JP2016013660A - Laminated microporous film and separator for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated microporous film that is suitable for continuous massive discharge without decreasing high energy density and achieves a safe lithium secondary battery with a long service life, and a separator for a lithium ion secondary battery.SOLUTION: The laminated microporous film includes two outermost layers at the top and back of the film and at least one intermediate layer located between the outermost layers. At least one layer in the outermost layers and the intermediate layer contains a resin composition (I) comprising 100 parts by mass of a polyolefin resin and 1 to 90 parts by mass of a polyamide resin; and at least one of the outermost layers contains a resin composition (II) comprising a polypropylene resin but substantially no polyamide resin.

Description

  The present invention relates to a laminated microporous film and a separator for a lithium ion secondary battery.

  With the rapid development of the electronics field in recent years, electronic devices have become more sophisticated, smaller and more portable, and the demand for rechargeable high energy density secondary batteries used in these electronic devices has increased. Conventionally, secondary batteries mounted on these electronic devices include lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc. Recently, carbon materials that can occlude / release lithium ions, lithium alloys, etc. are used as active materials. A lithium ion secondary battery in which the negative electrode used and a positive electrode using a lithium-containing composite oxide or the like as an active material is researched, developed, and put into practical use. This type of battery has a high battery voltage and a large energy density per unit weight and volume as compared with the conventional battery, and is the most expected secondary battery in the future.

  Recently, it has been found that lithium ion secondary batteries are excellent not only in energy density but also in output characteristics due to their device structure, and are beginning to be put into practical use as batteries suitable for power tools such as electric tools. On the other hand, as a technique for improving the energy density of this type of battery, a sheet-like battery using a lightweight and thin laminated film for an outer case, which is formed by laminating and integrating a metal foil such as aluminum on a pair of insulating resin films. Is being considered. This battery has been studied and partially put into practical use as a battery for portable devices, a battery for hybrid automobiles, and a battery for electric assist bicycles.

Further, not only energy density but also improvement of life is desired, and lithium hexafluorophosphate (LiPF ₆ ) is often used as an electrolyte salt in terms of charge / discharge characteristics. However, those using lithium hexafluorophosphate (LiPF ₆ ) as the electrolyte salt of the electrolytic solution have a problem in that lithium fluoride (LiF) is deposited on the negative electrode and deactivates the negative electrode with use. That is, this battery has low moisture stability and thermal stability. For example, water mixed in a small amount in the battery, electrolyte anion decomposes even at a weak high temperature of about 60 ° C., and hydrogen fluoride (HF) or similar in the battery. An acid is produced, which combines with Li ions present in the battery to produce lithium fluoride (LiF), and a passive film that inhibits the charge / discharge reaction is deposited on the negative electrode surface. This phenomenon is a major cause of performance deterioration of this type of battery.

  For this reason, it is important to promptly fix hydrogen fluoride (HF) or the like generated at the beginning of battery operation from the viewpoint of extending the life of this type of battery. In order to solve this problem, Patent Document 1 discloses a method of fixing HF by using a polyolefin resin layer in which polyamide resin (PA) is dispersed as an innermost layer in contact with a power generation element in an exterior case.

JP 2010-40227 A

  However, in the method of Patent Document 1, since the polyamide capable of fixing HF exists only in the vicinity of the outer case of the battery, it is difficult to fix HF generated near the center of the battery, and the life of the battery is reduced. It is difficult to stretch sufficiently.

  The present invention has been made in view of the above problems, and is suitable for continuous large discharge without impairing high energy density, and a laminated microporous film that realizes a safe and long-life lithium secondary battery, and It aims at providing the separator for lithium ion secondary batteries.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by a laminated microporous film having a predetermined configuration, and have completed the present invention.

That is, the present invention is as follows.
[1]
Two front and back outermost layers, and one or more intermediate layers located between the outermost layers,
At least one of the outermost layer and the intermediate layer includes a resin composition (I) containing 100 parts by mass of a polyolefin resin and 1 to 90 parts by mass of a polyamide resin,
A laminated microporous film in which at least one of the outermost layers contains a resin composition (II) containing substantially no polyamide resin and containing a polypropylene resin.
[2]
At least one of the intermediate layers contains the resin composition (I),
The laminated microporous film according to [1] above, wherein the two outermost layers contain the resin composition (II).
[3]
The laminated microporous film according to [1] or [2] above, wherein the resin composition (I) further contains an admixture.
[4]
A separator for a lithium ion secondary battery comprising the laminated microporous film according to any one of [1] to [3].

  According to the present invention, there are provided a laminated microporous film and a lithium ion secondary battery separator that are suitable for continuous large discharge and realize a safe and long-life lithium secondary battery without impairing a high energy density. be able to.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

[Laminated microporous film]
The laminated microporous film of this embodiment is
Two front and back outermost layers, and one or more intermediate layers located between the outermost layers,
At least one of the outermost layer and the intermediate layer includes a resin composition (I) containing 100 parts by mass of a polyolefin resin and 1 to 90 parts by mass of a polyamide resin,
At least one of the outermost layers contains a resin composition (II) containing substantially no polyamide resin and containing a polypropylene resin.

