KR101942640B1 - Separator for energy storage device - Google Patents

Abstract

A problem to be solved by the present invention is to provide a separator for a power storage device capable of improving the electrical resistance of the separator and the safety (CID start time) and the battery characteristics (initial charge / discharge efficiency) of the electrical storage device provided with the separator .
[MEANS FOR SOLVING PROBLEMS] A separator for a power storage device comprises a polyolefin microporous membrane containing one or more polyethylenes, and the microporous membrane contains 1.0 wt% or more and 2.0 wt% or less of paraffin.

Description

SEPARATOR FOR ENERGY STORAGE DEVICE

The present invention relates to a separator for a power storage device.

In recent years, development of a non-aqueous electrolyte cell typified by a lithium ion battery has been actively conducted. Normally, in a non-aqueous electrolyte cell, a separator including a microporous membrane is provided between the pair of electrodes. The separator has a function of preventing direct contact between the positive electrode and permeating the ions through the electrolytic solution retained in the micropores.

In general, a polyolefin microporous membrane is used as a separator. As the hole-forming material thereof, liquid paraffin, paraffin wax and the like are used, and the amount or the melting point of liquid paraffin contained in the separator is measured (Patent Documents 1 and 2).

However, as a means for securing the safety of the battery, a CID (Current Interrupt Device) that mechanically cuts off the current path when the internal pressure of the battery rises may be used. Also, as one of the characteristics of the battery, there is an initial charging / discharging efficiency. However, in Patent Documents 1 and 2, the CID start time and the initial charging / discharging efficiency of the battery using the separator are not evaluated.

In some separators, polyolefin resins containing an inorganic filler for adjusting the pore diameter, porosity and the like are used (Patent Documents 3 and 4). However, in Patent Documents 3 and 4, the CID start time and the initial charging / discharging efficiency of the battery using the separator are not evaluated.

Japanese Patent Application Laid-Open No. 2002-234963 Japanese Patent Application Laid-Open No. 2001-96614 WO 09/099088 Japanese Patent Application Laid-Open No. 2010-202829

The separators described in Patent Documents 1 to 4 are superior in safety (CID start time) and battery characteristics (initial charge / discharge efficiency) of the nonaqueous electrolyte battery including the amount of liquid paraffin and silica contained in the separator and the separator, The withstand voltage of the separator still remains to be investigated.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a separator for a power storage device capable of improving the electrical resistance of a separator and the safety (CID start time) and battery characteristics (initial charge and discharge efficiency) will be.

The present inventors have found that the above problems can be solved by controlling the content of liquid paraffin and the content of silicon (Si) contained in a separator for a power storage device, thereby completing the present invention. That is, the present invention is as follows.

[One]

A separator for power storage device comprising a polyolefin microporous membrane containing one or more polyethylenes,

Wherein the microporous membrane contains paraffin in an amount of 1.0 wt% or more and 2.0 wt% or less.

[2]

The separator for a power storage device according to [1], wherein the microporous membrane contains 1 ppm or more and 1000 ppm or less of silicon atoms (Si).

[3]

The separator for a power storage device according to [1] or [2], wherein the microporous membrane contains 1.5% by weight or more and 2.0% by weight or less of the paraffin.

[4]

The separator for a power storage device according to [2], wherein the microporous membrane contains 1 ppm or more and 500 ppm or less of the silicon atoms (Si).

[5]

A separator for power storage device according to any one of [1] to [4]

The positive electrode,

Negative

≪ / RTI >

[6]

The laminate according to [5] is wound.

[7]

A laminate according to [5] or a secondary battery comprising the winding described in [6] and an electrolyte.

According to the present invention, there is provided a separator for a power storage device, which is excellent in the electrical resistance of the separator, the safety (CID start time) of the electrical storage device provided with the separator, and the battery characteristics (initial charge / discharge efficiency).

Hereinafter, a mode for carrying out the present invention (hereinafter referred to as " present embodiment ") will be described in detail. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

The microporous membrane of the present embodiment includes a polyolefin resin and is also used for a separator of a battery device such as a battery, a condenser, and a capacitor.

It is essential that the microporous membrane according to the present embodiment contains paraffin in an amount of 1.0 wt% or more and 2.0 wt% or less. The microporous membrane of the present embodiment draws attention to the paraffin content of the separator, and when the content is within a suitable range, the cell safety and the battery characteristics are improved. When the paraffin content is within the above range, the microporous membrane can realize a battery separator for a power storage device that is excellent in safety (CID start time) and battery characteristics (initial charge / discharge efficiency) of a battery device having a separator.

It is preferable that the microporous membrane according to the present embodiment has a silicon atom (Si) content of 1 ppm or more and 1000 ppm or less. The microporous membrane of this embodiment focuses attention on the Si content of the separator, and when the content is within a suitable range, the separator characteristics, the cell safety, and the battery characteristics are improved. When the Si content is within the above range, the microporous membrane can realize a separator for power storage devices having excellent insulation properties of the separator, safety (CID start time) and battery characteristics (initial charge / discharge efficiency) of a power storage device having a separator .

<Microscopic>

The porous membrane in the present invention will be described.

As the porous film, it is preferable that the porous film has a small electron conductivity, an ionic conductivity, a high resistance to an organic solvent, and a small pore diameter.

