KR20200060353A - Polyolefin microporous membrane, battery separator and secondary battery - Google Patents

Abstract

Provided is a microporous membrane made of polyolefin having excellent mechanical strength such as protruding strength and shape retention characteristics at high temperature, and high uniformity and safety.
The semi-crystallization time (T _1/2 ) of 126 ° C is 200 seconds or less, the puncture strength in terms of 50% porosity and film thickness of 20 µm is 6.0 N / 20 µm or more, and the MD direction of the melt heat shrink stress is P _MD , When the TD direction is P _TD , at least one of the microporous membranes made of polyolefin, characterized in that at least one side is 0.8 MPa or less and P _{MD + TD, which} is the sum of P _MD and P _TD , is 1.5 MPa or less.

Description

Polyolefin microporous membrane, battery separator and secondary battery

The present invention relates to a microporous membrane made of polyolefin, a separator for a battery, and a secondary battery, which are suitably used for a separator of a secondary battery, a coated separator substrate, and the like.

In recent lithium ion secondary batteries, high capacity and high output are required. In addition, since it is appropriate to shorten the distance between electrodes in order to advance the high capacity and high output, thinning of the separator for a lithium ion secondary battery is in progress. With the thinning of the separator, high mechanical strength is required to prevent local short-circuiting due to shortening between the electrode or the film due to foreign matter in the battery manufacturing process. In addition, with the high performance of the secondary battery, safety at the time of overcharging or battery runaway due to external impact is also required at a high level, and the separator has a high shape retention characteristic in the thermal runaway temperature range in order to prevent short circuit of the electrode at the thermal runaway temperature. In order to provide a, reduction in heat shrinkage when melting the separator is required. For this reason, the separator for lithium ion secondary batteries is required to have higher mechanical strength and high shape retention properties under high temperature in order to prevent breakage or short circuit.

Patent Literature 1 discloses a technique related to a microporous membrane using polyolefin having a viscosity average molecular weight of 150000 to 1000000 and improving the stretching method, and having excellent mechanical strength and film shrinkage (hereinafter, TD) thermal contraction characteristics. However, according to the manufacturing method, since the heat shrinkage in one direction of TD was suppressed, when the foreign material was stuck from the outside of the battery and thermal runaway occurred, the contraction of the other side could not be suppressed, and there was a fear of short circuit.

In addition, many proposals have been made to add a crystal nucleating agent to increase the strength of the microporous membrane as in Patent Document 2 or Patent Document 3. Among them, the microporous membrane added with a crystal nucleating agent has an effect of increasing strength and improving withstand voltage characteristics, but there are cases in which compatibility with heat shrinking, particularly melting heat shrinking under high temperature, is insufficient.

Japanese Patent No. 531275 Publication of International Patent Publication 2016/104790 Japanese Patent Publication No. Hei 8-12799

The problem of the present invention is to solve the problems of the prior art, and to provide a microporous membrane made of polyolefin having excellent mechanical strength such as swell strength and shape retention properties at high temperature, and high uniformity and stability.

The present inventors carefully studied the materials and properties of the microporous membrane, and as a result, semi-crystallization time, protruding strength, and melt heat-shrinking stress were specified within a specific range, thereby making polyolefins having excellent mechanical strength, shape-retaining properties at high temperatures, and structural uniformity Discovering what can be done with a microporous membrane has led to the completion of the present invention. The polyolefin microporous membrane according to the embodiment of the present invention can suppress the melt shrinkage stress of the polyolefin microporous membrane by adjusting the polyolefin having a weight average molecular weight of 1.0 × 10 ⁶ or more in the polyolefin microporous membrane to a content within a certain range. In addition, by increasing the crystallization rate of the polyolefin composition by adding a crystal nucleating agent, etc., it is possible to make the crystal structure and the higher order structure finer and uniform, thereby efficiently propagating stress during stretching. The microporous membrane made of polyolefin promotes structural changes to crystals that greatly affect mechanical strength by stretching in a specific condition range to be described later, and has excellent mechanical strength, shape-retaining properties at high temperatures, and structural uniformity.

In order to solve the above problems, the present invention adopts the following configuration. In other words,

(1) The semi-crystallization time (T _1/2 ) at 126 ° C is 200 seconds or less, and the puncture strength in terms of 50% porosity and film thickness of 20 µm is 6.0 N / 20 µm or more, and the melting heat shrinkage stress is When the MD direction is P _MD and the TD direction is P _TD , at least one is 0.8 MPa or less, and P _{MD + TD, which} is the sum of P _MD and P _TD , is 1.5 MPa or less.

(2) In the above (1),

A microporous membrane made of polyolefin, characterized in that the maximum pore diameter observed from the forometer is 45 nm or less, and the ratio of the average flow rate pore diameter / maximum pore diameter is 0.6 or more.

(3) In the above (1) or (2),

A microporous membrane made of polyolefin, characterized in that the melting peak area ratio of 141 ° C or higher observed from DSC is 25% or higher.

(4) In any one of (1) to (3) above,

A microporous membrane made of polyolefin, characterized in that the porosity is 30% or more.

(5) In any one of (1) to (4) above,

Polyolefin microporous membrane, characterized in that the MD + TD tensile breaking strength is 350 MPa or more.

(6) In any one of (1) to (5) above,

A polyolefin microporous membrane comprising polyethylene as a main component.

(7) In any one of (1) to (6) above,

A microporous membrane made of polyolefin, characterized in that the air permeation resistance in terms of a film thickness of 20 µm is 50 to 600 sec / 100 cc.

(8) In any one of (1) to (7) above,

A microporous membrane made of polyolefin, comprising a crystal nucleating agent.

(9) In any one of (1) to (8) above,

A polyolefin microporous membrane, wherein the polyolefin microporous membrane has a weight average molecular weight of 1.0 × 10 ⁶ or more and a polyethylene-containing ratio of 25% by mass or less.

(10) A battery separator comprising a polyolefin microporous membrane according to any one of (1) to (9) above.

(11) A secondary battery using the separator according to (10) above.

The microporous membrane made of polyolefin according to the embodiment of the present invention is a separator for lithium ion secondary batteries as compared to a conventional microporous membrane made of polyolefin, while maintaining basic performance, mechanical strength such as stiffness, shape retention properties under high temperature, and hole structure It becomes a microporous membrane excellent in uniformity. As a result, the battery capacity can be expected to be improved by thinning than the conventional polyolefin microporous membrane, and the possibility of short-circuiting is lowered by increasing the strength and improving the shape retention characteristics at high temperatures, so that the battery safety can also be expected to be improved. have.

1. Polyolefin microporous membrane

The present invention will be described in detail below. The microporous membrane made of polyolefin according to the embodiment of the present invention has a semi-crystallization time (T _1/2 ) of 126 ° C. or less and a puncture strength of 6.0 N / 20 μm in terms of 50% porosity and 20 μm film thickness. When the MD direction of the melt heat-shrinking stress is P _MD and the TD direction is P _TD , at least one is 0.8 MPa or less, and P _{MD + TD, which} is the sum of P _MD and P _TD , is 1.5 MPa or less.

The 126 ° C semi-crystallization time (T _1/2 ) in the polyolefin microporous membrane is 200 seconds or less, preferably 180 seconds or less, and more preferably 165 seconds or less. When the 126 ° C semi-crystallization time (T _1/2 ) exceeds 200 seconds, the crystal structure becomes non-uniform during crystallization, and the efficiency of stress distribution against the load on the microporous membrane made of polyolefin decreases, resulting in low mechanical strength. It is easy to become a sclera. Although the lower limit of the 126 ° C semi-crystallization time (T _1/2 ) is not particularly limited, it is preferably 1 second or more, and more preferably 10 seconds or more.

In order for the 126 ° C semi-crystallization time (T _1/2 ) to be _less than 1 second, it is necessary to add an excessive amount of a crystal nucleating agent, etc. As a result, it is not preferable because there is a possibility of adversely affecting economic efficiency, film properties and productivity. The 126 ° C semi-crystallization time (T _1/2 ) can be controlled by addition of a crystal nucleating agent, etc., and details of the control method will be described later.

