JP2016193613A - Laminated porous film, and non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a laminated porous film, in which a filler-containing coating liquid is applied to a base material porous film having different surface roughnesses to form heat resistant layers and powder dropping can be suppressed much more.SOLUTION: The method for producing the laminated porous film, which film has the heat resistant layers each containing an inorganic filler and a binder on both surfaces of the polyolefin base material porous film having different surface roughnesses on the front and back surfaces, comprises the steps of: applying the coating liquid, which contains the inorganic filler, the binder and a solvent, to the small-roughness surface of the base material porous film; distilling the solvent to form the first heat resistant layer; applying the coating liquid to the large-roughness surface of the base material porous film; and distilling the solvent to form the second heat resistant layer.SELECTED DRAWING: None

Description

  The present invention relates to a laminated porous film suitable as a separator for a nonaqueous electrolyte secondary battery, a method for producing the same, and a nonaqueous electrolyte secondary battery having the laminated porous film.

  Non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are widely used as batteries for personal computers, mobile phones, portable information terminals and the like because of their high energy density.

Non-aqueous electrolyte secondary batteries represented by these lithium secondary batteries are high in energy density, and when internal short circuit / external short circuit occurs due to damage of the battery or equipment using the battery, A large current flows and generates intense heat. Therefore, non-aqueous electrolyte secondary batteries are required to prevent heat generation beyond a certain level and ensure high safety.
As a means for ensuring such safety, a method of providing a shutdown function for preventing further heat generation by blocking the passage of ions between the positive and negative electrodes by a separator in the event of abnormal heat generation is common. As a method for imparting a shutdown function to the separator, a method in which a porous film made of a material that melts when abnormal heat is generated is used as the separator. That is, in the battery using the separator, the porous film melts and becomes non-porous when abnormal heat is generated, and the passage of ions can be blocked and further heat generation can be suppressed.

  As the separator having such a shutdown function, for example, a polyolefin porous film is used. The separator made of the porous film made of polyolefin suppresses further heat generation by blocking (shutdown) the passage of ions by melting and making non-porous at about 80 to 180 ° C. during abnormal heat generation of the battery. However, when the heat generation is severe, the separator made of the porous film may cause a short circuit due to direct contact between the positive electrode and the negative electrode due to shrinkage or film breakage. Thus, the separator made of a polyolefin porous film has insufficient shape stability and sometimes cannot suppress abnormal heat generation due to a short circuit.

Several separators for non-aqueous electrolyte secondary batteries excellent in shape stability at high temperatures have been proposed. As one of the means, a laminate in which a heat-resistant layer containing fine particle filler and a porous film mainly composed of polyolefin as a substrate (hereinafter sometimes referred to as “substrate porous film”) are laminated. A separator for a non-aqueous electrolyte secondary battery made of a porous film has been proposed (for example, Patent Document 1).
In addition, recently, in order to diversify the usage scene as separators and increase functionality, as a porous substrate film, laminated porous films in which multiple films with different surface roughness are laminated, and processing of the front and back sides Substrate porous films with different surface roughness such as single layer porous films with different conditions are also used.

  In such a separator, one of the problems is suppression of filler falling off from the surface of the laminated porous film, so-called “powder falling”. When powder falls from the separator, there are problems that the expected properties of the separator do not appear and that the device is contaminated by the powder (filler) dropped when assembled into a battery.

As a method for suppressing powder falling, a method of performing surface treatment on the filler (for example, Patent Document 2), a method for characterizing the chemical structure of the binder resin and binding the filler (for example, Patent Document 3), and fixing the filler There has been proposed a method (for example, Patent Document 4) for controlling the average fiber diameter of the fibers to be processed and the particle diameter of the filler to a predetermined relationship.
However, suppression of powder omission by these methods is not sufficient, and further improvement is required.
In addition, in the case of a substrate porous film having a different surface roughness, compared to a substrate porous film having the same surface roughness, powder falling tends to occur more easily. In some cases, variations occurred.