As such a porous film, for example, a porous film including a polyolefin resin, a porous film such as a polyethylene terephthalate, a polycycloolefin, a polyether sulfone, a polyamide, a polyimide, a polyimide amide, a polyaramid, a polycycloolefin, A porous film containing a resin such as ethylene, a woven fabric of polyolefin fibers (woven fabric), a nonwoven fabric of polyolefin fibers, a paper, and a collection of particles of insulating material. Among them, in the case of obtaining a multilayer porous film, that is, a separator for a power storage device through a coating process, the coating liquid is excellent in coatability, the thickness of the separator is made thinner, the ratio of the active material in a battery device such as a battery is increased, From the viewpoint of increasing the capacity, a porous film containing a polyolefin resin (hereinafter also referred to as a &quot; polyolefin resin porous film &quot; or a &quot; polyolefin microporous film &quot;) is preferable.

The polyolefin resin porous film will be described.

The polyolefin resin porous film is formed by a polyolefin resin composition in which the polyolefin resin occupies 50% by mass or more and 100% by mass or less of the resin component constituting the porous film from the viewpoint of improving the shutdown performance when the separator for power storage device is improved It is preferably a porous film. The proportion of the polyolefin resin in the polyolefin resin composition is more preferably 60% by mass or more and 100% by mass or less, and still more preferably 70% by mass or more and 100% by mass or less.

Examples of the polyolefin resin contained in the polyolefin resin composition include a homopolymer, a copolymer or a copolymer obtained by using ethylene, propylene, 1-butene, 4-methyl-1-pentene, Multistage polymers, and the like. These polyolefin resins may be used alone or in combination of two or more.

In the present embodiment, the polyolefin microporous membrane contains one or more polyethylenes, that is, one or more kinds of polyethylenes, from the viewpoint of the shutdown characteristics of the separator, the cell safety, and the battery characteristics.

If desired, the polyolefin microporous membrane may contain polypropylene, a copolymer of propylene and other monomers, and a mixture thereof, in view of the shutdown characteristics of the separator for power storage devices.

Specific examples of the polyethylene include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high molecular weight polyethylene,

Specific examples of the polypropylene include isotactic polypropylene, syndiotactic polypropylene, atactic polypropylene,

Specific examples of the copolymer include ethylene-propylene random copolymer, ethylene propylene rubber and the like.

Among them, it is preferable to use high-density polyethylene as the polyolefin resin from the viewpoint of satisfying required performance of low melting point and high strength when the polyolefin microporous membrane is used as a battery separator. In the present specification, high-density polyethylene refers to polyethylene having a density of 0.942 to 0.970 g / cm3. The density of the high-density polyethylene is preferably 0.960 to 0.969 (g / cm3) or 0.950 to 0.958 (g / cm3) from the viewpoint of the strength of the porous film. In this specification, the density of polyethylene refers to a value measured according to JIS K7112 (1999). The ratio of polyethylene to the total polyolefin resin in the polyolefin resin composition is preferably 65 to 99 mass%, more preferably 80 to 97 mass%, and still more preferably 90 to 96 mass% from the viewpoint of both high strength and heat resistance Mass%.

From the viewpoint of improving the heat resistance of the porous film, it is preferable to use a mixture of polyethylene and polypropylene as the polyolefin resin. In this case, the ratio of polypropylene to the total polyolefin resin in the polyolefin resin composition is preferably 1 to 35 mass%, more preferably 3 to 20 mass%, further preferably 3 to 20 mass% from the viewpoint of achieving both heat resistance and a good shutdown function And preferably from 4 to 10 mass%.

The polyolefin resin composition may contain an optional additive. As the additive, for example, a polymer other than a polyolefin resin; Inorganic filler; Antioxidants such as phenol, phosphorus, and sulfur; Metal soaps such as calcium stearate and zinc stearate; Ultraviolet absorber; Light stabilizer; An antistatic agent; Antifogging agents; And coloring pigments.

Since the porous film has a porous structure in which a large number of very small holes are gathered to form dense communication holes, the porous film has excellent ion conductivity, good withstand voltage characteristics, and high strength.

The porous film may be a single layer film containing the above-described material or a laminated film.

The film thickness of the porous film is preferably 0.1 占 퐉 or more and 100 占 퐉 or less, more preferably 1 占 퐉 or more and 50 占 퐉 or less, further preferably 3 占 퐉 or more and 25 占 퐉 or less. From the viewpoint of mechanical strength, the thickness is preferably 0.1 mu m or more, and from the viewpoint of high capacity of the battery, 100 mu m or less is preferable. The film thickness of the porous film can be adjusted by controlling the diagonal gap or the stretch ratio in the stretching process. The film thickness of the porous membrane can be measured by a dial gauge (PEACOCK No. 25 (trademark) by Ozaki Seisakusho).

The average pore diameter of the porous film is preferably 0.03 to 0.70, more preferably 0.04 to 0.20, still more preferably 0.05 to 0.10, and particularly preferably 0.06 to 0.09 . From the viewpoint of high ion conductivity and withstand voltage, it is preferable that the thickness is 0.03 mu m or more and 0.70 mu m or less. The average pore diameter of the porous membrane can be measured by a measurement method described later.

The average pore diameter can be adjusted by controlling the composition ratio, the cooling rate of the extruded sheet, the stretching temperature, the stretching ratio, the heat fixing temperature, the stretching ratio at the time of heat fixation, the relaxation rate at the time of heat fixation, or a combination thereof.