  As long as the above conditions are satisfied, the mode of lamination is not particularly limited. Specific embodiments of the laminated microporous film are not particularly limited. For example, at least one of the intermediate layers includes the resin composition (I), and the two outermost layers include the resin composition (II). It is preferable to include. Further, the intermediate layer may be one or more layers or two or more layers. By having such an aspect, the discharge capacity retention rate tends to be further improved.

In this way, the laminated microporous film (separator) separating the positive electrode and the negative electrode, in particular, by introducing polyamide into the inner layer, hydrogen fluoride (HF) generated with use of the battery is laminated microporous. It can be quickly fixed inside the film. Thereby, deposition of lithium fluoride (LiF) on the negative electrode can be prevented. This is because the polyamide resin (PA) dispersed in the inner layer of the laminated microporous film has strong interaction with hydrogen fluoride (HF). Here, if only to secure the hydrogen fluoride (HF), F scavenger were charged hydrogen fluoride (HF) ^- a method is also conceivable to immobilize, in which hydrogen gas is generated in the battery There is a problem that the safety of the battery is lowered. On the other hand, in the laminated microporous film of this embodiment, hydrogen fluoride (HF) is not reacted, but the interaction between hydrogen fluoride (HF) and polyamide resin (PA) is used for battery performance. By collecting in a site with little influence, it is possible to achieve both a longer life of the lithium ion secondary battery by fixing hydrogen fluoride and ensuring the safety of the battery.

  From such a viewpoint, the inner layer portion of the separator is suitable as a portion that has a particularly small influence on the battery performance. Since deposition of lithium fluoride (LiF) occurs in the negative electrode, it is not preferable to collect hydrogen fluoride (HF) in the vicinity of the negative electrode, and the positive electrode active material used in the lithium ion secondary battery is stable in an alkaline environment. In an acidic environment, the cathode active material is decomposed and another deterioration mode starts, which is not preferable. Therefore, the innermost layer of the laminated microporous film (separator) that does not contact the negative electrode and the positive electrode is the most preferable place.

[Resin composition (I)]
The resin composition (I) contains 100 parts by mass of a polyolefin resin and 1 to 90 parts by mass of a polyamide resin.

[Polyolefin resin]
The polyolefin resin contained in the resin composition (I) is not particularly limited, but a polypropylene resin or a polyethylene resin described later is preferable from the viewpoint of obtaining a laminated microporous film having good permeability.

[Polypropylene resin]
The melt flow rate (MFR) of the polypropylene resin contained in the resin composition (I) is preferably 0.10 to 1.0 g / 10 minutes, more preferably 0.30 to 0.70 g / 10 minutes. More preferably, it is 0.40 to 0.50 g / 10 minutes. When the MFR is 0.10 g / 10 min or more, there is a tendency that breakage in the film forming process hardly occurs. Moreover, when the MFR is 1.0 g / 10 min or less, the puncture strength of the obtained laminated microporous film tends to be further improved.

  The pentad fraction of the polypropylene resin is preferably 90% to 100%, more preferably 92 to 97%, and further preferably 94 to 97%. The pentad fraction indicates the abundance ratio of isotactic chains in pentad units in the molecular chain, and is the fraction of propylene monomer units at the center of a chain in which five propylene monomer units are continuously meso-bonded. When the Pendat fraction is 90% or more, the stereoregularity of the polymer becomes high and the polypropylene resin has high crystallinity. Thereby, the openability at the time of extending | stretching improves and it exists in the tendency for favorable permeability to be obtained. On the other hand, when the pentad fraction is 100% or less, the puncture strength tends to be further improved.

  The polypropylene resin may be a propylene-based copolymer obtained by copolymerizing propylene having a propylene content of 98 to 99.5 mol% and ethylene and / or α-olefin. The α-olefin is not particularly limited, and examples thereof include butene-1, pentene-1, 4-methyl-pentene-1, hexene-1, and octene-1. When the polypropylene resin is a propylene-based copolymer, ethylene or α-olefin serves as a tie molecule that connects the polypropylene crystals, and tends to suppress breakage occurring between the crystals. From such a viewpoint, ethylene and α-olefin are preferably present dispersed in the molecular chain, and the propylene-based copolymer is preferably a random copolymer.

[Polyethylene resin]
The MFR of the polyethylene resin contained in the resin composition (I) is preferably 0.10 to 2.0 g / 10 minutes, more preferably 0.10 to 1.0 g / 10 minutes, and still more preferably 0. 20 to 0.50 g / 10 min. When the MFR is 0.1 g / 10 min or more, a laminated microporous film with few fish eyes tends to be obtained. Moreover, it exists in the tendency for permeability to improve more because MFR is 2.0 g / 10min or less.