The microporous membrane made of polyolefin according to the embodiment of the present invention (hereinafter sometimes referred to as microporous membrane) has a porosity in terms of 50% porosity and a film thickness of 20 µm, which is 6.0 N / 20 µm or more. More preferably, it is 6.5 N / 20 µm or more, more preferably 7.0 N or more, and most preferably 7.5 N / 20 µm or more. 50% porosity and film thickness When the puncture strength in terms of 20 µm is less than 6.0 N / 20 µm, the sharp portion of the coating material or electrode material protrudes on the microporous membrane, and cracks are likely to occur, or the distance between electrodes becomes shorter when thinned. In the case, a local short circuit is likely to occur.

Here, the porosity in 50% porosity and the film thickness of 20 µm refers to the puncture strength when the film thickness of the polyolefin microporous membrane having a porosity of 50% of the polyolefin microporous film is converted to 20 µm (hereinafter, referred to as “protrusion strength”). In other words, it may be abbreviated as 50% porosity and projected strength in terms of film thickness of 20㎛).

The higher the protruding strength in terms of 50% porosity and 20 µm film thickness, the better, but considering the limiting strength of the polyolefin microporous membrane, the upper limit of the microporous membrane according to the embodiment of the present invention is 30 N / 20 µm. The 50% porosity of the microporous membrane made of polyolefin according to the embodiment of the present invention and the protruding strength in terms of the film thickness of 20 µm are controlled by controlling the crystallization rate of the polyolefin mixture by adding a crystal nucleating agent, etc. It can adjust by controlling a crystal structure by extending | stretching conditions. Details of the control method will be described later.

In the microporous membrane made of polyolefin according to the embodiment of the present invention, when the MD direction (winding direction) of the melt heat-shrinking stress is P _MD and the TD direction (winding direction) is P _TD , at least one of one of the directions (P _MD , P _TD) ) Is 0.8 MPa or less. More preferably, it is 0.75 MPa or less, More preferably, it is 0.7 MPa or less, More preferably, it is 0.5 MPa or less, More preferably, it is 0.4 MPa or less. When P _MD and P _TD together exceed 0.8 MPa, it is not preferable because the likelihood of short-circuiting due to shrinkage increases at high battery temperatures. The lower the melt heat shrinkage stresses P _MD and P _TD , the better. However, in order to be less than 0.1 MPa, the film width may need to be excessively narrowed by a stress relaxation mechanism, or the film formation speed may need to be lowered. It is preferably 0.1 MPa or more. The melt heat shrinkage stresses P _MD and P _TD of the polyolefin microporous membrane according to the embodiment of the present invention can be adjusted by the molecular weight of the polyolefin mixture or the temperature or condition of stretching. Details of the control method will be described later.

In the microporous membrane made of polyolefin according to the embodiment of the present invention, P _{MD + TD, which} is the sum of P _MD and P _TD , is 1.5 MPa or less. It is more preferably 1.3 MPa or less, and still more preferably 1.2 MPa or less. When P _{MD + TD} exceeds 1.5 MPa, it is not preferable in the case of battery nail penetration test, etc., because the crack or hole shrinkage may increase and the degree of ignition may increase. P _{MD + TD, which} is the sum of the MD direction and the TD direction of the melt heat-shrinking stress, is better when it is low. However, in order to be less than 0.2 MPa, productivity may be lowered as described above. The P _{MD + TD} of the polyolefin microporous membrane according to the embodiment of the present invention can be adjusted by the molecular weight of the polyolefin mixture or the temperature and conditions of stretching. Details of the control method will be described later.

It is preferable that the microporous membrane made of polyolefin according to the embodiment of the present invention has a maximum pore diameter of 45 nm or less observed from a forometer. The maximum pore size is more preferably 42 nm or less, and even more preferably 40 nm or less. When the maximum pore size exceeds 45 nm, the possibility of non-uniformity in the cell reaction or the growth of dendrites increases, which is not preferable. When the maximum pore diameter is less than 10 nm, the air permeation resistance is remarkably increased, and thus may adversely affect the output of the battery, so it is preferably 10 nm or more. The maximum pore size of the microporous membrane made of polyolefin according to the embodiment of the present invention can be adjusted by controlling the crystallization rate of the polyolefin mixture by adding a crystal nucleating agent, etc., and controlling the crystal structure by miniaturizing crystals or by temperature or stretching conditions. . Details of the control method will be described later.

In the microporous membrane made of polyolefin according to the embodiment of the present invention, it is preferable that the ratio of the average flow pore diameter / maximum pore diameter observed from the forometer is 0.6 or more. The ratio of the average flow rate pore size / maximum pore size is more preferably 0.65 or more, more preferably 0.67 or more, still more preferably 0.7 or more, and still more preferably 0.73 or more. When the value of the average flow rate pore size / maximum pore size is less than 0.6, the tendency for coarse pores to exist inside the microporous membrane is increased, which is not preferable because the withstand voltage characteristics may be lowered or the battery reaction may be uneven. The ratio of the average flow rate pore size / maximum pore size is more preferable from the viewpoint of uniformity, but in order to be larger than 0.9, it is necessary to add an excessive amount of a crystal nucleating agent, and as a result, film properties and productivity may be adversely affected. It is not preferable, and 0.9 or less is preferable. The ratio of the average flow rate / maximum pore size of the polyolefin microporous membrane according to the embodiment of the present invention is to control the crystallization rate of the polyolefin mixture by adding a crystal nucleating agent, etc., and to refine the crystals or to determine the temperature or stretching conditions. It can be adjusted by controlling the structure. Details of the control method will be described later.

In the microporous membrane made of polyolefin according to the embodiment of the present invention, it is preferable that an area ratio of a melting peak of 141 ° C. or higher observed by DSC is 25% or more. The area ratio of the melting peak of 141 ° C or higher is more preferably 30% or higher, further preferably 33% or higher, and particularly preferably 35% or higher. If the area ratio of the melting peak of 141 ° C or higher is less than 25%, the strength of the crystal structure is insufficient, and the strength of the microporous membrane is lowered, which is undesirable. When the area ratio of the melting peak of 141 ° C or higher exceeds 70%, the crystallization component of the low melting point decreases, and the shutdown characteristics may deteriorate, which is not preferable, and 70% or less is preferable. The area ratio of the melting peak of 141 ° C or higher of the polyolefin microporous membrane according to the embodiment of the present invention controls the crystallization rate of the polyolefin mixture by adding a nucleating agent, etc., and refines the crystal structure by temperature or stretching conditions. It can be adjusted by controlling. Details of the control method will be described later.

The polyolefin microporous membrane according to the embodiment of the present invention preferably has a porosity of 30% or more. The porosity is more preferably 40% or more, still more preferably 43% or more, and particularly preferably 45% or more. The upper limit of the porosity of the microporous membrane made of polyolefin is preferably 70% or less, more preferably 60% or less from the viewpoint of improving the film strength and withstand voltage characteristics. When the porosity is less than 30%, it is not preferable because there is a fear of lowering the electrolyte content or secondary battery output. The porosity of the microporous membrane made of polyolefin according to the embodiment of the present invention can be adjusted by controlling the crystallization rate of the polyolefin mixture by adding a crystal nucleating agent, etc., and controlling the crystal structure by miniaturizing crystals or by temperature or stretching conditions. Details of the control method will be described later.