The porosity of the porous film is preferably 25% or more and 95% or less, more preferably 30% or more and 65% or less, and still more preferably 35% or more and 55% or less. Is preferably 25% or more from the viewpoint of improving the ionic conductivity, and is preferably 95% or less from the viewpoint of the withstand voltage characteristic. The porosity of the porous membrane was determined by employing a square sample having a side length of 10 cm,

Porosity (%) = {Volume (cm 3) - Mass (g) / Density (g / cm 3) of polyolefin resin composition} / Volume

. &Lt; / RTI &gt;

The porosity of the porous film can be adjusted by controlling the mixing ratio of the polyolefin resin composition and the plasticizer, the drawing temperature, the drawing magnification, the heat setting temperature, the drawing magnification at the time of heat setting, the releasing rate at the time of heat setting, .

When the porous membrane is a polyolefin resin porous membrane, the viscosity average molecular weight of the polyolefin resin porous membrane is preferably from 30,000 to 12,000,000, more preferably from 50,000 to less than 2,000,000, and still more preferably from 100,000 to less than 1,000,000. If the viscosity average molecular weight is 30,000 or more, the melt tension during melt molding becomes large, so that moldability tends to be good and tends to become high due to entanglement of the polymers. On the other hand, when the viscosity average molecular weight is 12,000,000 or less, melt kneading can be easily performed uniformly, and sheet formability, particularly, thickness stability tends to be excellent. When the polyolefin resin porous film is used as a separator for a battery, a viscosity average molecular weight of less than 1,000,000 is preferable because it tends to close the pores when the temperature rises and a good shutdown function tends to be obtained.

<Manufacturing Method>

The method for producing the porous film is not particularly limited, and a known manufacturing method can be employed. E.g

(1) a method in which a polyolefin resin composition and a hole-forming material are melt-kneaded to form a sheet, after stretching if necessary, and then the plasticizer is extracted to make it porous;

(2) a method in which the polyolefin resin composition is melt-kneaded and extruded at a high draw ratio, and then the polyolefin crystal interface is peeled off by heat treatment and stretching,

(3) a method in which the polyolefin resin composition and the inorganic filler are melt kneaded and formed into a sheet, and then the interface between the polyolefin and the inorganic filler is peeled off by stretching,

(4) a method in which the polyolefin resin composition is dissolved and then immersed in a poor solvent for the polyolefin to coagulate the polyolefin and simultaneously remove the solvent to make the polyolefin resin porous

And the like.

Hereinafter, as an example of a method for producing a porous membrane, a method of melt-kneading a polyolefin resin composition and a hole forming material to form a sheet, and then extracting the hole forming material will be described.

First, the polyolefin resin composition and the hole forming material are melt-kneaded. As the melt kneading method, for example, a polyolefin resin and, if necessary, other additives may be added to a resin kneading apparatus such as an extruder, a kneader, a Labo Plastomill, a kneading roll or a Banbury mixer to melt and melt the resin component By introducing a plasticizer into the mixture.

Examples of the hole-forming material include a plasticizer and an inorganic material.

The plasticizer is not particularly limited, but it is preferable to use a nonvolatile solvent capable of forming a uniform solution at a melting point of the polyolefin or higher. Specific examples of such nonvolatile solvents include hydrocarbons such as liquid paraffin and paraffin wax; Esters such as dioctyl phthalate and dibutyl phthalate; And higher alcohols such as oleyl alcohol and stearyl alcohol. These plasticizers may be recovered by recycling after distillation. Preferably, the polyolefin resin, the other additives, and the plasticizer are pre-kneaded beforehand in a predetermined ratio using a Henschel mixer or the like before being fed into the resin kneading apparatus. More preferably, in the pre-kneading, only a part of the plasticizer is added, and the remaining plasticizer is kneaded while being fed to the resin kneader by sideways. By using such a kneading method, the dispersibility of the plasticizer is increased, and when the sheet-shaped formed article of the melt-kneaded product of the resin composition and the plasticizer is stretched in the subsequent step, there is a tendency to be able to stretch at a high magnification without breaking.

Of the plasticizers, when the polyolefin resin is polyethylene or polypropylene, the liquid paraffin is highly compatible with these resins, and even when the melt-kneaded product is stretched, the interface separation between the resin and the plasticizer hardly occurs, Therefore, it is desirable.

The ratio of the polyolefin resin composition to the plasticizer is not particularly limited as long as they can be uniformly melted and kneaded to form a sheet. For example, the mass fraction of the plasticizer in the composition comprising the polyolefin resin composition and the plasticizer is preferably 20 to 90 mass%, more preferably 30 to 80 mass%. When the mass fraction of the plasticizer is 90 mass% or less, the melt tension at the time of melt molding tends to be insufficient, and the moldability tends to be improved. On the other hand, when the mass fraction of the plasticizer is 20% by mass or more, even when the mixture of the polyolefin resin composition and the plasticizer is stretched at a high magnification, the polyolefin chain is not cut off and a uniform and fine pore structure is easily formed and the strength tends to increase .

The inorganic material is not particularly limited, and examples thereof include oxide ceramics such as alumina, silica (silicon oxide), titania, zirconia, magnesia, ceria, yttria, zinc oxide and iron oxide; Nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; Calcium carbonate, calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate , Ceramics such as magnesium silicate, diatomaceous earth and silica sand; Glass fibers. These may be used singly or in combination of two or more. Of these, silica, alumina and titania are preferable from the viewpoint of electrochemical stability, and silica is particularly preferable in terms of easiness of extraction.