  Moreover, the molecular weight distribution of the polyethylene resin is preferably 10 to 20, more preferably 12 to 16, and further preferably 13 to 15. When the molecular weight distribution is 12 or more, the air permeability of the obtained laminated microporous film tends to be further improved. Moreover, when the molecular weight distribution is 20 or less, a laminated microporous film with less fish eyes tends to be obtained. The molecular weight distribution is the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (hereinafter referred to as “Mw / Mn”). Mw and Mn are determined from gel permeation chromatography (hereinafter referred to as “GPC”) of a laminated microporous film using polystyrene as a standard sample, and in detail according to the method described in the following examples. Measured.

The density of the polyethylene resin is preferably 955 to 970 kg / m ³ , more preferably 958 to 967 kg / m ³ , still more preferably 961 to 965 kg / m ³ , and most preferably 962 to 964 kg / m ^3. m is ^3. When the density is 955 kg / m ³ or more, a laminated microporous film having good air permeability tends to be obtained. Moreover, when it is 970 kg / m ³ or less, it tends to be difficult for the membrane to break during stretching. Furthermore, when the density is 962 to 964 kg / m ³ , a laminated microporous film having low electric resistance and extremely good lithium ion permeability tends to be obtained.

[Polyamide resin]
The polyamide resin is a polymer having an amide bond (—NHCO—) in the main chain. The polyamide resin is not particularly limited. For example, polycaprolactam (nylon 6), polytetramethylene adipamide (nylon 46), polyhexamethylene adipamide (nylon 66), polyhexamethylene sebacamide (nylon 610) ), Polyhexamethylene dodecamide (nylon 612), polyundecamethylene adipamide (nylon 116), polyundecalactam (nylon 11), polydodecalactam (nylon 12), polytrimethylhexamethylene terephthalamide (nylon TMHT) ), Polyhexamethylene isophthalamide (nylon 6I), polynonanemethylene terephthalamide (9T), polyhexamethylene terephthalamide (6T), polybis (4-aminocyclohexyl) methane dodecamide (nylon) ACM12), polybis (3-methyl-aminocyclohexyl) methane dodecamide (nylon dimethyl PACM12), polymetaxylylene adipamide (nylon MXD6), polyundecamethylene hexahydroterephthalamide (nylon 11T (H)), and these Among them, polyamide copolymers containing at least two different polyamide components, and mixtures thereof can be mentioned.

  Among the polyamides, polyhexamethylene adipamide (nylon 66) is preferable from the viewpoint of heat resistance.

[Admixture]
The resin composition (I) preferably further contains an admixture. The admixture refers to an agent that acts as a dispersant for dispersing the polyamide resin in the polypropylene resin matrix. By using the admixture, the puncture strength of the laminated microporous film is further improved. There is a tendency.

  As the admixture, an epoxy group-containing ethylene copolymer is preferable from the viewpoint of dispersibility of the polyamide resin. Although it does not specifically limit as this epoxy group containing ethylene copolymer, For example, ethylene unit 50-99 mass%, unsaturated carboxylic acid glycidyl ester unit or unsaturated glycidyl ether unit 0.1-30 mass%, and ethylene It is a copolymer containing 0-50 mass% of a system unsaturated ester compound unit.

  Although it does not specifically limit as said unsaturated carboxylic acid glycidyl ester, For example, the compound normally represented by following Chemical formula (1) is mentioned.

（上記式（１）中、Ｒ¹は、エチレン系不飽和結合を有する、炭素数２〜１８の炭化水素基を表す。）

(In the above formula (1), R ¹ has the ethylenically unsaturated bond, a hydrocarbon group having 2 to 18 carbon atoms.)

  Although it does not specifically limit as a compound represented by the said Formula (1), For example, glycidyl acrylate, glycidyl methacrylate, itaconic acid glycidyl ester, etc. are mentioned.

  Moreover, although it does not specifically limit as said unsaturated glycidyl ether, For example, the compound represented by following Chemical formula (2) is mentioned.

（上記式（２）中、Ｒ²は、エチレン系不飽和結合を有する、炭素数２〜１８の炭化水素基を表し、Ｘは、メチレンオキシ基又はフェノキシ基を表す。）

(In the above formula (2), R ^2, having an ethylenically unsaturated bond, a hydrocarbon group having 2 to 18 carbon atoms, X is a methylene group or a phenoxy group.)

  Although it does not specifically limit as a compound represented by the said Formula (2), For example, allyl glycidyl ether, 2-methylallyl glycidyl ether, styrene-p-glycidyl ether, etc. are mentioned.