The polyolefin microporous membrane according to the embodiment of the present invention preferably has an MD + TD tensile breaking strength of 350 MPa or more. It is more preferably 370 MPa or more, and still more preferably 400 MPa or more. The higher the MD + TD tensile breaking strength, the better, but considering the basic performance of the polyolefin microporous membrane, the upper limit of the microporous membrane according to the embodiment of the present invention is 800 MPa or less. When the MD + TD tensile breaking strength is less than 350 MPa, it is not preferable because it is unable to withstand tensile stress due to winding in a coating or battery production process or compression in the winding width direction, and thus yields in production such as cracks and film breakage are likely to occur. The MD + TD tensile breaking strength of the microporous membrane made of polyolefin according to the embodiment of the present invention controls the crystallization rate of the polyolefin mixture by addition of a crystal nucleating agent, etc., and refines the crystal or controls the crystal structure by temperature or stretching conditions. I can adjust it. Details of the control method will be described later.

It is preferable that the microporous membrane made of polyolefin according to the embodiment of the present invention has an air permeation resistance of 50 to 600 sec / 100 cc in terms of a film thickness of 20 mu m. The air permeation resistance in terms of a film thickness of 20 µm is more preferably 50 to 400 sec / 100 cc, more preferably 50 to 300 sec / 100 cc, and particularly preferably 50 to 250 sec / 100 cc. When the air permeation resistance in terms of a film thickness of 20 μm is 600 sec / 100 cc or less, sufficient ion permeability is obtained and electrical resistance is reduced, which is preferable. In addition, if the air permeation resistance in terms of a film thickness of 20 µm is 50 sec / 100 cc or more, it is preferable because the ion permeability is excellent and the possibility of internal short circuit is reduced. The permeation resistance at the time of converting a microporous membrane made of polyolefin according to an embodiment of the present invention to a thickness of 20 µm controls the crystallization rate by adding a crystal nucleating agent or the like of a polyolefin mixture, and refines the crystallization temperature, stretching conditions, and heat setting. It can be adjusted by conditions. Details of the control method will be described later.

The thickness of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 1 to 2000 µm, more preferably 1 to 1000 µm.

The solid thermal shrinkage of MD and TD of the polyolefin microporous membrane according to the embodiment of the present invention is preferably 30% or less, more preferably 20% or less, and even more preferably 15% or less. When the individual heat shrinkage ratio of MD and TD is 30% or less, when the microporous membrane is used as a separator for a lithium battery, the possibility of short-circuiting may be reduced because the end of the separator does not shrink even when heated, which is preferable. The lower limit of the solid heat shrinkage is not particularly limited, but is preferably 0% or more (not expanding) because it can prevent defects such as wrinkles during heating in the coating process.

The polyethylene-containing ratio of the weight average molecular weight (Mw) of 1.0 × 10 ⁶ or more in the polyolefin microporous membrane according to the embodiment of the present invention is preferably 25% by mass or less, more preferably 1 to 20% by mass, and more preferably 1-15 mass%, most preferably 5-10 mass%. When the polyethylene-containing ratio of the weight average molecular weight of 1.0 × 10 ⁶ or more exceeds 25% by mass, mechanical strengths such as the protruding strength and tensile breaking strength of the microporous membrane may increase, while the melt shrinkage stress may increase. When the content of polyethylene having a weight average molecular weight of 1.0 × 10 ⁶ or more is less than 1% by mass, mechanical strengths such as the protruding strength and tensile breaking strength of the microporous membrane may be lowered. When the content of polyethylene having a weight average molecular weight of 1.0 × 10 ⁶ or more is included in the above range, a microporous membrane having high strength and excellent melt heat shrinkage stress can be obtained without impairing the productivity of the polyolefin microporous membrane.

The details of the preferred embodiment of the present invention will be described below. In addition, this invention is not limited to the following embodiment, It can be implemented by changing within the range of the summary.

The microporous membrane made of polyolefin according to the embodiment of the present invention is made of a mixture containing a polyolefin resin as a main component. Here, the main component in this application contains 50 mass% or more of polyolefin resin. And, it is preferably 70% by mass or more, more preferably 80% by mass or more, particularly preferably 90% by mass or more.

Hereinafter, the present invention will be described for each item.

(1) Polyolefin resin

As the polyolefin resin according to the embodiment of the present invention, polyethylene, polypropylene, poly (4-methyl-pentene-1), ethylene-propylene copolymer, polytetrafluoroethylene, polytetrafluoroethylene, polyvinylidene fluoride, polychloride Vinylidene, polyvinyl fluoride, polyvinyl chloride, polysulfone, and polycarbonate are exemplified.

The polyolefin resin may be a mixture of two or more polyolefins.

It is preferable that the said polyolefin resin contains a polyethylene resin. The content of the polyethylene resin is preferably 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 99% by mass or more in the polyolefin resin. When the ratio of the polyethylene resin in the polyolefin resin is within the above range, the strength of the resulting polyolefin microporous membrane can be improved.

As the polyethylene resin, a copolymer of (I) ethylene homopolymer or (II) ethylene and comonomers such as propylene, butene-1, and hexene-1, and mixtures thereof can be used.

Especially, it is preferable that it is ethylene homopolymer from a viewpoint of economical efficiency and film strength, and it is preferable that it is high density polyethylene whose weight average molecular weight (Mw) is 1x10 ⁴ or more and less than 1x10 ⁶ . As a molecular weight dispersion (MwD) of a polyethylene resin, 1-20 are preferable, for example, 3-10 are more preferable from a viewpoint of extrusion moldability and physical property control by stable crystallization control.

The content of the comonomer in the copolymer as the polyethylene resin is preferably 10 mol% or less based on 100 mol% of the copolymer. Such a copolymer can be produced by any favorable polymerization process, such as a process using a Ziegler Natta catalyst or a single site catalyst. The comonomer may be an A-olefin, for example, if desired, the comonomer is propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, octene-1, vinyl acetate, methyl methacrylate , One or more of styrene or other monomers.

Moreover, as the said polyethylene resin, you may contain 25 mass% or less of polyolefin resin in ultra high molecular weight polyethylene whose weight average molecular weight (Mw) is 1.0x10 ⁶ or more, More preferably, it is 20 mass% or less, More preferably, it is 15 mass. % Or less, and most preferably 10% by mass or less. When the content of the ultra-high molecular weight polyethylene having a weight average molecular weight of 1.0 × 10 ⁶ or more exceeds 25% by mass, mechanical strength such as protruding strength and tensile breaking strength of the microporous membrane may increase while melt heat shrinkage stress may increase. When the ultra-high molecular weight polyethylene having 1.0 × 10 ⁶ or more is included in the above range, a microporous membrane having high strength and excellent melt heat shrinkage stress can be obtained without impairing the productivity of the polyolefin microporous membrane.

The polyethylene resin may be either a single polyethylene or a mixture of two or more polyethylenes.

(2) Other resin components

The polyolefin resin may contain other resin components other than the polyethylene resin as necessary. The other resin component is preferably a heat-resistant resin, and as the heat-resistant resin, for example, a crystalline resin having a melting point of 150 ° C or higher (including partially crystalline resin), and / or a glass point transfer (TG) of 150 ° C. The above amorphous resin is exemplified. Here, TG is a value measured according to JIS K7121.

Specific examples of other resin components include polyester, polymethylpentene [PMP or TPX (transparent polymer X), melting point: 230 to 245 ° C], polyamide (PA, melting point: 215 to 265 ° C), polyallylene sulfide ( Fluorine-containing resins such as polyvinylidene fluoride homopolymers such as PAS) and polyvinylidene fluoride (PVDF), fluorinated olefins such as polytetrafluoroethylene (PTFE), and copolymers thereof; Polystyrene (PS, melting point: 230 ° C), polyvinyl alcohol (PVA, melting point: 220 to 240 ° C), polyimide (PI, Tg: 280 ° C or higher), polyamideimide (PAI, Tg: 280 ° C), polyether Sulfone (PES, Tg: 223 ° C), polyether ether ketone (PEEK, melting point: 334 ° C), polycarbonate (PC, melting point: 220 to 240 ° C), cellulose acetate (melting point: 220 ° C), cellulose triacetate (melting point) : 300 ° C), polysulfone (Tg: 190 ° C), polyetherimide (melting point: 216 ° C), and the like. The resin component is not limited to being composed of a single resin component, and may be composed of a plurality of resin components. The preferred weight average molecular weight (Mw) of other resin components varies depending on the type of resin, but is generally 1 × 10 ^{3 to} 1 × 10 ⁶ , and more preferably 1 × 10 ⁴ to 7 × 10 ⁵ . Further, the content of other resin components in the polyolefin resin is suitably adjusted within a range not departing from the gist of the present invention, but is contained within a range of approximately 10% by mass or less of the polyolefin resin.