JP 2004-227972 A JP 2007-31151 A International Publication No. 2009/123168 Pamphlet JP 2006-331760 A

As described above, in the production of a laminated porous film in which a heat-resistant layer is formed by applying a coating liquid containing a filler to a porous substrate film having a different surface roughness, the powder fall is further suppressed. The development of a method that can do this is desired.
Under such circumstances, an object of the present invention is to further reduce powder in the production of a laminated porous film in which a heat-resistant layer is formed by applying a coating liquid containing a filler to a substrate porous film having a different surface roughness. It is providing the manufacturing method of the laminated porous film which can suppress that, and the nonaqueous electrolyte secondary battery which has as a laminated porous film separator manufactured by this manufacturing method.

  As a result of intensive studies to solve the above problems, the present inventor has found that the following inventions meet the above object, and have reached the present invention.

That is, the present invention relates to the following inventions.
<1> A method for producing a laminated porous film in which a heat-resistant layer containing an inorganic filler and a binder is laminated on one side of a polyolefin-based porous film having different surface roughnesses on the front and back, wherein the inorganic filler, binder and solvent are used. A method for producing a laminated porous film in which a coating liquid is applied to a surface having a small surface roughness in the polyolefin-based porous film, and then the solvent is distilled off to form a heat-resistant layer.
<2> A method for producing a laminated porous film in which a heat-resistant layer containing an inorganic filler and a binder is laminated on both surfaces of a polyolefin-based porous film having different surface roughnesses on the front and back sides, wherein the inorganic filler, binder and solvent are used. After the coating liquid is applied to the surface of the polyolefin-based porous film having a small surface roughness, the solvent is distilled off to form the first heat-resistant layer, and then the coating liquid is added to the polyolefin group. The manufacturing method of the laminated porous film which coats on the surface with the large surface roughness in a material porous film, distills a solvent off, and forms a 2nd heat resistant layer.
<3> The method for producing a laminated porous film according to <1> or <2>, wherein the surface roughness of the surface having a small surface roughness is a root mean square roughness (rms) of 700 nm or less.
<4> The method for producing a laminated porous film according to any one of <1> to <3>, wherein the polyolefin-based porous film is a laminate of polyolefin porous films having different surface roughnesses.
<5> The method for producing a laminated porous film according to any one of <1> to <4>, wherein the polyolefin-based porous film is a porous film mainly composed of polyethylene.
<6> The method for producing a laminated porous film according to any one of <1> to <5>, wherein the binder is a water-soluble binder.
<7> The method for producing a laminated porous film according to any one of <1> to <6>, wherein the inorganic filler is alumina.
<8> A nonaqueous electrolyte secondary battery comprising a laminated porous film produced by the method according to any one of <1> to <7> as a separator.

  According to the present invention, powder falling off during film production is suppressed, and a laminated porous film suitable as an excellent separator for a nonaqueous electrolyte secondary battery can be produced with high reproducibility and high efficiency. Become.

Hereinafter, the present invention will be described in detail.
1st aspect of the manufacturing method of the lamination | stacking porous film of this invention is the lamination | stacking porous film by which the heat resistant layer containing an inorganic filler and a binder was laminated | stacked on the single side | surface of the polyolefin base porous film from which front and back differ in surface roughness. A coating solution containing an inorganic filler, a binder and a solvent is applied to the surface of the polyolefin substrate porous film having a small surface roughness, and then the solvent is distilled off to form a heat-resistant layer. It is characterized by doing.

The second aspect of the method for producing a laminated porous film of the present invention is a laminated porous film in which a heat resistant layer containing an inorganic filler and a binder is laminated on both surfaces of a polyolefin-based porous film having different surface roughnesses on the front and back sides. A coating solution containing an inorganic filler, a binder and a solvent is applied to the surface of the polyolefin-based porous film having a small surface roughness, and the solvent is distilled off to remove the first heat-resistant layer. Then, the coating liquid is applied to the surface of the polyolefin-based porous film having a large surface roughness, and the solvent is distilled off to form a second heat-resistant layer.
In the following description, “polyolefin-based porous film” is sometimes abbreviated as “base-material porous film”.

Hereinafter, the component of a laminated porous film, a manufacturing method, etc. are demonstrated in detail about the manufacturing method of the laminated porous film of this invention.
In the following, the substrate porous film “A layer”, the coating liquid containing the inorganic filler, the binder and the solvent may be referred to as “B liquid”, and the heat-resistant layer formed from the coating liquid may be referred to as “B layer”. is there.