  Further, the ethylenically unsaturated ester compound is not particularly limited, but examples thereof include saturated carboxylic acid vinyl esters such as vinyl acetate and vinyl propionate; methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, methacryl And α, β-unsaturated carboxylic acid alkyl esters such as ethyl acrylate and butyl methacrylate. In particular, vinyl acetate, methyl acrylate, and ethyl acrylate are preferable from the viewpoints of productivity and mechanical properties of the laminated microporous film. The ethylenically unsaturated ester compound unit is a constituent component that can be appropriately used in the epoxy group-containing ethylene copolymer, and is not an essential component.

  As a method for producing an epoxy group-containing ethylene copolymer, a known method, for example, a monomer to be subjected to copolymerization, in the presence of a radical generator, at 500 to 4000 atm and 100 to 300 ° C., in an appropriate solvent or chain. Examples thereof include a method of copolymerization in the presence or absence of a transfer agent, a method of mixing an unsaturated epoxy compound and a radical generator in polyethylene, and a melt graft copolymerization in an extruder.

  The content of the admixture is preferably 1.0 to 20% by mass, more preferably 1.0 to 15% by mass, and still more preferably based on 100% by mass of the total amount of the resin composition (I). It is 1.0-5.0 mass%. When the content of the admixture is within the above range, the puncture strength of the laminated microporous film tends to be further improved.

[Resin composition (II)]
The resin composition (II) substantially does not contain a polyamide resin and contains a polypropylene resin. Although it does not specifically limit as a polypropylene resin used by resin composition (II), For example, what was illustrated by resin composition (I) is mentioned. The polypropylene resin used in the resin composition (II) and the polypropylene resin used in the resin composition (I) may be the same or different.

  In the resin composition (II), the polyamide resin is not particularly limited, and examples thereof include those exemplified for the resin composition (I). Here, “substantially free of polyamide resin” means that the content of the polyamide resin is preferably 0 to 1.0% by mass with respect to 100% by mass of the total amount of the resin composition (II), More preferably, it is 0-0.5 mass%, More preferably, it means 0-0.1 mass%.

(Other additives)
Resin composition (I) and resin composition (II) may contain other additives as necessary in addition to the components described above. Other additives include, but are not particularly limited to, for example, olefin elastomers, antioxidants, metal deactivators, heat stabilizers, flame retardants (organic phosphate ester compounds, ammonium polyphosphate compounds, aromatics Halogen flame retardants, silicone flame retardants, etc.), fluorine polymers, plasticizers (low molecular weight polyethylene, epoxidized soybean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weather resistance (light ) Property improver, polyolefin nucleating agent, slip agent, inorganic or organic filler and reinforcing material (polyacrylonitrile fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.), various types A coloring agent, a mold release agent, etc. are mentioned.

[Method for producing laminated microporous film]
The production method of the laminated microporous film of the present embodiment is not particularly limited.
Film formation in which the resin composition (II) is the outermost layer and the resin composition (I) is the intermediate layer is obtained by cooling and solidifying the molten resin extruded from the two-type three-layer coextrusion die installed at the tip of the extruder. Process,
A cold stretching process in which the laminated film obtained in the film forming process is stretched 1.05 times to 2.0 times in at least one direction in a state where the laminated film is held at −20 ° C. to 80 ° C .;
And a heat stretching step of stretching the laminated film obtained in the cold stretching step at a temperature of 90 ° C. to 130 ° C. at least 1.05 times to 5.0 times in one direction.

[Film formation process]
In the film forming step, a raw film in which a layer containing a polypropylene resin and a layer containing a polyethylene resin are laminated is obtained by a coextrusion method using, for example, a T die or a circular die. From the viewpoint of physical properties and applications required for the laminated microporous film of the present embodiment, a coextrusion method using a T die is preferable.

  The die temperature in the film forming step (hereinafter also referred to as film forming temperature) is preferably 180 to 260 ° C. When the film forming temperature is 180 ° C. or higher, the thickness of the raw film tends to be uniform. Moreover, when the film forming temperature is 260 ° C. or lower, the puncture strength of the laminated microporous film tends to be further improved.

  The draw ratio after extrusion in the film forming step, that is, the film winding speed (unit: m / min) is the resin extrusion speed (linear velocity in the flow direction of the molten resin passing through the die lip. Unit: m / min). The divided value is preferably 10 to 500, more preferably 100 to 300, and further preferably 160 to 230. When the draw ratio is in the above range, the permeability of the obtained laminated microporous film tends to be further improved.

  Moreover, the winding speed of the film when winding the raw fabric film is preferably 2.0 to 400 m / min, more preferably 10 to 200 m / min.

[Heat treatment process]
The raw film produced as described above is preferably subjected to a heat treatment step of performing a heat treatment before a cold stretching step described later. The heat treatment method is not particularly limited. For example, a method of bringing the original film into contact with a heated roll, a method of exposing the original film in a heated gas phase before winding, and a method of winding the original film on a core and heating heated gas phase. Or the method of exposing in a heating liquid phase and the method of combining these are mentioned.