Moreover, as another resin component, you may contain the polyolefin other than the said polyethylene as needed, and polybutene-1 polybutene-1, polypentene-1, polyhexene whose Mw is 1.0x10 <^4> -4.0x10 <^6> . You may use at least 1 sort (s) selected from the group which consists of -1, polyethylene wax whose polyoctene-1 and Mw are 1.0x10 <^3> -1.0x10 <^4> .

The content of the polyolefin other than the polyethylene can be appropriately adjusted within a range that does not impair the effects of the present invention, but in the polyolefin resin, preferably 10% by mass or less, more preferably less than 5% by mass, and 0% by mass It is more preferable.

(3) Crystalline nucleating agent

It is preferable that the microporous membrane of this embodiment contains a crystal nucleating agent.

The crystal nucleating agent that can be used for the microporous membrane of the present embodiment is not particularly limited, and general compound-based and microparticle-based crystal nucleating agents used for polyolefin resins can be used. The crystal nucleating agent may be a master batch in which the crystal nucleating agent is previously mixed and dispersed in a polyolefin resin.

There is no particular limitation on the method for mixing the above-described crystal nuclei with the polyolefin resin, and it may be mixed with a polyolefin resin raw material or a film forming agent solvent in advance before melt-kneading, or may be introduced and mixed during melt-kneading.

The blending amount of the crystal nucleating agent is not particularly limited, but the upper limit is preferably 10 parts by mass or less with respect to 100 parts by mass of the polyolefin resin, more preferably 5 parts by mass or less, and the lower limit is 0.00001 parts by mass with respect to 100 parts by mass of the polyolefin resin. The above is preferable, and 0.0001 parts by mass or more is more preferable. If the amount of the crystal nucleating agent is within the above range, good dispersibility in the polyolefin resin, good handling workability in the manufacturing process, and economical efficiency can be expected.

[Compound-based crystal nucleating agent]

Examples of the compound-based crystal nucleating agent include cyclic hydrocarbon carbohydrates such as sodium benzoate, aluminum 4-tert-butylbenzoate, sodium adipic acid and bisodium bicyclo [2.2.1] heptan-2,3-dicarboxylate. Aliphatic carboxylic acid metal salts such as carboxylic acid metal salts, sodium laurate and zinc stearate, sodium bis (4-tert-butylphenyl) phosphate, sodium-2,2'-methylenebis (4,6-di-tert-butylphenyl) ) Acetal skeletons such as dibenzylidene sorbitol, bis (methylbenzylidene) sorbitol, and bis (dimethylbenzylidene) sorbitol. It is possible to use compounds having. Moreover, it is preferable to use an aromatic phosphate ester metal salt and an aliphatic metal salt from a viewpoint of strength improvement.

[Particulate magnetic nucleating agent]

As the microparticle-based nucleating agent, for example, a microparticle-based crystal nucleating agent such as silica or alumina can be used.

As a commercially available crystal nucleating agent, for example, "Gel All D" (manufactured by New Japan Chemical Co., Ltd.), "ADEKA STAB" (manufactured by ADEKA CORPORATION), "HYPERFORM" (manufactured by Milliken Chemical) "Pine Crystal" ( Arakawa Chemical Industries, Ltd.) or "IRGACLEAR D" (manufactured by Ciba Specialty Chemicals) or the like can be used. Further, as the polyethylene resin masterbatch in which the crystal nucleating agent is blended, "Rikemaster" (manufactured by Riken Vitamin Co., Ltd.) is commercially available, for example.

(4) Other additives

In addition, antioxidants (for example, tetrakis [methylene-3- (3,5-di-tertarybutyl-4-hydroxyphenyl) -propionate] methane, etc.) are necessary for the polyolefin resin as described above. , UV absorbers, pigments, dyes and other additives can be blended within a range that does not impair the object of the present invention.

When adding an additive other than a crystal nucleating agent to the polyolefin resin, the blending amount is preferably 0.01 parts by mass to 10 parts by mass relative to 100 parts by mass of the polyolefin resin. This is because a sufficient effect may not be obtained when the content is less than 0.01 parts by mass, it may be difficult to control the amount of addition at the time of manufacture, or it may be inferior to the economic efficiency when it exceeds 10 parts by mass.

2. Manufacturing method of microporous membrane made of polyolefin

The method for producing the microporous membrane made of polyolefin according to the embodiment of the present invention is not particularly limited as long as it can produce a microporous membrane made of polyolefin having the above-described characteristics, and a conventionally known method can be used. For example, Japanese Patent No. 2132327 And Japanese Patent No. 3347835, International Patent Publication 2006/137540, and the like. Specifically, it is preferable to include the following steps (1) to (5), and may further include the following steps (6), and may also include the following steps (7) and / or (8).

(1) A step of melt-kneading the polyolefin resin, a crystal nucleating agent, and a film forming solvent and preparing a polyolefin resin composition.

(2) Process of extruding and cooling the polyolefin resin composition to form a gel-like sheet

(3) First stretching step of stretching the gel sheet

(4) Step of removing solvent for film formation from gel sheet after stretching

(5) Step of drying sheet after removal of solvent for film formation

(6) A second stretching step for stretching the dried sheet.

(7) Process of heat-treating the sheet after drying

(8) A step of crosslinking and / or hydrophilizing the sheet after the stretching step.

Hereinafter, each process is demonstrated.

(1) Process for preparing polyolefin resin composition

After mixing the polyolefin resin with a crystal nucleating agent and a suitable film-forming solvent, the mixture is melt-kneaded to prepare a polyolefin resin composition. As the melt kneading method, for example, a method using a twin-screw extruder described in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is known, the description is omitted.

In the polyolefin resin composition, the blending ratio of the polyolefin resin and the solvent for forming a film is not particularly limited, but is preferably 50 to 80 parts by mass of a film forming solvent relative to 20 to 50 parts by mass of a polyolefin resin. More preferably, it is 60 to 75 parts by mass of the film forming solvent relative to 25 to 40 parts by mass of the polyolefin resin.

The amount of the crystal nucleating agent to be blended in the polyolefin resin composition is as described above.

(2) Formation process of gel sheet

The polyolefin resin composition is fed from the extruder to the die, and extruded into a sheet shape. A plurality of polyolefin resin compositions of the same or different composition may be fed from an extruder to a die, laminated thereon in layers, and extruded in a sheet form.

The extrusion method may be either a flat die method or an inflation method. The extrusion temperature is preferably 140 to 250 ° C, and the extrusion speed is preferably 0.2 to 15 m / min. The film thickness can be adjusted by controlling the amount of each extrusion of the polyolefin resin composition.

As the extrusion method, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

A gel-like sheet is formed by cooling the obtained extruded body. As a method for forming a gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of at least 50 ° C / min up to at least the gelation temperature, more preferably at least 100 ° C / min, more preferably at least 150 ° C / min. Cooling of the gel sheet is preferably performed at 50 ° C or lower, more preferably 40 ° C or lower, more preferably 30 ° C or lower, and particularly preferably 20 ° C or lower. The polyolefin composition according to the embodiment of the present invention is capable of increasing the structural uniformity of the gel sheet by cooling the gel sheet under conditions within the above range, in addition to increasing the crystallization rate by the addition of a crystal nucleating agent, etc. It becomes possible to promote further strength improvement in the stretching process of the.