Next, the melt-kneaded product is formed into a sheet. Examples of the method for producing the sheet-shaped molded product include a method of extruding a melt-kneaded product through a T-die or the like into a sheet form and bringing the melt-kneaded product into contact with a heat conductor to cool the melt to a temperature sufficiently lower than the crystallization temperature of the resin component have. Examples of the heat conductor used for cooling and solidifying include metal, water, air, or plasticizer itself, but it is preferable to use a metal roll because heat conduction efficiency is high. Further, when the extruded kneaded material is brought into contact with a metal roll, it is preferable to sandwich the extruded kneaded material between the rolls because the efficiency of heat conduction is further increased and the sheet is oriented to increase the film strength and also to improve the surface smoothness of the sheet More preferable. When the molten kneaded product is extruded from the T-die into the sheet, the die-lap interval is preferably 200 占 퐉 or more and 3,000 占 퐉 or less, more preferably 500 占 퐉 or more and 2,500 占 퐉 or less. When the diagonal gap is 200 mu m or more, gums and the like are reduced and the influence on the film quality such as streaks and defects is small, and the risk of film breakage or the like in the subsequent drawing process can be reduced. On the other hand, if the die lip interval is 3,000 탆 or less, the cooling speed is fast and cooling unevenness can be prevented, and the thickness stability of the sheet can be maintained.

Alternatively, the sheet-shaped formed body may be rolled. Rolling can be performed by a pressing method using, for example, a double belt press machine or the like. By performing rolling, it is possible to increase the orientation of the surface layer portion in particular. The rolled surface magnification is preferably 1 to 3 times, more preferably 1 to 2 times. If the rolling magnification exceeds 1, the surface orientation tends to increase and the film strength of the finally obtained porous film tends to increase. On the other hand, if the rolling magnification is three times or less, the orientation difference between the surface layer portion and the inside of the center is small, and a uniform porous structure can be formed in the film thickness direction.

Subsequently, the hole forming material is removed from the sheet-shaped formed body to form a porous film. Examples of the method for removing the hole forming material include a method in which the sheet forming material is immersed in an extraction solvent to extract the hole forming material and sufficiently dried. The method of extracting the hole forming material may be either a batch method or a continuous method. In order to suppress the contraction of the porous film, it is preferable to restrain the end portion of the sheet-shaped formed body during a series of processes of immersion and drying.

The extraction solvent used when extracting the hole forming material is preferably a poor solvent for the polyolefin resin, a good solvent for the hole forming material, and a boiling point lower than the melting point of the polyolefin resin. Examples of the extraction solvent include hydrocarbons such as n-hexane and cyclohexane; Halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane; Non-chlorinated halogenated solvents such as hydrofluoroether, hydrofluorocarbon and the like; Alcohols such as ethanol and isopropanol; Ethers such as diethyl ether and tetrahydrofuran; And ketones such as acetone and methyl ethyl ketone. These extraction solvents may be recovered and reused by an operation such as distillation. When an inorganic material is used as the hole forming material, an aqueous solution of sodium hydroxide, potassium hydroxide or the like can be used as an extraction solvent.

It is also preferable to stretch the sheet-shaped formed article or the porous film. The stretching may be performed before extracting the hole forming material from the sheet-form molding. Further, it may be performed on the porous film from which the hole forming material is extracted from the sheet-form molding. It may also be performed before and after the hole forming material is extracted from the sheet-form molding.

As the stretching treatment, both uniaxial stretching and biaxial stretching can be suitably used, but biaxial stretching is preferable from the viewpoint of improving the strength and the like of the obtained porous film. When the sheet-like formed body is stretched at a high magnification in the biaxial direction, the molecules are oriented in the plane direction, and the finally obtained porous film is hardly torn and has a high sticking strength. Examples of the stretching method include simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching. Examples of the stretching method include simultaneous biaxial stretching in terms of enhancement of stab strength, uniformity of stretching, and shut- desirable.

Here, the simultaneous biaxial stretching refers to a stretching method in which stretching of MD (machine direction of continuous microporous membrane) and stretching of TD (direction of MD of microporous membrane at an angle of 90 DEG) are simultaneously performed, The stretching magnification may be different. Sequential biaxial stretching is a stretching method in which MD or TD stretching is carried out independently. When stretching is performed in MD or TD, the other direction is set to be unrestrained or fixed at a certain length.

The drawing magnification is preferably in the range of 20 times to 100 times, more preferably in the range of 25 times to 50 times. The stretching magnification in each axial direction is preferably in the range of not less than 4 times and not more than 10 times in MD, not less than 4 times and not more than 10 times in TD, not less than 5 times and not more than 8 times in MD, not less than 5 times and not more than 8 times in TD Is more preferable. When the total area multiplication factor is 20 or more, sufficient strength can be given to the resulting porous film. On the other hand, if the total area multiplication factor is 100 or more, the film tends to be prevented from being broken in the drawing step, have.

In order to suppress shrinkage of the porous film, heat treatment may be performed after the stretching process or after the formation of the porous film for the purpose of fixing the heat. Further, the porous film may be subjected to a post-treatment such as a hydrophilic treatment with a surfactant or the like, or a crosslinking treatment with ionizing radiation or the like.