<Substrate porous film (A layer)>
A layer is a porous film which consists of polyolefin resin, has the structure which has the pore connected to the inside, and can permeate | transmit gas and a liquid from one surface to the other surface.
When layer A is used as a separator, a laminated porous film obtained by laminating layer A and a heat-resistant layer (layer B) obtained by the production method of the present invention has the property of becoming non-porous by melting at a high temperature. In the event of abnormal heat generation when a battery accident occurs, the laminated porous film is given a shutdown function by melting and making it non-porous.

  The B layer is formed by applying a liquid B containing an inorganic filler and a binder resin as a main component to the A layer and then drying, and the main skeleton is formed by the inorganic filler and the binder resin. And since it is a heat-resistant layer whose shape does not change at the temperature at which the A layer becomes nonporous, it imparts a shape maintaining function to the laminated porous film.

The substrate porous film constituting the A layer will be described.
A layer is a porous film which consists of polyolefin resin, and surface roughness differs in the front and back. More specifically, the surface roughness differs between the front and back surfaces of a laminated film obtained by laminating a plurality of porous films having different surface roughness, and processing conditions such as T-die design and roll temperature during molding. A single-layer film formed as described above can be mentioned. In any form, the substrate porous film has a structure having pores connected to the inside thereof, and gas or liquid can be transmitted from one surface to the other surface.
In addition, for example, in order to give the base porous film a high piercing strength and a shutdown function in a low temperature range, a laminated film obtained by laminating a plurality of porous films having different properties may be used as the A layer. However, when the A layer is such a laminated film, the surface roughness inevitably differs between the front and back surfaces, and therefore the production method of the present invention can be suitably applied.

In the present invention, the surface roughness is a value based on the root mean square (hereinafter sometimes referred to as rms value), and the lower the rms value, the smoother the surface. Specifically, the rms value can be calculated by measuring a mean square value in a range of 255 μm × 185 μm using a confocal microscope.
Here, in the present invention, the “polyolefin-based porous film having different surface roughness between the front and back surfaces” constituting the A layer means that the rms value of the surface having a large surface roughness is the surface roughness. Refers to a film that is 1.05 times or more of the smaller surface.

  In the present invention, a heat resistant layer (B layer) having excellent uniformity can be formed by using the A layer having an rms value of 700 nm or less, preferably 500 nm or less, on the surface having a small surface roughness.

  In the present invention, the proportion of the polyolefin component in the polyolefin-based porous film is 50% by volume or more of the entire A layer, preferably 90% by volume or more, and more preferably 95% by volume or more.

The polyolefin component preferably contains a high molecular weight component having a weight average molecular weight of 5 × 10 ⁵ to 15 × 10 ⁶ , and in particular, by containing a polyolefin component having a weight average molecular weight of 1 million or more, The strength of the entire laminated porous film including the A layer, and thus the A layer, can be improved.

  Examples of the polyolefin include a high molecular weight homopolymer or copolymer obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Among these, high molecular weight polyethylene mainly having ethylene and having a weight average molecular weight of 1,000,000 or more is preferable.

  The pore size of the A layer is preferably 3 μm or less, and more preferably 1 μm or less in terms of ion permeability and prevention of particles entering the positive electrode and the negative electrode when used as a battery separator.

  The thickness of the A layer is preferably 4 to 40 μm, and more preferably 7 to 30 μm. In addition, when A layer is a laminated body of several types of polyolefin porous films, these values point out the total thickness of a laminated body.

The porosity of the A layer is preferably 20 to 80% by volume, more preferably 30 to 70% by volume. In addition, when A layer is a laminated body of several types of polyolefin porous films, these values are the average values of the porosity of each film.
When the porosity of the A layer is in the above range, the ion permeability is excellent, and when used as a separator for a non-aqueous electrolyte secondary battery, excellent characteristics are exhibited. If the porosity is less than 20% by volume, the amount of electrolyte retained may be small, and if it exceeds 80% by volume, non-porous formation at a high temperature causing shutdown will be insufficient, that is, when the battery generates heat due to an accident. The current may not be cut off.