  As for these heat treatment conditions, for example, heat treatment is preferably performed at a heating temperature of 110 ° C. or higher and lower than 130 ° C. for 10 seconds to 100 hours. When the heating temperature is 120 ° C. or higher, the permeability of the obtained laminated microporous film tends to be further improved. Moreover, even if it anneals in the state which wound the original film on the core body because heating temperature is less than 130 degreeC, it exists in the tendency for films to become difficult to fuse | melt. The heating temperature during the heat treatment is more preferably 120 ° C to 128 ° C. By performing annealing in such a temperature range, the crystallinity of the raw film is increased, and a laminated microporous film having good permeability tends to be obtained.

[Cold drawing process]
Although it does not specifically limit as a extending | stretching method in a cold extending process and the hot extending process mentioned later, For example, the method of extending | stretching to a uniaxial direction and / or a biaxial direction with a roll, a tenter, an autograph, etc. is mentioned. In particular, two or more stages of uniaxial stretching with a roll are preferable from the viewpoint of physical properties and applications required for the laminated microporous film of the present embodiment.

  In the cold drawing step, the raw film obtained in the film forming step or the heat treatment step is a step of drawing 1.03 times to 2.0 times in at least one direction at a temperature of −20 ° C. to 80 ° C.

  The stretching temperature for cold stretching in the cold stretching step is more preferably 0 ° C to 50 ° C. When the stretching temperature of cold stretching is −20 ° C. or higher, the film tends to be difficult to break. Moreover, when the drawing temperature of cold drawing is less than 80 ° C., the permeability of the obtained laminated microporous film tends to be further improved. Here, the “cold stretching temperature” refers to the surface temperature of the film in the cold stretching step. The surface temperature of the film can be measured by providing a non-contact type thermocouple in the stretching roll machine.

  The draw ratio of cold drawing in the cold drawing step is 1.03 to 2.0 times, more preferably 1.1 to 1.3 times. When the draw ratio is 1.03 times or more, the permeability tends to be good. Moreover, it exists in the tendency for a film to be hard to fracture | rupture because a draw ratio is 2.0 times or less. The cold stretch of the raw film may be performed in at least one direction and may be performed in two directions. Preferably, the original film is uniaxially stretched only in the film extrusion direction (hereinafter also referred to as “MD direction”).

[Heat drawing process]
The hot stretching step is a step of performing cold stretching as described above and then hot stretching at least 1.05 times to 5.0 times while maintaining the temperature at 90 ° C to 130 ° C. When the draw ratio is 1.05 times or more, the permeability tends to be further improved. Moreover, it exists in the tendency for a film to become difficult to fracture | rupture because a draw ratio is 5.0 times or less. The thermal stretching may be performed in at least one direction and may be performed in two directions, but preferably, uniaxial stretching is performed only in the “MD direction”.

  The stretching temperature of the hot stretching is more preferably 110 ° C to 125 ° C. When the stretching temperature for hot stretching is 90 ° C. or higher, the film tends to be difficult to break. Moreover, it exists in the tendency for the permeability | transmittance of the laminated microporous film obtained to improve more because the extending | stretching temperature of heat stretching is 130 degrees C or less. Here, the “stretching temperature for heat stretching” refers to the surface temperature of the film in the heat stretching process. The surface temperature of the film can be measured by providing a non-contact type thermocouple in the stretching roll machine.

[Heat setting process]
It is preferable that the manufacturing method of the lamination | stacking microporous film of this embodiment includes the heat setting process which heat-processes at 0 to 30 degreeC higher than heat stretching temperature after a heat stretching process. As this heat setting method, a method of heat shrinking to such an extent that the length of the laminated microporous film after heat setting is reduced by 3 to 50% with respect to the length of the laminated microporous film before heat setting (hereinafter referred to as “the heat fixing method”) , This method is referred to as “relaxation”), and a method of heat setting so that the dimension in the stretching direction does not change. By this heat setting, it is possible to obtain a laminated microporous film having excellent dimensional stability, that is, having a small heat shrinkage rate.

[Separator for lithium ion secondary battery]
The separator for a lithium ion secondary battery of the present embodiment is composed of the laminated microporous film.

[Lithium ion secondary battery]
Next, the lithium ion secondary battery of the present invention will be described. Examples of the positive electrode active material of the lithium ion secondary battery include a spinel structure compound such as LiMn ₂ O ₄ and a lithium-containing transition metal composite oxide having an α-NaFeO ₂ structure generally represented by LiMO ₂ (where M is A single or two or more metal elements selected from Co, Ni, Al, Mn, Ti, Fe, etc.), lithium-containing phosphoric acid compounds, and the like can be used. Furthermore, if the battery manufacturing method is devised, metal oxides such as MnO ₂ and V ₂ O ₅ into which lithium can be inserted, metal sulfides such as TiS ₂ and ZnS _2, and polyaniline having electrochemical redox activity It is also possible to use a π-conjugated polymer such as polypyrrole or a disulfide compound utilizing the formation-cleavage of a sulfur-sulfur bond in the molecule.