It is preferable that the porous film is subjected to heat treatment for the purpose of heat fixation from the viewpoint of suppressing shrinkage. Examples of the heat treatment method include a relaxation operation performed with a predetermined temperature atmosphere and a predetermined relaxation rate for the purpose of stretching operation performed at a predetermined temperature atmosphere and a predetermined elongation rate and / or stress reduction for the purpose of adjusting physical properties have. The relaxation operation may be performed after the stretching operation. These heat treatments can be performed using a tenter or a roll stretcher.

It is preferable that the stretching operation is performed by stretching at least 1.1 times, more preferably at least 1.2 times, MD and / or TD of the film in view of obtaining a porous film having further high strength and high porosity.

The relaxation operation is a shrinking operation of the membrane to MD and / or TD. The relaxation rate is a value obtained by dividing the dimension of the membrane after the relaxation operation by the dimension of the membrane before the relaxation operation. Also, when both MD and TD are relaxed, it is a value obtained by multiplying the relaxation rate of MD and the relaxation rate of TD. The relaxation rate is preferably 1.0 or less, more preferably 0.97 or less, still more preferably 0.95 or less. The relaxation rate is preferably 0.5 or more from the viewpoint of film quality. The relaxation operation may be performed in both MD and TD directions, but only one of the MD and TD directions may be performed.

The stretching and the relaxation after the plasticizer is preferably performed by TD. The temperature in the stretching and relaxation operation is preferably lower than the melting point (hereinafter, also referred to as "Tm") of the polyolefin resin, more preferably 1 ° C to 25 ° C lower than Tm and 3 ° C to 23 ° C lower than Tm , And a range of 5 占 폚 to 21 占 폚 lower than the Tm is particularly preferable. When the temperature in the stretching and the relaxation is in the above range, it is preferable from the viewpoint of the reduction of the heat shrinkage rate and the balance of the porosity.

<CID>

CID (Current Interrupt Device) is a device that cuts off the current itself when a change in pressure inside a sealed battery, that is, a pressure rise, is detected and becomes equal to or higher than a certain pressure. The time from the start of charge / discharge of the battery to the start of the CID is referred to as the CID start time. It is preferable that the CID start time is short in view of securing the safety of a higher battery.

<Paraffin content of microporous membrane>

The content of paraffin contained in the polyolefin microporous membrane is preferably 1.0 wt% or more and 2.0 wt% or less, more preferably 1.5 wt% or more and 2.0 wt% or less from the viewpoint of shortening the CID start time of the cylindrical battery using the separator containing the microporous membrane Do. If the content of the paraffin is less than 1.0% by weight, the CID start time can not be shortened. If the content is more than 2.0% by weight, the initial charge / discharge efficiency is deteriorated. The details of the mechanism that shortens the CID start time when the paraffin content is within the above range is unknown but is estimated as follows. In the cylindrical battery using the separator including the microporous membrane, by repeating charge and discharge, it is possible to generate gas by decomposition of paraffin and to start the CID at an early stage, thereby making it possible to securely insulate the battery .

The paraffin contained in the polyolefin microporous membrane may be either solid or liquid as long as it contains one or more alkanes having 20 or more carbon atoms. Paraffin is generally referred to as "paraffin" or "paraffin wax" when it is solid and "liquid paraffin" when it is liquid. When the polyolefin microporous membrane contains plural kinds of paraffins, it is assumed that the total content of all the paraffins is in the range of 1.0 wt% or more and 2.0 wt% or less. The paraffin content is determined according to the methods and conditions described in the examples.

The paraffin can be identified by analytical methods typified by chromatography or infrared absorption spectrometry.

In the above-described method for producing a polyolefin microporous membrane, by adjusting the mixing ratio of the resin composition and paraffin, the dispersibility of paraffin to the resin composition, and the extraction conditions of paraffin from the sheet containing the resin composition and paraffin, It is preferable to control the paraffin content of the sclera to 1.0 wt% or more and 2.0 wt% or less.

<Si content of microporous film>

The content of silicon atoms (Si) contained in the polyolefin microporous membrane is preferably 1 ppm or more and 1000 ppm or less based on the weight of the microporous membrane in view of the CID startup time of the battery using the separator containing the microporous membrane, Or more and 500 ppm or less. The details of the mechanism for shortening the CID start time when the Si content is within the above range are unknown, but are estimated as follows. In a cylindrical battery using a separator including the microporous membrane, a small amount of hydrofluoric acid is generated by repeating charge and discharge, and it reacts with a Si compound (for example, silica) to generate silicon fluoride (SiF ₄ ). This makes it possible to start the CID at an early stage, thereby making it possible to securely insulate the battery.

The Si content in the polyolefin microporous membrane is preferably 1000 ppm or less from the viewpoint of the withstand voltage of the separator. If the Si content is more than 1000 ppm, it is presumed that the moisture contained in the Si compound (for example, silica) deteriorates the withstand voltage of the separator.

(Battery separator and battery)

The microporous membrane of the present embodiment is preferably used particularly for use as a battery separator. That is, the battery separator of the present embodiment includes the microporous membrane of the present embodiment. The battery of this embodiment also includes the microporous membrane of the present embodiment.

By bonding the separator according to the present embodiment to an electrode, a laminate in which a separator and an electrode are laminated can be obtained.