As the basis weight of the A layer, the strength, film thickness, handling property and weight of the laminated porous film, and further, the weight energy density and volume energy density of the battery when used as a battery separator can be increased. usually from ^{4~15g / m 2, 5~12g / m} 2 is preferred. In addition, when A layer is a laminated body of a multiple types of polyolefin porous film, these values are total values of the basis weight of each film.

  In the present invention, the production method of the substrate porous film in the case where the A layer is a laminate of a plurality of types of polyolefin porous films is not particularly limited. For example, the draw ratio, the film forming temperature, etc. It is obtained by laminating a plurality of films prepared by changing the conditions.

For example, as described in JP-A-7-29563, such individual films may be formed by adding a plasticizer to a thermoplastic resin to form a film, and then removing the plasticizer with an appropriate solvent. As described in Kaihei 7-304110, a film made of a thermoplastic resin produced by a known method is used to selectively stretch a structurally weak amorphous portion of the film to form micropores. A method is mentioned. For example, when the base porous film is formed from a polyolefin resin containing ultrahigh molecular weight polyethylene and a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, from the viewpoint of production cost, the following method is used. It is preferable to manufacture.
Specifically, (1) 100 parts by weight of ultrahigh molecular weight polyethylene, 5 to 200 parts by weight of a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, and 100 to 400 parts by weight of an inorganic filler such as calcium carbonate are kneaded to polyolefin resin. Step (2) for obtaining a composition (2) Step for molding a sheet using the polyolefin resin composition (3) Step (4) for removing inorganic filler from the sheet obtained in step (2) (4) Step (3) It can be obtained by a method including a step of obtaining the A layer by stretching the obtained sheet.
Moreover, a commercially available film can also be used.

<Coating liquid (B liquid)>
Hereinafter, the coating liquid according to the present invention will be described in detail.

  As the inorganic filler contained in the liquid B, a filler generally called a filler can be used. Specifically, calcium carbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate, barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, Examples include fillers such as titanium oxide, alumina, mica, zeolite, and glass. In addition, these fillers can be used individually or in mixture of 2 or more types.

Among these, as the inorganic filler, inorganic oxides are more preferable and alumina is particularly preferable from the viewpoints of heat resistance and chemical stability.
Alumina has many crystal forms such as α-alumina, β-alumina, γ-alumina, and θ-alumina, and any of them can be suitably used. Of these, α-alumina is most preferred because it has particularly high thermal and chemical stability.
These inorganic filler materials can be used alone. Two or more materials can be mixed and used.

  Various inorganic fillers, such as spherical, oval, short, and saddle shapes, and irregular shapes that do not have a specific shape, depending on the manufacturing method of the inorganic filler material and the dispersion conditions during preparation of the B liquid Can be used.

The binder resin contained in the liquid B has the ability to bind inorganic fillers, inorganic fillers and base porous films, is insoluble in battery electrolytes, and is electrochemical within the battery usage range. Resins that are stable in nature are preferred.
For example, polyolefins such as polyethylene and polypropylene, fluorine-containing resins such as polyvinylidene fluoride and polytetrafluoroethylene, fluorine-containing resins such as vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer and ethylene-tetrafluoroethylene copolymer Rubbers such as rubber, styrene-butadiene copolymer and its hydride, methacrylic acid ester copolymer, acrylonitrile-acrylic acid ester copolymer, styrene-acrylic acid ester copolymer, ethylene propylene rubber, polyvinyl acetate, The melting point and glass transition temperature of polyphenylene ether, polysulfone, polyethersulfone, polyphenylene sulfide, polyetherimide, polyamideimide, polyetheramide, polyester, etc. are 18 ℃ above resins, polyvinyl alcohol, polyethylene glycol, cellulose ethers, sodium alginate, polyacrylic acid, polyacrylamide, a binder resin such as polymethacrylic acid.
Among the binder resins, the water-soluble polymer is preferable in terms of process and environmental load because a solvent mainly composed of water can be used as a solvent. Among water-soluble polymers, at least one selected from the group consisting of carboxyalkyl cellulose, alkyl cellulose, hydroxyalkyl cellulose, starch, polyvinyl alcohol and sodium alginate is preferable, and cellulose ether is particularly preferably used.
Specific examples of the cellulose ether include carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), carboxyethyl cellulose, methyl cellulose, ethyl cellulose, cyanethyl cellulose, oxyethyl cellulose, and the like. CMC is particularly preferable.