(3) 1st stretching process

Next, the obtained gel-like sheet is stretched in at least one axial direction. Since the gel sheet contains a solvent for film formation, it can be stretched uniformly. After heating, it is preferable to stretch the gel sheet at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

In the case of uniaxial stretching, the stretching ratio (area stretching ratio) in this step is preferably 5 times or more, and more preferably 10 to 100 times. In the case of biaxial stretching, 30 times or more is preferable, 45 times or more is more preferable, and 75 times or more is particularly preferable. Moreover, 5 times or more is preferable in both the length and the transverse direction (MD and TD direction), and the draw ratios in the MD direction and the TD direction may be the same or different. When the draw ratio is less than 30 times, mechanical strength may be lowered, which is not preferable. Moreover, when the draw ratio becomes 150 times or more, the possibility of rupture increases, which is not preferable. In addition, the draw ratio in this process means the area draw ratio of the microporous membrane immediately before being provided in the next step based on the microporous membrane immediately before this step.

The stretching temperature in this step is preferably within the range of the crystal dispersion temperature (TCD) to TCD + 30 ° C of the polyolefin resin, and within the range of the crystal dispersion temperature (TCD) + 5 ° C to the crystal dispersion temperature (TCD) + 28 ° C. It is more preferable to do it, and it is particularly preferable to be within the range of TCD + 10 ° C to TCD + 26 ° C. When the stretching temperature is within the above range, breakage due to polyolefin resin stretching is suppressed, and high-magnification stretching is possible.

The crystal dispersion temperature (TCD) is obtained by measuring the temperature characteristics of dynamic viscoelasticity according to ASTM D4065. Since ultra high molecular weight polyethylene, polyethylene other than ultra high molecular weight polyethylene, and polyethylene compositions have a crystal dispersion temperature of about 90 to 100 ° C, the stretching temperature is preferably 90 to 130 ° C, more preferably 110 to 120 ° C, More preferably, it is set at 114 to 117 ° C.

By the above-described stretching, cleavage occurs between the polyethylene lamellae, and the polyethylene phase is refined to form a large number of fibrils. Fibrils form a three-dimensional irregularly connected network structure.

(4) Removal of solvent for film formation

The solvent for film formation is removed (washed) using a cleaning solvent. Since the polyolefin phase is phase-separated from the solvent phase for film formation, the removal of the solvent for film formation consists of fibrils forming a fine three-dimensional network structure, and a porous membrane having pores (voids) communicating irregularly in three dimensions is obtained. The method of removing the cleaning solvent and the solvent for forming a film using the same is well known, and thus the description is omitted. For example, the method disclosed in Japanese Patent No. 2132327 or Japanese Patent Laid-Open No. 2002-256099 can be used.

(5) Dry

The microporous membrane from which the solvent for film formation is removed is dried by a heating drying method or an air drying method. The drying temperature is preferably equal to or less than the crystal dispersion temperature (TCD) of the polyolefin resin, and particularly preferably 5 ° C or more lower than TCD. Drying is preferably performed with the microporous membrane being 100 parts by mass (dry weight) until the residual cleaning solvent is 5 parts by mass or less, and more preferably 3 parts by mass or less.

(6) 2nd stretching process

It is preferable to stretch the microporous membrane after drying in at least one axial direction. The extending | stretching of a microporous membrane can be performed by a tenter method, a roll method, etc. similarly to the above, while heating. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used.

Although the extending | stretching temperature in this process is not specifically limited, Usually, it is 90-135 degreeC, More preferably, it is 95-130 degreeC.

The draw ratio (area draw ratio) in the uniaxial direction of the stretching of the microporous membrane in this step is preferably a lower limit of 1.0 times or more, more preferably 1.1 times or more, still more preferably 1.2 times or more. Moreover, it is preferable to make an upper limit into 5.0 times or less. In the case of uniaxial stretching, the stretching ratio is 1.0 to 5.0 times in the MD direction or the TD direction. In the case of biaxial stretching, the area draw ratio is preferably a lower limit of 1.0 times or more, more preferably 1.1 times or more, still more preferably 1.2 times or more. The upper limit is preferably 16.0 times or less, 1.0 to 4.0 times in the MD direction and the TD direction, respectively, and the stretching ratios in the MD direction and the TD direction may be the same or different. In addition, the draw ratio in this process means the thing of the draw ratio of the microporous membrane immediately before being provided to the next process based on the microporous membrane immediately before this process.

(7) Heat treatment

In addition, the microporous membrane after drying can be heat treated. The crystal is stabilized by heat treatment, and the lamella is homogenized. As a heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is a heat treatment that heats while maintaining the membrane dimensions unchanged. The thermal relaxation treatment is a heat treatment that heat-shrinks the film in the MD or TD direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as the heat relaxation treatment method, as disclosed in Japanese Patent Laid-Open No. 2002-256099, it is possible to reduce the melt shrinkage stress in each direction by performing a reduction of 0.95 times or less in the MD direction or the TD direction. desirable. However, it is not preferable because shrinkage of 0.7 times or less tends to cause looseness of the film. The heat treatment temperature is preferably in the range of TCD to TM of the polyolefin resin, more preferably in the range of the stretching temperature ± 5 ° C of the microporous membrane, and particularly preferably in the range of the second stretching temperature of ± 3 ° C of the microporous membrane.

(8) Crosslinking treatment, hydrophilization treatment

In addition, crosslinking treatment and hydrophilization treatment may be further performed on the microporous membrane after bonding or stretching.

For example, crosslinking treatment is performed by irradiating the microporous membrane with ionizing radiation such as A, B, Γ, and electron beams. In the case of irradiation with electron beams, an electron dose of 0.1 to 100 MRAD is preferred, and an acceleration voltage of 100 to 300 KV is preferred. The meltdown temperature of the microporous membrane is increased by the crosslinking treatment.

In addition, hydrophilization treatment can be performed by monomer graft, surfactant treatment, corona discharge, or the like. It is preferable to perform monomer graft after crosslinking treatment.

3. Laminated microporous membrane

A porous layer may be formed by forming a porous layer on at least one surface of the polyolefin microporous membrane. As the porous layer, for example, a porous layer formed using a filler-containing resin solution or a heat-resistant resin solution containing a filler and a resin binder can be exemplified.

4. Battery separator

The microporous membrane made of polyolefin according to the embodiment of the present invention can be suitably used for either a battery using an aqueous electrolyte or a battery using a non-aqueous electrolyte. Specifically, it can be preferably used as a separator for secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium secondary batteries, and lithium polymer secondary batteries. Especially, it is preferable to use it as a separator of a lithium ion secondary battery.

In a lithium ion secondary battery, a positive electrode and a negative electrode are stacked through a separator, and the separator contains an electrolysis solution (electrolyte). The structure of the electrode is not particularly limited, and a conventionally known structure can be used. For example, the electrode structure (coin type), which is disposed as opposed to the disk-shaped positive electrode and the negative electrode, and the plate-shaped positive electrode and the negative electrode alternately It can be made into a stacked electrode structure (stacked type), an electrode structure in which a stacked strip-shaped positive electrode and a negative electrode are wound (wound type), or the like.

The current collector, the positive electrode, the positive electrode active material, the negative electrode, the negative electrode active material, and the decomposition liquid used in the lithium ion secondary battery are not particularly limited and can be used by appropriately combining conventionally known materials.

In addition, this invention is not limited to the said embodiment, It can be implemented by various modification within the scope of the summary.

Example

The present invention will be described in more detail by examples, but the embodiments of the present invention are not limited to these examples.

In addition, each method and material of the evaluation method and analysis used in the Example are as follows.

(1) Film thickness (㎛)

The film thickness of 5 points in the range of 95 mm x 95 mm of the microporous membrane was measured with a contact thickness meter (LITEMATIC manufactured by Mitutoyo Corporation), and the average value of the film thickness T was obtained.

(2) Public rate (%)

Samples of 5 cm x 5 cm were cut out from the microporous membrane, the volume (cm 3) and weight (g) were determined, and calculated from them and the polymer density (g / cm 3) using the following equation. The above measurements were performed at three points in different places in the same microporous membrane to obtain an average porosity value.