On the other hand, as the negative electrode, metallic lithium or various lithium alloys, various metal oxides such as SnO ₂ , or a carbon material capable of occluding and releasing lithium can be used. As the carbon material, naturally produced graphite or baked raw material is fired at a high temperature of 2000 ° C. or higher, and a graphite-based carbon material having a flat potential characteristic with a developed graphite structure, or an organic raw material is relatively heated to 1000 ° C. or lower. Coke-based carbon materials that are fired at a low temperature and can be expected to have a larger charge / discharge capacity than graphite-based materials are used.

  The positive electrode current collector is preferably an aluminum foil having a thickness of 5 to 60 μm. The positive electrode active material, a conductive aid such as scaly graphite or carbon black, and a binder such as polyvinylidene fluoride are provided on at least one surface of the current collector. A paste formed with a solvent can be applied and dried to form a positive electrode active material-containing coating film having a thickness of 30 to 300 μm.

  As the negative electrode current collector, a copper foil having a thickness of 5 to 60 μm is preferable, and the negative electrode active material and a binder such as polyvinylidene fluoride are pasted into a paste with a solvent on at least one surface of the current collector and dried. And what formed the 30-300 micrometer-thick negative electrode active material containing coating film can be used.

  As a solvent for the electrolytic solution, a solvent usually used in an electrolytic lithium ion secondary battery, for example, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), sulfolane (SL), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dimethoxyethane (DME), diethoxyethane (DEE), 2-methyl-tetrahydrofuran (2MeTHF), various glymes, etc. alone or in a mixed system Can be used.

The lithium salt used as the solute of the electrolytic solution is usually a lithium salt used in an electrolytic lithium ion secondary battery, such as lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO _4). ), Inorganic lithium salts such as lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethylsulfonate (LiOSO ₂ CF ₃ ), lithium bis (trifluoromethylsulfonyl) imide (LiN (CF ₃ SO ₂ ) ₂ ) An organic lithium salt such as bis (perfluoroethylsulfonyl) imidolithium (LiN (C ₂ F ₅ SO ₂ ) ₂ ) can be appropriately selected and used in accordance with the method for synthesizing the electrolyte gel.

  Next, although an example and a comparative example are given and explained more concretely, the present invention is not limited to the following examples. In addition, about the evaluation method of the various characteristics in an Example and a comparative example, it is as follows.

(1) MFR of polypropylene resin
Based on JIS K7210, it is a value measured under conditions of a temperature of 210 ° C. and a load of 2.16 kg, and its unit is g / 10 minutes.

(2) MFR of polyethylene resin
Based on JIS K7210, it is a value measured under conditions of a temperature of 190 ° C. and a load of 2.16 kg, and its unit is g / 10 minutes.

(3) Pentad fraction (%)
The pentad fraction of the polypropylene resin was calculated by the peak height method from the 13C-NMR spectrum assigned based on the description in the Polymer Analysis Handbook (edited by the Japan Analytical Chemical Society). The 13C-NMR spectrum was measured by using ECS-400 manufactured by JEOL Ltd., dissolving the laminated microporous film in o-dichlorobenzene-d, under the conditions of a measurement temperature of 130 ° C. and a cumulative number of 10,000 times. went.

(4) Molecular weight distribution of polyethylene resin (Mw / Mn)
The molecular weight distribution of the resin in the polyethylene resin composition is a value of the ratio Mw / Mn of the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained from gel permeation chromatography (GPC). GPC measurement was performed using a Waters 150-C ALC / GPC apparatus, using Shodex AT-807S and Tosoh TSK-gelGMH-H6 in series as a column, and trichlorobenzene containing 10 ppm Irganox 1010 as a solvent. Was measured at 140 ° C. A calibration curve was prepared using commercially available monodisperse polystyrene as a standard substance. Mw and Mn were determined from the molecular weight in terms of polystyrene, and the molecular weight distribution was derived.

(5) Film thickness (μm)
The film thickness of the laminated microporous film was measured using a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”).

(6) Air permeability (sec / 100cc)
The air permeability of the laminated microporous film was measured with a Gurley air permeability meter according to JIS P-8117.

(7) Puncture strength (g)
Using a “KES-G5 Handy Compression Tester” (trademark) manufactured by Kato Tech, a piercing test was performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec, and the maximum piercing load (g) was measured. .