The solvent used for the B liquid is a solvent or a dispersion medium for dissolving or dispersing the inorganic filler and the binder resin, and any solvent that can uniformly and stably dissolve or disperse the inorganic filler and the binder resin may be used. Specifically, alcohols such as water, methanol, ethanol, isopropanol, acetone, toluene, xylene, hexane, N-methylpyrrolidone, N, N-dimethylacetamide, N, N-dimethylformamide, etc. are used alone or in combination. A plurality of them may be mixed within the range.
The solvent can be only water, but it is a mixed solvent of water and an organic polar solvent in that the drying and removal rate is fast and the binder resin has sufficient solubility in the above-mentioned water-soluble polymer. preferable. In the case of using only an organic solvent, there is a possibility that the drying speed becomes too fast and the leveling is insufficient, or the solubility is insufficient when a water-soluble polymer is used as the binder resin.
As the organic polar solvent used for the mixed solvent, an alcohol which is compatible with water at an arbitrary ratio and has an appropriate polarity is suitable, and among these, methanol and ethanol are used. The ratio of the water and the polar solvent is selected in consideration of the leveling property and the type of the binder resin to be used within the range in which the above contact angle range is achieved, and usually contains 50% by weight or more of water.

  Moreover, in this B liquid, components other than an inorganic filler and binder resin may be included in the range which does not impair the objective of this invention as needed. Examples of such components include a dispersant, a plasticizer, and a pH adjuster.

The method for obtaining the liquid B by dissolving or dispersing the inorganic filler or the binder resin is not particularly limited as long as it is a method necessary for obtaining a homogeneous liquid B.
For example, a mechanical stirring method, an ultrasonic dispersion method, a high-pressure dispersion method, a media dispersion method, and the like can be mentioned, but the high-pressure dispersion method is preferable in that it can be uniformly dispersed.
As long as there is no particular problem such as the occurrence of precipitates, the mixing order may be mixed by adding inorganic filler, binder resin and other components to the solvent at once, or after each dissolved or dispersed in the solvent Mixing is optional.

  The method for applying the liquid B to the porous substrate film is not particularly limited as long as it can be uniformly wet coated, and a conventionally known method can be adopted. For example, a capillary coating method, a spin coating method, a slit die coating method, a spray coating method, a roll coating method, a screen printing method, a flexographic printing method, a bar coater method, a gravure coater method, a die coater method, and the like can be employed. The thickness of the heat-resistant layer to be formed can be controlled by adjusting the coating amount, the concentration of the binder resin in the B liquid, and the ratio of the filler to the binder resin.

It is preferable to hydrophilize the base porous film in advance before applying the liquid B on the base porous film. Liquid B can be applied directly to the substrate porous film, but by applying a hydrophilic treatment to the substrate porous film, the coating properties are improved and a more uniform heat-resistant layer can be obtained. . This hydrophilization treatment is particularly effective when the concentration of water in the solvent is high.
The hydrophilic treatment of the substrate porous film may be any method, and specific examples include chemical treatment with an acid or alkali, corona treatment, plasma treatment and the like.
Here, in the corona treatment, in addition to being able to hydrophilize the substrate porous film in a relatively short time, modification of the polyolefin resin by corona discharge is limited only to the vicinity of the surface of the membrane, It exhibits the advantage that high coatability can be secured without changing the properties.

In the case where the B liquid is applied to the A layer, when the B layer is formed only on one surface of the A layer, the surface having a high surface roughness (smooth surface) is applied to the surface having a low surface roughness (smooth surface) ( It is possible to obtain a laminated porous film that is more difficult to powder off than when applied to a rough surface).
Moreover, when forming B layer on both surfaces of A layer, a surface with a low surface roughness (smooth surface) is used as the first coating surface, and a surface with a high surface roughness (rough surface) is used as the second coating surface. By coating, it is possible to obtain a laminated porous film that is more difficult to fall off than when applied in the reverse order.