The laminate is excellent in not only handling properties at the time of winding, rate characteristics and cycle characteristics of the electrical storage device but also adhesiveness and permeability. Therefore, the laminate can be suitably used, for example, in batteries such as nonaqueous electrolyte secondary batteries, power storage devices such as capacitors, capacitors, and the like.

The method of producing the laminate is not particularly limited, but may include, for example, a step of superposing the separator according to the present embodiment and the electrode, and heating and / or pressing, if necessary. Heating and / or pressing may be performed when overlapping the electrode and the separator. After the electrode and the separator are superimposed, a wound body can be obtained by winding the electrode or separator in a circular or flat manner. Heating and / or pressing may be performed on the winding body.

A separator having a length of 10 to 500 mm (preferably 80 to 500 mm) and a length of 200 to 4000 m (preferably 1000 to 4000 m) may be prepared and overlapped with the electrode.

The laminate may be produced by laminating in the order of a positive electrode-separator-negative electrode-separator or negative electrode-separator-positive electrode-separator in the form of a flat plate, and pressing and supplementarily heating as required.

The pressure at the time of pressing is preferably 1 MPa to 30 MPa. The pressing time is preferably 5 seconds to 30 minutes. The heating temperature is preferably 40 to 120 ° C. The heating time is preferably 5 seconds to 30 minutes. Even if heating is performed and then pressure is applied, pressurization and heating may be performed at the same time even if heating is performed after pressurization. Among them, it is preferable to simultaneously perform pressurization and heating.

In the case where the electrical storage device is a secondary battery, the laminated body is wound in a circular or flat wrapping form to obtain a winding body, the winding body is housed in a housing such as a can, a pouch- And / or a press may be further performed to obtain a secondary battery.

When a nonaqueous electrolyte secondary battery is manufactured using the separator according to the present embodiment, a known positive electrode, negative electrode, and nonaqueous electrolyte solution may be used.

Examples of the positive electrode material include, but are not limited to, lithium-containing complex oxides such as LiCoO ₂ , LiNiO ₂ , spinel type LiMnO ₄ and olivine type LiFePO ₄ .

The negative electrode material is not particularly limited, and examples thereof include carbon materials such as graphite, non-graphitized carbonaceous, graphitized carbonaceous material and composite carbonaceous material; Silicon, tin, metal lithium, various alloy materials, and the like.

The non-aqueous electrolyte is not particularly limited, but an electrolytic solution in which an electrolyte is dissolved in an organic solvent can be used. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and the like. Examples of the electrolyte include lithium salts such as LiClO ₄ , LiBF ₄ and LiPF ₆ .

As the battery using the separator according to the present embodiment, it is possible to use a polyolefin microporous membrane (separator) containing not less than 1.0 wt% and not more than 2.0 wt% of paraffin, a positive electrode, a negative electrode, an electrolytic solution, Element) is preferable as the battery. The cylindrical secondary battery having such a configuration can improve the safety by shortening the CID startup time and easily attain both the safety and the battery characteristics (initial charge / discharge efficiency).

As described above, the CID start time of the battery using the separator according to the present embodiment is preferably as short as possible from the viewpoint of securing safety of a higher battery. The CID start time of the battery using the separator is measured according to the method and conditions described in the embodiment. In order to measure the CID startup time, a separator is interposed between the positive electrode and the negative electrode and is wound to manufacture an electrode assembly, which will be described in detail in the embodiment. As the startup time of the CID is shorter, the change in the internal pressure of the battery, that is, the pressure rise is detected, and the current is cut off more quickly, so that the safety of the battery can be secured.

The initial charging and discharging efficiency of the battery using the separator according to the present embodiment is preferably 80% or more for increasing the output of the battery. The initial charge-discharge efficiency of the battery using the separator is measured according to the method and conditions described in the embodiment.

The withstand voltage of the separator according to the present embodiment is preferably 1.7 kV or more and more preferably 2.3 kV or more from the viewpoint of reduction of the internal short circuit using the separator. The withstand voltage of the separator is measured according to the method and conditions described in the embodiment.

Example

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the examples. The measurement methods and evaluation methods of various physical properties used in the following Production Examples, Examples and Comparative Examples are as follows. Unless otherwise specified, various measurements and evaluations were carried out under the conditions of a room temperature of 23 캜, 1 atm and a relative humidity of 50%.

&Lt; Measurement method and evaluation method >

&Lt; viscosity average molecular weight &

ASTM-D4020. The polyolefin raw material or microporous membrane was dissolved in a decalin solution at 135 占 폚 to measure the intrinsic viscosity [?] And the viscosity average molecular weight (Mv) was calculated by the following equation.

^{[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

<Measurement of liquid paraffin content>

The content of the liquid paraffin in the separator of Examples and Comparative Examples was measured by the following method.

The separator sampled at 100 x 100 mm was de-electrified and weighed using a precision balance (W ₀ (g)). Subsequently, 200 ml of methylene chloride was added to the sealed container, and the separator was immersed at room temperature for 15 minutes. Thereafter, the separator was taken out, dried at room temperature for 3 hours, discharged similarly to the above, and weighed using a precision balance (W ₁ (g)). The liquid paraffin content was determined by the following formula.

Paraffin content (wt%) = (W ₀ -W ₁ ) / W ₀ 100

<Measurement of Si Content>

The Si contents in the separators of Examples and Comparative Examples were measured by the following methods.