(8) Discharge capacity maintenance rate (%)
A lithium ion secondary battery was produced according to the procedure shown below, and a cycle test was performed under the following conditions in order to compare the life characteristics of the lithium ion secondary battery. That is, a cycle in which each battery was discharged for 2 hours with a current of 0.5 CA and a voltage of 4.2 V for 2 hours, and then discharged until the cell voltage became 2.5 V with a current of 2 CA was repeated 1000 cycles. The ratio of the 2CA capacity at the 1000th cycle to the 2CA discharge capacity at the 10th cycle was defined as the discharge capacity maintenance rate (%) (average value of 5 batteries).

[Production of positive electrode]
In a dispersing machine, 91 parts by mass of LiCoO ₂ powder as a positive electrode active material, 4.0 parts by mass of polyvinylidene fluoride resin as a binder, 5.0 parts by mass of graphite powder as a conductive agent, and N-methylpyrrolidone as a dispersing agent Then, a slurry for coating the positive electrode active material mixture was prepared by stirring and mixing.

  Next, the positive electrode active material mixture coating slurry is applied simultaneously on both sides to a current collector made of aluminum foil using a die coater, and dried in an oven to remove the dispersant, thereby removing the positive electrode active material mixture. An agent coating was formed. This was pressed to a predetermined density, cut to a predetermined size, and a positive electrode plate having a plain part with a predetermined width was obtained.

[Production of negative electrode]
Application of negative electrode active material mixture by mixing 90 parts by weight of artificial graphite powder, 10 parts by weight of polyvinylidene fluoride resin as a binder, and N-methylpyrrolidone as a dispersant by stirring and mixing with a disperser A slurry was prepared.

  Next, the negative electrode active material mixture coating slurry is simultaneously coated on both sides of a copper foil current collector using a die coater, and dried in an oven to remove the dispersant, thereby removing the negative electrode active material mixture. An agent coating was formed. This was pressed to a predetermined density and cut to a predetermined size to obtain a negative electrode plate having a plain part with a predetermined width.

[Battery assembly]
Two laminated microporous films (any of Examples 1 to 4 and Comparative Examples 1 and 2 described later) were partially heat-sealed to produce an envelope-like separator bag, which was produced as described above. The positive electrode plate was inserted leaving the plain portion to obtain a separator-covered positive electrode plate.

  Next, the negative electrode plate and the separator-covered positive electrode plate are alternately stacked so that the plain portions of the respective electrode plates are in opposite directions to produce an electrode group of 10 positive electrode plates and 11 negative electrode plates. did.

  Next, a positive electrode terminal made of an aluminum plate having a thickness of 0.2 mm and a width of 40 mm is formed on the uncoated positive electrode plate of the electrode group thus produced, and a nickel plate having a thickness of 0.2 mm and a width of 40 mm is formed on the uncoated negative electrode plate. A negative electrode terminal to be prepared was attached by an ultrasonic welding method under respective predetermined conditions to produce a battery element.

  Next, the battery element produced in this manner is housed in an outer case made of a laminate film, vacuum-dried under predetermined conditions, and then mixed with ethylene carbonate and diethyl carbonate having a weight mixing ratio of 3: 7 to hexafluorophosphoric acid. After injecting an organic electrolytic solution in which lithium was dissolved to 1.3 mol / L, the inside of the cell was depressurized and the opening was heat-sealed and sealed, then a predetermined initial charge with a current of 0.1 CA, The battery was stored for a predetermined time, and then discharged at a current of 0.2 CA until the cell voltage reached 2.75 V. Finally, an activation treatment was performed to produce a sheet-like lithium ion secondary battery with a rated capacity of 15 Ah.

The resins used in the examples and comparative examples are as follows.
Polypropylene (A): propylene homopolymer, MFR = 0.7 g / min, pentad fraction = 96%
Polyethylene (A): MFR = 0.2 g / min, density = 963 kg / m ³ , Mw / Mn = 12.
Polyamide (A): Leona 1300S manufactured by Asahi Kasei Chemicals Corporation
Polyamide (B): Leona 1500 manufactured by Asahi Kasei Chemicals Corporation
Admixture (A): Epoxy group-containing ethylene copolymer Sumitomo Chemical Co., Ltd. Bondfast E

[Example 1]
90 parts by mass of polypropylene (A), 10 parts by mass of polyamide (A) and 3 parts by mass of admixture (A) were melt-kneaded using a twin screw extruder set at a temperature of 260 ° C. and a screw rotation speed of 300 rpm, and a resin composition (I) was obtained as a pellet. Moreover, polypropylene (A) was used as it is as the resin composition (II).

  Resin composition (II) having a diameter of 20 mm, L / D = 30 (L: distance from raw material supply port to discharge port of twin screw extruder (m), D: inner diameter of twin screw extruder (m). The same applies to the single screw extruder having a cylinder temperature of 220 ° C. through the feeder, and the resin composition (I) is fed to the single screw extruder having another diameter of 20 mm, L / D = 30 and cylinder temperature of 220 ° C. Through. Next, the resin composition (II) was extruded as an outermost layer and the resin composition (I) as an intermediate layer from a two-type three-layer coextrusion T die having a lip thickness of 4.0 mm installed at the tip of the extruder. The ratio of the extrusion weight of the outermost layer and the intermediate layer was 2: 1, and the T die temperature (also referred to as “film formation temperature”) was 230 ° C. Immediately apply cold air of 25 ° C to the molten resin after extrusion, and then wind it on a core with a cast roll cooled to 95 ° C at a draw ratio of 200 and a winding speed of 15 m / min. Obtained (film formation step).