The removal of the medium from the liquid B applied on the substrate porous film is generally performed by drying.
In addition, when apply | coating B liquid on a base material porous film, it is preferable that the drying temperature of a medium is below the temperature which does not reduce the air permeability of a base material porous film, ie, the temperature which a shutdown produces.

The thickness of the B layer as the heat-resistant layer obtained by drying the B liquid after coating is usually 0.1 μm or more and 10 μm or less, preferably 2 μm or more and 6 μm or less. If the B layer is too thick, the load characteristics of the battery may be reduced when a non-aqueous electrolyte secondary battery is manufactured. If it is too thin, the battery will generate heat due to an accident or the like. There is a risk that the separator may contract without being able to resist the heat shrinkage of the polyolefin porous membrane.
When the B layer is formed on both sides of the A layer, the thickness of the B layer is the total thickness of both sides.

The B layer is a porous film in which inorganic fillers are connected by a binder, pores formed in the gaps between the inorganic fillers are connected, and gas or liquid can pass from one surface to the other surface. The pore diameter is preferably 3 μm or less, more preferably 1 μm or less, as an average value of the diameters of the spheres when the pores are approximated to a sphere. When the average pore diameter exceeds 3 μm, there is a possibility that problems such as short-circuiting easily occur when the carbon powder, which is the main component of the positive electrode or the negative electrode, or a small piece thereof falls off.
The porosity of the B layer is preferably 30 to 90% by volume, more preferably 40 to 85% by volume.

  The thickness of the entire laminated porous film (A layer + B layer) of the present invention is usually 5 to 80 μm, preferably 5 to 50 μm, and particularly preferably 6 to 35 μm. When the thickness of the entire laminated porous film is less than 5 μm, the membrane is easily broken. On the other hand, if the thickness is too thick, the electric capacity of the battery tends to be small when used as a separator for a non-aqueous secondary battery.

  In addition, the laminated porous film of the present invention may include a porous film other than the base porous film and the heat-resistant layer, for example, a porous film such as an adhesive film and a protective film as long as the object of the present invention is not impaired. Good.

The laminated porous film of the present invention thus prepared has a small amount of powder falling off. In addition, that the amount of powder falling here is small means that the peeling strength (henceforth peel strength) in a T-type peel test using a tape is high.
The T-type peel test is a method for mainly evaluating the interfacial adhesive force between the A layer and the heat-resistant layer. In particular, when the laminated porous film is used as a separator for a non-aqueous electrolyte secondary battery, the heat-resistant layer easily peels off from the A layer in a process of cutting the film in accordance with the battery shape, and the peel strength is high. Highness is important as a property of such a separator.

  The higher the T-type peel strength of the laminated porous film, the less the influence of the powder falling on the battery production line, and the easier the handling becomes.

<Nonaqueous electrolyte secondary battery>
The laminated porous film of the present invention can be suitably used as a separator for batteries, particularly nonaqueous electrolyte secondary batteries such as lithium secondary batteries.
As a preferred use example of the laminated porous film of the present invention, a non-aqueous electrolyte secondary battery will be described by taking as an example a case of using it for a non-aqueous electrolyte secondary battery such as a lithium battery. The method of using the quality film is not limited to these.

  The non-aqueous electrolyte secondary battery of the present invention includes the above-described laminated porous film of the present invention as a separator, and an electrode group in which a negative electrode sheet, a separator, and a positive electrode sheet are laminated as other components. Contains non-aqueous electrolyte.

  When a non-aqueous electrolyte secondary battery is manufactured using the laminated porous film of the present invention as a separator, the separator has a high load characteristic, and even if the battery generates intense heat due to an accident, the separator exhibits a shutdown function. Contact between the positive electrode and the negative electrode due to shrinkage is avoided, and a highly safe non-aqueous electrolyte secondary battery is obtained.

  The shape of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited, and may be any of a paper type, a coin type, a cylindrical type, a square type, a laminate type, and the like.