About 0.2 g of the separator was weighed into a closed decomposition vessel made of a fluororesin, 5 mL of high purity nitric acid was added thereto, and the mixture was treated by a microwave decomposition apparatus (manufactured by Milestone General Corporation, trade name "ETHOS TC", instrument number 125571) After heating at 200 ° C for 20 minutes, the solution was adjusted to 50 ml with ultrapure water. Thereafter, measurement was carried out by an ICP mass spectrometer (trade name: X series X7 ICP-MS, manufactured by Thermo Scientific Co., Ltd., instrument No. X0126). The quantification method was carried out according to the internal standard method using a four-point calibration curve of 0, 2, 10 and 20 μg / L of each element concentration. In addition, the test sample solution was diluted so as to fall within the calibration curve range. Cobalt (Co) was used as an internal standard element.

<CID start time evaluation>

94% by weight of LiCoO ₂ (positive electrode active material), 3% by weight of carbon black (conductive agent) and 3% by weight of polyvinylidene fluoride (binder) were mixed in N-methylpyrrolidone to prepare a positive electrode active material slurry. This slurry was applied to an aluminum foil (current collector), dried and rolled to produce a positive electrode.

Natural graphite and polyvinylidene fluoride (binder) were mixed in N-methylpyrrolidone solvent in a mass ratio of 96: 4 to prepare negative electrode slurry. The negative electrode slurry was coated on a copper foil with a thickness of 14 탆 to form a thin electrode plate. The negative electrode slurry was dried at 135 캜 for 3 hours or more and rolled to prepare a negative electrode.

A mixed solvent obtained by mixing fluoroethylene carbonate, ethylene carbonate, ethyl methyl carbonate and diethyl carbonate in a ratio of 20 vol%: 10 vol%: 20 vol%: 50 vol% was mixed with 1.3M LiPF ₆ was added. To this mixture, succinonitrile was added in an amount of 5% by weight based on the total weight of the mixture to prepare an electrolyte.

A lithium secondary battery having a capacity per volume of 730 Wh / l was prepared using the positive electrode, the negative electrode, the electrolyte, and the separator of Examples and Comparative Examples.

The CID (Current Interrupt Device) startup time was measured while continuously charging the produced lithium secondary battery at a constant voltage of 4.35 V at 60 캜. The shortening time (min.) Of the battery using the separator of Example and Comparative Example 2 with respect to the startup time of Comparative Example 1 was calculated using the CID startup time of the battery using the separator according to Comparative Example 1 as a blank.

&Lt; Evaluation of initial charging / discharging efficiency &

(Production of coin battery)

98 parts of graphite having a particle diameter of 20 mu m and a specific surface area of 4.2 m &lt; 2 &gt; / g and 5 parts of PVDF (polyvinylidene fluoride) as a binder for the negative electrode active material layer (corresponding to solid content) were mixed as the negative electrode active material, And the mixture was mixed with a planetary mixer to prepare an electrode composition for a negative electrode on a slurry. The negative electrode active material was used to produce a coin-shaped secondary battery. In this secondary battery, the test electrode using the negative electrode active material is accommodated in the can 1, the counter electrode is attached to the can 2, and then the can 1 and 2 are laminated via the separator of the embodiment and the comparative example in which the electrolyte is impregnated And then caulked through a gasket. The electrode composition for negative electrode was applied to a copper foil current collector so as to have a thickness of 60 mu m after drying, dried and punched into a pellet shape having a diameter of 16.4 mm. As the counter electrode, a lithium cobalt oxide (LiCoO ₂ ) plate punched with a diameter of 15.2 mm was used. In the case of producing the electrolytic solution, ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) were mixed as a solvent, and lithium hexafluorophosphate (LiPF ₆ ) as an electrolytic salt was dissolved. In this case, the composition of the solvent was set to EC: EMC: DMC = 30: 10: 60 by mass ratio, and the concentration of the electrolyte salt was 1 mol / dm 3 (= 1 mol / l).

(Evaluation of initial charging / discharging efficiency)

The first charge capacity was evaluated using the coin-type thus prepared. The battery was charged at a constant current until the battery voltage reached 4.2 V at a current of 1 mA and then charged at a constant voltage until the current reached 100 mu A at a voltage of 4.2 V. The mass of the copper foil current collector and the binder And the charging capacity per unit mass was obtained. Here, charging refers to a lithium insertion reaction into the negative electrode active material.

In the case of checking the discharge capacity for the first time, the battery was charged in the same procedure as that for the first charging capacity, and then discharged at a constant current until the battery voltage reached 2.5 V at a current of 1 mA. The discharge capacity per unit mass excluding the mass of the copper foil collector and the binder was determined. Here, the discharge means a lithium removal reaction from the negative electrode active material.

The initial charge-discharge efficiency was calculated according to the following formula.

Initial charge / discharge efficiency (%) = (first discharge capacity / first charge capacity) × 100

&Lt; Measurement of withstand voltage &

The dielectric strength of the separator in Examples and Comparative Examples was measured by the following method.

A separator sample of 50 mm x 50 mm was put on an electrode of? 35 mm which made the surface clean, a voltage was applied to the electrode, the voltage was gradually increased, and a voltage value at the time of sparking Respectively. This measurement was taken at at least 20 different points within the plane of the same film sample and the average value thereof was recorded. At this time, the withstand voltage resistance was evaluated based on the following evaluation criteria.