  After the obtained laminated raw film is wound on the core body, it is annealed at a temperature of 128 ° C. for 5 hours, then cooled to 25 ° C. (heat treatment step), and 1.3 ° in the MD direction at a temperature of 25 ° C. Uniaxially stretched twice (cold stretching process), then uniaxially stretched 2.5 times in the MD direction at a temperature of 110 ° C. (hot stretching process), and further relaxed 0.9 times at a temperature of 130 ° C. Then, heat setting was performed (heat setting step) to obtain a laminated microporous film. The resulting laminated microporous film was measured for film thickness, air permeability, puncture strength, and discharge capacity retention rate. The results are shown in Table 1.

[Example 2]
A laminated microporous film was obtained in the same manner as in Example 1 except that the composition of the resin composition (I) was changed as shown in Table 1 without using an admixture. The measurement results are shown in Table 1.

Example 3
A laminated microporous film was obtained in the same manner as in Example 1 except that the composition of the resin composition (I) was changed as shown in Table 1. The measurement results are shown in Table 1.

Example 4
A laminated microporous film was obtained in the same manner as in Example 1 except that the composition of the resin composition (I) was changed as shown in Table 1. The sono measurement results are shown in Table 1.

[Comparative Example 1]
Polypropylene (A) was introduced into a single screw extruder having a diameter of 20 mm, L / D = 30 and cylinder temperature of 220 ° C. through a feeder, and extruded from a single layer T die having a lip thickness of 4.0 mm at a film forming temperature of 230 ° C. The subsequent molten resin was immediately applied with cold air at 25 ° C., and then wound on a core with a cast roll cooled to 95 ° C. under a draw ratio of 200 and a winding speed of 15 m / min to obtain an original film.

  The obtained raw film is wound on a core body, annealed at a temperature of 128 ° C. for 5 hours, cooled to 25 ° C. (heat treatment process), and 1.3 times in the MD direction at a temperature of 25 ° C. Next, it is uniaxially stretched (cold stretching process) at a temperature of 110 ° C. and then uniaxially stretched 2.5 times in the MD direction (hot stretching process), and further relaxed 0.9 times at a temperature of 130 ° C. Heat setting was performed (heat setting step) to obtain a single-layer microporous film. The evaluation results are shown in Table 1. The discharge capacity retention rate of the battery using the single layer microporous film of Comparative Example 1 was extremely inferior to that of the Example.

[Comparative Example 2]
The resin composition (I) produced in Example 1 was introduced into a single screw extruder having a diameter of 20 mm, L / D = 30, and a cylinder temperature of 220 ° C. through a feeder, and from a single layer T die having a lip thickness of 4.0 mm. 25 ° C cold air is immediately applied to the molten resin after extrusion at a film forming temperature of 230 ° C, and then wound on a core with a draw roll of 200 and a winding speed of 15 m / min. A raw film was obtained.

  The obtained raw film is wound on a core body, annealed at a temperature of 128 ° C. for 5 hours, cooled to 25 ° C. (heat treatment process), and 1.3 times in the MD direction at a temperature of 25 ° C. Next, it is uniaxially stretched (cold stretching process) at a temperature of 110 ° C. and then uniaxially stretched 2.5 times in the MD direction (hot stretching process), and further relaxed 0.9 times at a temperature of 130 ° C. Heat setting was performed (heat setting step) to obtain a single-layer microporous film. The evaluation results are shown in Table 1. The discharge capacity retention rate of the battery using the single-layer microporous film of Comparative Example 2 was inferior to that of the Example.

  The laminated microporous film of the present invention has industrial applicability as a battery separator, more specifically as a lithium secondary battery separator.

Claims (4)

Two front and back outermost layers, and one or more intermediate layers located between the outermost layers,
At least one of the outermost layer and the intermediate layer includes a resin composition (I) containing 100 parts by mass of a polyolefin resin and 1 to 90 parts by mass of a polyamide resin,
A laminated microporous film in which at least one of the outermost layers contains a resin composition (II) containing substantially no polyamide resin and containing a polypropylene resin. At least one of the intermediate layers contains the resin composition (I),
The laminated microporous film according to claim 1, wherein the two outermost layers contain the resin composition (II).   The laminated microporous film according to claim 1 or 2, wherein the resin composition (I) further contains an admixture.   The separator for lithium ion secondary batteries which consists of a lamination | stacking microporous film of any one of Claims 1-3.