As the positive electrode sheet, a sheet in which a mixture containing a positive electrode active material, a conductive material, and a binder is carried on a current collector is usually used. Specifically, as the positive electrode active material, a material containing a material that can be doped / undoped with lithium ions, a carbonaceous material as a conductive material, and a thermoplastic resin as a binder can be used. Examples of the material that can be doped / undoped with lithium ions include lithium composite oxides containing at least one transition metal such as V, Mn, Fe, Co, and Ni. Among these, lithium composite oxides having an α-NaFeO ₂ type structure such as lithium nickelate and lithium cobaltate, and lithium composite oxides having a spinel type structure such as lithium manganese spinel are preferable in that the average discharge potential is high. Can be mentioned.

  The lithium composite oxide may contain various metal elements, particularly at least selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al, Ga, In, and Sn. The metal element is included so that the at least one metal element is 0.1 to 20 mol% with respect to the sum of the number of moles of one metal element and the number of moles of Ni in lithium nickelate. It is preferable to use composite lithium nickelate because the cycle performance in use at a high capacity is improved.

  As the binder, polyvinylidene fluoride, vinylidene fluoride copolymer, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, Examples include ethylene-tetrafluoroethylene copolymers, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, thermoplastic resins such as thermoplastic polyimide, polyethylene, and polypropylene.

  Examples of the conductive agent include carbonaceous materials such as natural graphite, artificial graphite, cokes, and carbon black. As the conductive material, each may be used alone, for example, artificial graphite and carbon black may be mixed and used.

As the negative electrode sheet, a sheet in which a mixture containing a negative electrode active material, a conductive material and a binder is supported on a current collector is usually used. As the negative electrode active material, for example, a material capable of doping and dedoping lithium ions, lithium metal, or a lithium alloy can be used.
Materials that can be doped / undoped with lithium ions include carbonaceous materials such as natural graphite, artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fibers, and fired organic polymer compounds, and lower potential than the positive electrode. And chalcogen compounds such as oxides and sulfides for doping and dedoping lithium ions.
As a carbonaceous material, a carbonaceous material mainly composed of graphite materials such as natural graphite and artificial graphite, because it has a high potential flatness and a low average discharge potential, so that a large energy density can be obtained when combined with a positive electrode. Is preferred.

  As the negative electrode current collector, Cu, Ni, stainless steel, or the like can be used. In particular, in a lithium secondary battery, Cu is preferable because it is difficult to form an alloy with lithium and it can be easily processed into a thin film. As a method of supporting the mixture containing the negative electrode active material on the negative electrode current collector, a method of pressure molding, or a method of pasting into a paste using a solvent or the like and applying pressure to the current collector by pressing after drying Is mentioned.

As the nonaqueous electrolytic solution, for example, a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent can be used. Examples of lithium salts include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , One or a mixture of two or more of lower aliphatic carboxylic acid lithium salts, LiAlCl4 and the like can be mentioned. The lithium salt is selected from the group consisting of LiPF ₆ containing fluorine, LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiC (CF ₃ SO ₂ ) ₃ among these. It is preferable to use those containing at least one selected from the above.

  More specifically, for example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di (methoxycarbonyloxy) ethane. Carbonates such as; ethers such as 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran Esters such as methyl formate, methyl acetate and Y-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N, N-dimethylformamide and N, N-dimethylacetamide; 3 Carbamates such as methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, 1,3-propane sultone, or those obtained by introducing a fluorine group into the above substances can be used. Two or more of these are mixed and used.

  Among these, those containing carbonates are preferred, and cyclic carbonates and acyclic carbonates, or mixtures of cyclic carbonates and ethers are more preferred. As a mixture of cyclic carbonate and acyclic carbonate, ethylene carbonate and dimethyl have a wide operating temperature range and are hardly decomposable even when a graphite material such as natural graphite or artificial graphite is used as the negative electrode active material. A mixture comprising carbonate and ethyl methyl carbonate is preferred.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

