteristic value is 0.90 or more.
B: The output characteristic value is 0.80 or more and less than 0.90.
C: The output characteristic value is less than 0.80.

c. Oven test (150 ° C)
Using the batteries assembled and selected for evaluation as described above, the charged batteries are placed in an oven, heated from room temperature to 150 ° C at 5 ° C / min, and left at 150 ° C for a predetermined time. , Confirmed the ignition status. The results of the oven test were evaluated according to the following criteria.
A: Those that did not ignite even after being left for 60 minutes or more.
B: Ignition occurred after leaving it for 30 minutes or more and less than 60 minutes.
C: Ignition occurred in less than 30 minutes.

d. Impact test FIG. 2 is a schematic diagram of an impact test.
In the impact test, a round bar is placed on the sample placed on the test table so that the sample and the round bar (φ = 15.8 mm) are approximately orthogonal to each other, and the round bar is placed at a height of 61 cm from the round bar. By dropping a 18.2 kg weight on the upper surface of the round bar, the effect of impact on the sample is observed.
With reference to FIG. 2, the procedure of the impact test in Examples and Comparative Examples will be described below.
For the laminated secondary battery assembled as described above and selected for evaluation, a constant current constant voltage (CCCV) for 3 hours under the conditions of a current value of 3000 mA (1.0 C) and a cutoff battery voltage of 4.2 V. Charged.
Next, in an environment of 25 ° C., the battery assembled as described above and selected for evaluation is placed sideways on a flat surface, and a stainless steel having a diameter of 15.8 mm is placed so as to cross the center of the battery. A round bar was placed. The round bar was arranged so that its long axis was parallel to the longitudinal direction of the separator. A weight of 18.2 kg was dropped from a height of 61 cm so that an impact was applied at a right angle to the vertical axis direction of the battery from a round bar arranged in the center of the battery. After the collision, the surface temperature of the battery was measured. The test was conducted for each of 5 cells and evaluated according to the following criteria. For this evaluation item, A (good) and B (acceptable) were used as acceptance criteria. The surface temperature of the battery means a temperature measured by a thermocouple (K-type seal type) at a position 1 cm from the bottom side of the outer body of the battery.
A (good): The surface temperature rise is 30 ° C. or less in all cells.
B (allowable): There are cells whose surface temperature exceeds 30 ° C and is 100 ° C or less, but the surface temperature is 100 ° C or less in all cells.
C (impossible): The surface temperature exceeds 100 ° C or ignites in one or more cells.

e. 4.2V micro-short circuit inspection test In this specification, the micro-short circuit of the laminated secondary battery is determined by the following method.
The above-mentioned a-6. For the battery for which the capacity measurement has been completed, the battery voltage is set to 4 by a method of continuously charging the battery with a constant current constant voltage (CCCV) for 3 hours under the condition of a cutoff battery voltage of 4.2 V and then continuing the charge with a constant voltage of 4.2 V for 2 hours. .Adjust to 2V. Subsequently, the battery is allowed to stand for one week in a constant temperature bath set at 25 ° C. under a pressure of 10 kPa, and when the voltage drops to 3.8 V or less, it is judged to be a slight short circuit. Ten batteries were prepared, and the result of the 4.2V micro short circuit inspection test was evaluated according to the following criteria based on the number of micro shorted batteries.
A: The number of micro-short-circuited batteries is 0 to 1. B: The number of micro-short-circuited batteries is 2 to 3 C: The number of micro-short-circuited batteries is 4 or more.

[Example 1]
According to the polyethylene (PE) and polypropylene (PP) species shown in Table 1 and the PE species, PP species and raw material composition ratios shown in Table 2, PE and PP are mixed with a Henshell mixer, and penta as an antioxidant. Erislithyl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] was added in an appropriate amount and premixed. The resulting mixture was fed by a feeder to the feed port of a twin-screw co-screw extruder. Further, the liquid paraffin is extruded in two steps so that the ratio of the amount of liquid paraffin (LP) to the total mixture (100.1 parts by mass) extruded by melt-kneading is 75.0 parts by mass. Added to the cylinder by side feed. The set temperature was 160 ° C. for the kneaded portion and 200 ° C. for the T-die. Subsequently, the melt-kneaded product was extruded into a sheet from a T-die and cooled with a cooling roll controlled to a surface temperature of 70 ° C. to obtain a sheet-shaped molded product having a thickness of 1200 μm.

The obtained sheet-shaped molded product was guided to a simultaneous biaxial stretching machine to obtain a primary stretching film (primary stretching step). The set stretching conditions were a preheating temperature of 118 ° C., a preheating coefficient of 153000 ° C. · m, an MD stretching ratio of 7 times, a TD stretching ratio of 7 times, and an MD and TD stretching temperature of 118 ° C. Next, the obtained primary stretched film was guided to a methylene chloride tank and sufficiently immersed to extract and remove liquid paraffin as a plasticizer, and then methylene chloride was dried and removed to obtain an extract film.

Subsequently, the extraction membrane was guided to the TD uniaxial tenter for heat fixation. As a heat fixing step, a stretching operation was performed under the conditions of a TD stretching temperature of 130 ° C. and a TD stretching ratio of 2.00 times, and then a relaxation operation was performed under the conditions of a relaxation temperature of 132 ° C. and a relaxation ratio of 0.80 times. Various characteristics of the obtained PO microporous membrane were evaluated by the above method. Table 2 shows the film forming conditions, and Table 4 shows the results of various evaluations.

[Examples 2 to 21 and Comparative Examples 1 to 7]
A PO microporous film was obtained in the same manner as in Example 1 except that the raw material species, raw material composition ratio, sheet thickness, stretching process conditions, and heat fixing process conditions were set as shown in Table 2 or Table 3, respectively. .. Various characteristics of the obtained PO microporous membrane and the battery containing the same were evaluated by the above method. The evaluation results are shown in Table 4 or Table 5.

Claims (7)

It is a polyolefin microporous film, and the average value of the withstand voltage values at 20 points measured in the region of 5 cm × 10 cm of the polyolefin microporous film is 0.125 kV / μm or more and 1.250 kV / μm or less, and is standard. A microporous polyolefin membrane with a deviation of 0 or more and 0.0060 or less. The polyolefin microporous film according to claim 1, wherein the polyolefin microporous film has a film thickness of 0.1 μm to 6.5 μm. The polyolefin microporous film according to claim 1 or 2, wherein the polyolefin microporous film has a basis weight equivalent strength of 80 gf / (g / m ² ) or more. The polyolefin microporous membrane according to any one of claims 1 to 3, wherein the polyolefin microporous membrane has a melting point (Tm) of 120.0 ° C to 135.0 ° C. The polyolefin microporous membrane according to any one of claims 1 to 4, wherein the polyolefin microporous membrane has a viscosity average molecular weight (Mv) of 600,000 to 2,000,000. The polyolefin microporous membrane according to any one of claims 1 to 5, wherein the average value of the smoothness of both surfaces of the polyolefin microporous membrane is 5,000 sec / 100 cm ³ to 1,000,000 sec / 100 cm ³ . .. The polyolefin microporous film according to any one of claims 1 to 6, wherein the heat shrinkage rate of the polyolefin microporous film at 120 ° C. is 20% or less for both MD and TD.

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