Porosity = [(volume-weight / polymer density) / volume] × 100

(3) Speculation resistance (sec./100cc)

For the microporous membrane having a film thickness (T1) (µm), the gas permeation resistance (G1) measured by a gas permeation resistance meter (EGO-1T, manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P 8117 (2009) (sec./100cc) was converted into the speculative resistance (G2) when the film thickness was converted to 20 µm by the formula: G2 = (G1 × 20) / T1. The above measurements were performed at three points in different locations in the same microporous membrane to determine the average value of the air permeation resistance (G2).

(4) Maximum hole diameter and average flow hole diameter (nm)

The following measurements were performed at three points in different locations in the same microporous membrane to determine the average value of the maximum pore diameter and the average pore diameter / maximum pore diameter. The maximum pore diameter and the average flow pore diameter were measured in the order of DRY-UP and WET-UP using a permometer (CFP-1500A manufactured by PMI). In the WET-UP, the pore diameter converted from the pressure at which air starts to penetrate by applying pressure to the microporous membrane sufficiently immersed by GALWICK (trade name) manufactured by PMI, whose surface tension is already known, is called the maximum pore diameter.

For the average flow hole diameter, the hole diameter was converted from the pressure at the point at which the curve of 1/2 of the pressure and the flow curve curve crosses the curve of the WET-UP measurement by DRY-UP measurement. The following formula was used for the conversion of pressure and pore diameter.

D = C · Γ / P

(In the above formula, "D (μm)" was the pore diameter of the microporous membrane, "Γ (mN / m)" was the surface tension of the liquid, "P (Pa)" was the pressure, and "C" was the constant.

(5) 50% porosity and film thickness 20㎛ equivalent projection strength (N / 20㎛)

A microporous membrane having a film thickness (T1) (µm) and a porosity (P1) (%) is formed by a needle having a diameter of 1 mm with a spherical surface (curvature radius (R): 0.5 mm) using a stone magnetic field manufactured by MARUBISHI. The maximum load when struck at a speed of mm / sec was measured. The maximum load when the film thickness is 20 µm and the porosity is 50% by the formula L2 = (L1 × 20) / T1 × 50 / (100-P1) is the measured value (L1) (N) of the maximum load. It was converted into (L2), and it was set as the protruding strength in 50% porosity and the film thickness conversion of 20 micrometers. The above measurements were performed at three points in different locations in the same microporous membrane to obtain an average value of 50% porosity and puncture strength in terms of film thickness of 20 µm.

(6) Weight average molecular weight (Mw) and polyethylene content ratio (mass%) of Mw 1.0 × 10 ⁶ or more in a polyolefin microporous membrane

The weight average molecular weight of the microporous membrane made of UHMwPE, HDPE and polyolefin was determined by gel permeation chromatography (GPC) under the following conditions.

Measuring device: GPC-150C made by WATERS CORPORATION

Column: SHODEX UT806M manufactured by Showa Denko K. K.

Column temperature: 135 ℃

Solvent (mobile phase): O-dichlorobenzene

Solvent flow rate: 1.0 ml / min

Sample concentration: 0.1 wt% (dissolution conditions: 135 ° C / 1H)

· Injection amount: 500 µl

Detector: Differential reflectometer (RI detector) made by WATERS CORPORATION

-Calibration curve: It was prepared using a predetermined conversion constant from a calibration curve obtained using a monodisperse polystyrene standard sample.

The polyethylene content ratio of Mw 1.0 × 10 ⁶ or more in the polyolefin microporous membrane was determined from the measurement results of the weight average molecular weight of the polyolefin microporous membrane in the above method.

(7) 126 ℃ semi-crystallization time (T _1/2 ) (sec)

In the following method, three points were measured at different points in the same microporous membrane, and the average value was set to 126 ° C semi-crystallization time (T _1/2 ). The 126 ° C semi-crystallization time (T _1/2 ) was measured by the following method. The microporous membrane made of polyolefin was sealed in a measuring pan, heated up to 230 ° C using a PYRIS DIAMOND DSC manufactured by PARKING ELMER, completely melted, and then kept at 230 ° C for 10 minutes. Then, the temperature was lowered to 126 ° C at 30 ° C / min and maintained at 126 ° C. The time change of the amount of heat after entering isothermal control at 126 ° C was recorded, and the time at which the peak area was halved was defined as the 126 ° C semi-crystallization time (T _1/2 ).

(8) Crystal melting peak area ratio (%) of 141 ° C or higher

The measurement was carried out in three points at different points in the same microporous membrane by the following method to make the area ratio of the crystal melting peak at 141 ° C or higher. A microporous membrane made of polyolefin was sealed in a measuring pan, and heated to 230 ° C using a PYRIS DIAMOND DSC manufactured by PARKING ELMER to measure the crystal melting peak. The ratio of the amount of heat of fusion of the whole of the obtained crystal melting peak and the amount of heat of fusion of 141 ° C or higher was defined as the crystalline melting peak area ratio of 141 ° C or higher (melting peak area ratio of 141 ° C or higher).

(9) MD and TD solid heat shrinkage (%)

The solid thermal contraction rate of the microporous membrane was a microporous membrane cut to 95 mm x 95 mm and heated at 105 ° C. for 8 hours, and the rate of change in the dimensions of MD and TD of the microporous membrane before and after heating was used. The above measurements were performed at three points in different locations in the same microporous membrane to obtain average values of MD and TD solid heat shrinkage.

(10) Melt heat shrink stress (MPa)

Using a thermomechanical analysis device (TMA / SS 6100 manufactured by Seiko Instruments), the temperature was increased and scanned to measure the shrinkage load (mN). Measurement conditions include sample shape; 3 mm wide x 10 mm long, initial load: 9.8 mN, temperature scan range 30-200 ° C, temperature increase rate; 10 ° C / min. Measurement was performed in both MD and TD, and the melt heat shrinkage stress (P) was calculated by substituting the maximum shrinkage load (N) (mN) of 130 ° C or higher in the obtained shrinkage load curve into the following equation. The above measurements were made for MD and TD by measuring each of three points at different locations in the same microporous membrane, and the average values thereof were taken as MD melt heat shrink stress and TD melt heat shrink stress, respectively, and their sums were taken as MD + TD melt heat shrink stress. .

P = (N-initial load) / initial cross-sectional area

(11) MD + TD tensile breaking strength (MPa)

Tensile testing was performed using a tensile tester (Shimazu autograph AGS-J type), and the strength at the time of sample blocking was divided by the sample cross-section before the test to obtain tensile breaking strength (MPa). Measurement conditions include temperature; 23 ± 2 ° C, sample shape; 10 mm wide x 50 mm long, distance between chucks; 20 mm, tensile speed; It is 100 mm / min. The above measurement was performed for each of three points at different locations in the same microporous membrane for MD and TD, and the sum of the average values of MD tensile breaking strength and TD tensile breaking strength was taken as MD + TD tensile breaking strength.

(12) Breakdown voltage test

In order to evaluate the withstand voltage between electrodes, a breakdown voltage test was conducted as follows.

Kikusui Electronics Co., Ltd. placed a microporous membrane cut into a circular shape with a diameter of 60 mm on a square aluminum plate of 150 mm on one side, and a cylindrical electrode having a diameter of 50 mm, a height of 30 mm, and a weight of 500 g on it. The TOS5051A dielectric breakdown resistance tester was connected. A voltage was applied at a step-up speed of 0.2 kV / sec, and the voltage V1 when the microporous membrane having a film thickness (T1) (µm) and a porosity (P1) (%) was dielectrically destroyed was read. The measured value V1 (kV) was converted into the breakdown voltage V2 at the unit film thickness when the porosity was 50% by the formula: V2 = V1 / T1 x 50 / (100-P1). The breakdown voltage was measured 15 times each to obtain an average value. When the average value was 0.15 kV / µm or more, the possibility of local short-circuiting was lowered, and therefore, it was said to be x when it was less than 0.15 kV / µm.