A (remarkably good): 2.3 kV or more

B (good): 1.7 kV or more and less than 2.3 kV

C (allowable): Less than 1.7 kV

[Example 1]

45 parts by mass of a homopolymer high density polyethylene having an Mv of 700,000, 45 parts by mass of a homopolymer having an Mv of 300,000, a mixture of a homopolymer polypropylene having Mv of 400,000 and a homopolymer polypropylene having an Mv of 150,000 (Mass ratio = 4: 3) and 0.0495 mass parts (500 ppm) of silica "LP" (manufactured by Dosso Silica) having an average primary particle diameter of 15 nm were dry blended using a tumbler blender to obtain a polyolefin mixture .

1 part by mass of tetrakis- [methylene- (3 ', 5'-di-t-butyl-4'-hydroxyphenyl) propionate] methane as an antioxidant was added to 99 parts by mass of the obtained polyolefin mixture, The mixture was obtained by dry blending using a blender.

The resulting mixture was fed into a twin-screw extruder under a nitrogen atmosphere by a feeder. In addition, liquid paraffin (kinematic viscosity at 37.78 ° C, 7.59 × 10 ^-5 m 2 / s) was plunged into the extruder cylinder by a pump. The operating conditions of the feeder and the pump were adjusted so that the ratio of the liquid paraffin occupied in the whole mixture to be extruded was 65 parts by mass, that is, the polymer concentration was 35 parts by mass.

Subsequently, the melt-kneaded product obtained by melting and kneading them while heating them in a twin-screw extruder at 230 DEG C was extruded through a T-die onto a cooling roll controlled at a surface temperature of 80 DEG C, and the extrudate was brought into contact with a cooling roll Cast and cooled and solidified to obtain a sheet-like molded product.

This sheet was stretched in a simultaneous biaxial stretching machine under the conditions of a magnification of 7 × 6.4 times and a temperature of 112 ° C. and then immersed in methylene chloride to extract liquid paraffin and dried, and then dried in a tenter stretcher at a temperature of 130 ° C. Fold. Thereafter, the stretched sheet was relaxed by about 10% in the width direction and heat treatment was performed to obtain a polyolefin microporous membrane. At this time, the extraction conditions were adjusted so that the paraffin content of the microporous membrane by the above measurement method was 1.0 wt%. Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Example 2]

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the extraction conditions of Example 1 were adjusted so that the paraffin content of the microporous membrane in the measurement method was adjusted to 1.5 wt%. Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Example 3]

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the extraction conditions of Example 1 were adjusted so that the paraffin content of the microporous membrane in the measurement method was 2.0 wt%. Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Example 4]

A polyolefin microporous membrane was obtained in the same manner as in Example 2 except that silica "LP" (manufactured by Doso Silica) having an average primary particle diameter of 15 nm was not added in Example 2.

Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Example 5]

A polyolefin microporous membrane was prepared in the same manner as in Example 2 except that the amount of silica "LP" (produced by Toso Silica) having an average primary particle diameter of 15 nm to be added in Example 2 was changed to 0.000099 parts by mass (1 ppm) . Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Example 6]

A polyolefin microporous membrane was prepared in the same manner as in Example 2 except that the amount of silica "LP" (manufactured by Doso Silica) having an average primary particle diameter of 15 nm added in Example 2 was changed to 0.099 mass parts (1000 ppm) . Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Example 7]

A polyolefin microporous membrane was prepared in the same manner as in Example 2 except that the amount of silica "LP" (manufactured by Doso Silica) having an average primary particle diameter of 15 nm to be added was changed to 0.198 parts by mass (2000 ppm) in Example 2 . Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Comparative Example 1]

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the extraction conditions of Example 1 were adjusted so that the paraffin content of the microporous membrane in the measurement method was adjusted to 0.2 wt%. Various properties were measured for the obtained microporous membrane by the above-mentioned method.

[Comparative Example 2]

A polyolefin microporous membrane was obtained in the same manner as in Example 1, except that the extraction conditions of Example 1 were adjusted so that the paraffin content of the microporous membrane in the measurement method was adjusted to 3.0 wt%. Various properties were measured for the obtained microporous membrane by the above-mentioned method.

The results of measurement of the physical properties of the microporous membranes obtained in Examples 1 to 7 and Comparative Examples 1 and 2 are shown in Table 1 below.

Claims (9)

A separator for power storage device comprising a polyolefin microporous membrane containing one or more polyethylenes,
Wherein the microporous membrane contains paraffin in an amount of 1.0 wt% or more and 2.0 wt% or less. The separator for a power storage device according to claim 1, wherein the microporous membrane contains 1 ppm or more and 2,000 ppm or less of silicon atoms (Si). The separator for a power storage device according to claim 1, wherein the microporous membrane contains 1 ppm or more and 1000 ppm or less of silicon atoms (Si). The separator for a power storage device according to claim 1, wherein the microporous membrane contains 1 ppm or more and 500 ppm or less of silicon atoms (Si). The separator for a power storage device according to any one of claims 1 to 4, wherein the microporous membrane contains 1.5% by weight or more and 2.0% by weight or less of the paraffin. A separator for power storage device according to any one of claims 1 to 4,
The positive electrode,
Negative
&Lt; / RTI &gt; The separator for power storage device according to claim 5,
The positive electrode,
Negative
&Lt; / RTI &gt; A secondary battery comprising the laminate according to claim 6 and an electrolyte. A secondary battery comprising the laminate according to claim 7 and an electrolyte.