(13) Battery shock short circuit test

In order to evaluate the short circuit resistance when an external shock was applied to the battery, a cylindrical battery was produced according to the following procedure, and an impact short circuit test was conducted.

<Production of the positive electrode>

92.2 mass% of lithium cobalt complex oxide LiCoO ₂ as the active material, 2.3 mass% of graphite and acetylene black, respectively, as a conductive agent, 3.2 mass% of polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) Dispersion was carried out to prepare a slurry. This slurry was applied by a die coater to one side of an aluminum foil having a thickness of 20 µm to be a positive electrode current collector with an active material coating amount of 250 g / m 2 and an active material bulk density of 3.00 g / cm 3. Then, the mixture was dried at 130 ° C. for 3 minutes, compression molded by a roll press machine, cut into a width of about 57 mm to form a strip.

<Production of the negative electrode>

A slurry was prepared by dispersing 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethylcellulose as a binder, and 1.7% by mass of styrene-butadiene copolymer latex in purified water. This slurry was applied by a die coater to one side of a copper foil having a thickness of 12 µm, which serves as a negative electrode current collector, with a high filling density of 106 g / m 2 of active material applied and a bulk density of 1.55 g / cm 3 of active material. Then, the mixture was dried at 120 ° C for 3 minutes, compression molded by a roll press machine, and cut into a width of about 58 mm to form a strip.

<Preparation of non-aqueous electrolyte solution>

It was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 1/2 (volume ratio) to a concentration of 1.0 mol / l.

<Separator>

The separators described in Examples and Comparative Examples were slit to 60 mm to form a strip.

<Battery assembly>

An electrode plate laminate was produced by overlapping in the order of a strip-shaped negative electrode, a separator, a strip-shaped positive electrode, and a separator, and winding a plurality of times in a vortex shape with a winding tension of 100 gf. The electrode plate laminate was stored in a stainless steel container having an outer diameter of 18 mm and a height of 65 mm, an aluminum tab derived from the positive electrode current collector was placed in the container lid terminal portion, and a nickel tab derived from the negative electrode current collector was placed in the container wall. Welded on. Then, after drying at 80 ° C under vacuum for 12 hours, the non-aqueous electrolyte was injected into a container in an argon box and sealed.

<Pretreatment>

The assembled battery was charged with a constant current of 1 / 3C to a voltage of 4.2V, and then charged with a constant voltage of 4.2V for 5 hours, and then discharged to a final voltage of 3.0V with a current of 1 / 3C. Subsequently, a constant current was charged to a voltage of 4.2 V at a current value of 1 C, and then a constant voltage charging of 4.2 V was performed for 2 hours, and then discharged to a final voltage of 3.0 V at a current of 1 C. Finally, after constant current charging up to 4.2V with a current value of 1C, constant voltage charging of 4.2V was performed for 2 hours to prepare as a pretreatment.

<Shock short circuit test>

After 20 batteries were produced by the above method, an impact test was conducted on each of them to evaluate impact resistance (safety) based on the number of the short-circuited batteries. First, the prepared battery was heated at 105 ° C for 1 hour, freely dropped from a height of 2 m, and then left for 10 minutes. Thereafter, the terminal voltage was measured, and it was judged that the terminal voltage was less than 90% before the test in the "short circuit" state. Then, the measurement results were counted, and the smaller the number of short-circuited batteries, the better the impact resistance was. The evaluation criteria are as follows.

If the number of short-circuited batteries is 0 to 4 out of 20: ○

5 to 10 of 20 short-circuited: △

Short circuit of 11 or more of 20: ×

[Example 1]

Tetrakis of 100 parts by weight of a polyethylene (PE) composition consisting of 10 parts by weight of ultra high molecular weight polyethylene (UHMwPE) having a weight average molecular weight (Mw) of 2.0 × 10 ⁶ and 90 parts by weight of high density polyethylene (HDPE) having an Mw of 2.8 × 10 ⁵ [Methylene-3- (3,5-di-tertarybutyl-4-hydroxyphenyl) -propionate] 0.375 parts by mass of methane, and masterbatch Rikemaster CN-002 (manufactured by Riken Vitamin Co., Ltd .: nuclear agent Content about 2 mass%) 1 mass part was dry-blended to obtain a mixture.

30 parts by mass of the mixture containing the obtained polyolefin resin was introduced into a twin-screw extruder of a steel kneading type, 70 parts by mass of liquid paraffin was supplied from a side feeder of a twin-screw extruder, and 210 ° C was maintained while maintaining the screw rotation speed (Ns) at 200 rpm. A polyethylene resin composition was prepared by melt-kneading at a temperature.

The obtained polyethylene resin composition was supplied to a T-die from a twin-screw extruder, cooled at a rate of 180 ° C / min, and extruded into a sheet-shaped molded body. The extruded molded body was cooled while being pulled out by a cooling roll temperature-regulated to 20 ° C to form a gel-like sheet. The obtained gel-like sheet was simultaneously biaxially stretched at a stretching temperature of 115 ° C so as to be 9 times MD and 9 times TD. The membrane after stretching was washed in a washing tank of methylene chloride temperature-adjusted to 25 ° C to remove liquid paraffin. The washed film was dried in a drying furnace adjusted to 60 ° C, and heat-treated at 125 ° C for 40 seconds in a tenter to obtain a microporous membrane made of polyolefin having a thickness of 7 µm. Table 1 shows the properties of the obtained microporous membrane.

[Example 2]

A microporous membrane made of a polyolefin having a thickness of 12 µm was obtained in the same manner as in Example 1 except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to be 7 times in MD and 7 times in TD. Table 1 shows the properties of the obtained microporous membrane.

[Example 3]

Tetrakis [methylene-3- (3,5-di-tertiary butyl-) with respect to 100 parts by mass of a polyethylene (PE) composition composed of 100 parts by mass of high density polyethylene (HDPE) having a weight average molecular weight (Mw) of 2.8 x 10 ⁵ 4-hydroxyphenyl) -propionate] methane 0.375 parts by mass, and the master batch Rikemaster CN-002 (manufactured by Riken Vitamin Co., Ltd .: nucleating agent content: about 2% by mass) 1 part by dry blend to obtain a mixture. Then, 35 parts by mass of the obtained mixture was put into a twin-screw extruder of a steel kneading type, 65 parts by mass of liquid paraffin was supplied from a side feeder of a twin-screw extruder, and melted at a temperature of 210 ° C while maintaining the screw rotation speed (Ns) at 200 rpm. A microporous membrane made of a polyolefin having a thickness of 7 µm was obtained in the same manner as in Example 1 except that the polyethylene resin composition was kneaded to prepare a polyethylene resin composition. Table 1 shows the properties of the obtained microporous membrane.

[Example 4]

A polyolefin microporous membrane having a thickness of 7 µm was obtained in the same manner as in Example 1 except that 100 parts by mass of a polyethylene (PE) composition composed of 100 parts by mass of high density polyethylene (HDPE) having a weight average molecular weight (Mw) of 2.8 × 10 ^{5 was} used. Table 1 shows the properties of the obtained microporous membrane.

[Example 5]

A polyolefin microporous membrane having a thickness of 12 µm was obtained in the same manner as in Example 3, except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to be 7 times in MD and 7 times in TD. Table 1 shows the properties of the obtained microporous membrane.

[Example 6]

Simultaneously biaxially stretching the gel sheet of the polyethylene resin composition to 7 times with MD and 7 times with TD, and after removing the liquid paraffin, the washed membrane was dried in a drying furnace adjusted to 60 ° C, and at 130 ° C in a tenter. After stretching in the MD direction at 1.21 times, stretching at 1.21 times in the TD direction, shrinking by 0.9 times in the TD direction, and heat treatment for 40 seconds, the same procedure as in Example 1 was carried out to obtain a microporous membrane made of polyolefin having a thickness of 11 µm. Table 1 shows the properties of the obtained microporous membrane.

[Comparative Example 1]

Master batch Rikemaster CN-002 (manufactured by Riken Vitamin Co., Ltd.), Example 4 and other than the simultaneous biaxial stretching so that the gel sheet of the polyethylene resin composition 5 times with MD and 5 times with TD Similarly, a microporous membrane made of polyolefin having a thickness of 20 µm was obtained. Table 2 shows the properties of the obtained microporous membrane.

[Comparative Example 2]

A microporous membrane made of a polyolefin having a thickness of 7 µm was obtained in the same manner as in Comparative Example 1 except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to be 7 times in MD and 7 times in TD. Table 2 shows the properties of the obtained microporous membrane.

[Comparative Example 3]

Masterbatch Rikemaster CN-002 (manufactured by Riken Vitamin Co., Ltd.) was used in the same manner as in Example 1, except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to 5 times with MD and 5 times with TD. Thus, a microporous membrane made of polyolefin having a thickness of 20 µm was obtained. Table 2 shows the properties of the obtained microporous membrane.

[Comparative Example 4]

A microporous membrane made of a polyolefin having a thickness of 20 µm was obtained in the same manner as in Example 1 except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to be 5 times in MD and 5 times in TD. Table 3 shows the properties of the obtained microporous membrane.

[Comparative Example 5]

Using 100 parts by weight of a polyethylene (PE) composition comprising 30 parts by weight of ultra high molecular weight polyethylene (UHMwPE) having a weight average molecular weight (Mw) of 2.0 × 10 ⁶ and 70 parts by weight of high density polyethylene (HDPE) having an Mw of 2.8 × 10 ⁵ . Except for Comparative Example 3, a microporous membrane made of polyolefin having a thickness of 20 µm was obtained. Table 3 shows the properties of the obtained microporous membrane.

[Comparative Example 6]

A microporous membrane made of a polyolefin having a thickness of 12 µm was obtained in the same manner as in Comparative Example 5, except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to 9 times in MD and 9 times in TD. Table 3 shows the properties of the obtained microporous membrane.

[Comparative Example 7]

Tetrakis of 100 parts by weight of a polyethylene (PE) composition consisting of 30 parts by weight of ultra high molecular weight polyethylene (UHMwPE) having a weight average molecular weight (Mw) of 4.15 × 10 ⁶ and 70 parts by weight of high density polyethylene (HDPE) having an Mw of 5.6 × 10 ⁵ . [Methylene-3- (3,5-di-tertarybutyl-4-hydroxyphenyl) -propionate] 0.375 parts by mass of methane was dry blended to obtain a mixture. A polyethylene solution was prepared by dissolving it in a mixed solvent of liquid paraffin (Moleco Corporation Sumoyle P-350: boiling point 480 ° C) and decalin (Wako Pure Chemical Corporation, boiling point 193 ° C) so that the polyethylene concentration was 30% by mass. The composition of the polyethylene solution is polyethylene: fluid paraffin: decalin = 30: 67.5: 2.5 (mass ratio). The polyethylene solution was extruded from the die at 148 ° C, and cooled in a water bath to produce a gel-like sheet. At this time, the gel sheet was extruded from the die, and then cooled to a cooling rate of 90 ° C / min. The said gel sheet was extended | stretched by biaxial stretching which performs longitudinal stretching and transverse stretching sequentially. Here, the longitudinal stretching was 6 times the stretching ratio, the stretching temperature was 90 ° C, the lateral stretching was 9 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 135 ° C. Subsequently, this was immersed in a methylene chloride bath, and liquid paraffin and decalin were extracted. Then, it dried at 50 degreeC and annealed at 120 degreeC, and the microporous membrane made from polyolefin was obtained. Table 3 shows the properties of the obtained microporous membrane.

[Comparative Example 8]

A microporous membrane made of polyolefin having a thickness of 11 µm was obtained in the same manner as in Example 6 except that the master batch Rikemaster CN-002 (manufactured by Riken Vitamin Co., Ltd.) was not blended. Table 4 shows the properties of the obtained microporous membrane.

[Comparative Example 9]

A microporous membrane made of a polyolefin having a thickness of 20 μm was obtained in the same manner as in Example 4, except that the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to be 5 times in MD and 5 times in TD. Table 4 shows the properties of the obtained microporous membrane.

[Comparative Example 10]

Master batch Rikemaster CN-002 (manufactured by Riken Vitamin Co., Ltd.) was blended, and the gel sheet of the polyethylene resin composition was simultaneously biaxially stretched to be 7 times as MD and 7 times as TD, as in Comparative Example 5. Thus, a microporous membrane made of polyolefin having a thickness of 12 µm was obtained. Table 4 shows the properties of the obtained microporous membrane.

As shown in Tables 1 to 4, in Examples, the balance of the values of the protruding strength and the melt shrinkage stress is clearly enhanced compared to the comparative example, and both the mechanical strength of the polyolefin microporous membrane and the shape-retaining properties under high temperature are shown. The improvement of was confirmed. In the comparative example, since the balance between the mechanical strength and the shape-retaining properties under high temperature was insufficient, both the breakdown voltage test and the impact short circuit test were not satisfied, but the breakdown voltage test and the impact short circuit were sufficient because the balance was high for the examples. Both sides of the test were met to confirm the superiority of battery safety. In addition, in the Example, the value of the average flow rate pore size / maximum pore size was clearly higher than that of the comparative example, and it was confirmed that the uniformity of the pore structure of the polyolefin microporous membrane was improved.

(Industrial availability)

Since the microporous membrane made of polyolefin according to the embodiment of the present invention has excellent mechanical strength and shape-retaining properties under high temperature, it can be suitably used for secondary batteries such as nonaqueous electrolyte-based secondary batteries, which are representative of lithium ion secondary batteries.

Although the present invention has been described in detail and with reference to specific embodiments, it is apparent to those skilled in the art that various changes or modifications can be added without departing from the spirit and scope of the present invention.

This application is based on the JP Patent application (Japanese Patent Application No. 2017-186142) of the application on September 27, 2017, The content is taken in here as a reference.

Claims (11)

The semi-crystallization time (T _1/2 ) of 126 ° C is 200 seconds or less, the puncture strength in terms of 50% porosity and film thickness of 20 µm is 6.0 N / 20 µm or more, and the MD direction of the melt heat shrink stress is P _MD , When the TD direction is P _TD , at least one side is 0.8 MPa or less, and the P _{MD + TD, which} is the sum of the P _MD and the P _TD , is 1.5 MPa or less. According to claim 1,
A microporous membrane made of polyolefin, characterized in that the maximum pore diameter observed from the forometer is 45 nm or less, and the ratio of the average flow rate pore diameter / maximum pore diameter is 0.6 or more. The method of claim 1 or 2,
A microporous membrane made of polyolefin, characterized in that the melting peak area ratio of 141 ° C or higher observed from DSC is 25% or higher. The method according to any one of claims 1 to 3,
A microporous membrane made of polyolefin, characterized in that the porosity is 30% or more. The method according to any one of claims 1 to 4,
Polyolefin microporous membrane, characterized in that the MD + TD tensile breaking strength is 350 MPa or more. The method according to any one of claims 1 to 5,
A polyolefin microporous membrane comprising polyethylene as a main component. The method according to any one of claims 1 to 6,
A microporous membrane made of polyolefin, characterized in that the air permeation resistance in terms of a film thickness of 20 µm is 50 to 600 sec / 100 cc. The method according to any one of claims 1 to 7,
A microporous membrane made of polyolefin, comprising a crystal nucleating agent. The method according to any one of claims 1 to 8,
A polyolefin microporous membrane, wherein the polyolefin microporous membrane has a weight average molecular weight of 1.0 × 10 ⁶ or more and a polyethylene-containing ratio of 25% by mass or less. A battery separator comprising the microporous membrane made of polyolefin according to any one of claims 1 to 9. A secondary battery using the separator according to claim 10.