In Examples and Comparative Examples, physical properties and the like of laminated porous films were measured by the following methods.
(1) Thickness measurement (unit: μm):
The thickness of the laminated porous film was measured with a high-precision digital length measuring machine manufactured by Mitutoyo Corporation.
(2) Weight per unit (unit: g / m ² ):
The laminated porous film was cut into a 10 cm long side and the weight W (g) was measured.
The weight per unit area (g / m ² ) = W / (0.1 × 0.1) was calculated. The basis weight of the heat resistant layer was calculated after subtracting the basis weight of the base porous film (A layer) from the basis weight of the laminated porous film.
(4) Surface roughness measurement:
It measured using confocal microscope PL (micro | micron | mu) 2300. Surface smoothness was expressed by means of root mean square roughness (rms value), which is an index of unevenness.
(5) Peel strength test In accordance with JIS standard K6854-3, a 3M Scotch clear tape was used as a peeling tape to measure the peeling speed at 100 mm / min.

<Substrate porous film (A layer)>
70% by weight of ultra high molecular weight polyethylene powder (340M, manufactured by Mitsui Chemicals), 30% by weight of polyethylene wax (FNP-0115, manufactured by Nippon Seiki Co., Ltd.) having a weight average molecular weight of 1000, and the ultra high molecular weight polyethylene and polyethylene wax The antioxidant (Irg1010, manufactured by Ciba Specialty Chemicals Co., Ltd.) 0.4% by weight, the antioxidant (P168, manufactured by Ciba Specialty Chemicals Co., Ltd.) 0.1% by weight, Add 1.3% by weight of sodium stearate, and add calcium carbonate (Maruo Calcium Co., Ltd.) with an average particle size of 0.1 μm so that the total volume is 38% by volume. Then, the mixture was melt kneaded with a biaxial kneader to obtain a polyolefin resin composition. The polyolefin resin composition was rolled with a pair of rolls having a surface temperature of 150 ° C. to produce a sheet.
Further, similarly, sheets with different draw ratios during rolling were prepared, and these two sheets were thermocompression bonded at 130 ° C. to obtain a laminated sheet.
This laminated sheet is immersed in an aqueous hydrochloric acid solution (hydrochloric acid 4 mol / L, nonionic surfactant 0.5 wt%) to remove calcium carbonate, and then stretched 6 times at 105 ° C. to make a polyethylene porous material. A substrate porous film A comprising a membrane was obtained. The results of calculating the rms value for the film are shown in Table 1. In the porous substrate A, a surface having a small rms value is referred to as a smooth surface, and a surface having a large rms value is referred to as an uneven surface.

Example 1
(1) Preparation of B liquid The B liquid of Example 1 was produced in the following procedures.
First, carboxymethylcellulose (CMC, Cellogen 3H manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was dissolved in an aqueous solution having a pure water: ethanol ratio of 70:30 as a medium to obtain a CMC solution having a concentration of 1%.
Next, 3500 parts by weight of alumina (manufactured by Sumitomo Chemical Co., Ltd .: AKP3000) is added to 100 parts by weight of CMC, added, mixed, and processed three times under high-pressure dispersion conditions (60 MPa) using a gorin homogenizer. B liquid was prepared.
(2) Production of laminated porous film Using a gravure coating machine, the B liquid was applied and dried on the smooth surface of the substrate PE porous film having different rms values on the front and back sides to produce a laminated porous film 1. Table 2 shows the physical properties of the laminated porous film 1 obtained.

Comparative Example 1
A laminated porous film 2 was obtained in the same manner as in Example 1 except that the coated surface was an uneven surface. Table 2 shows the physical properties.

Example 2
Using a gravure coating machine, the B liquid was applied to the uneven surface which is the uncoated surface of the laminated porous film 1 to obtain a laminated porous film 3. Table 2 shows the physical properties.

Comparative Example 2
Using a gravure coating machine, the B liquid was applied and dried on the smooth surface, which is the uncoated surface of the laminated porous film 2, to obtain a laminated porous film 4. Table 2 shows the physical properties.

  According to the present invention, there is provided a laminated porous film suitable as a separator for a non-aqueous secondary battery in which powder does not easily fall off from a laminated heat-resistant layer and is excellent in dimensional stability at high temperatures and ion permeability. The In the nonaqueous electrolyte secondary battery using the laminated porous film as a separator, the positive electrode and the negative electrode are prevented from coming into direct contact with the separator even when the battery generates heat intensely.

Claims (2)

A laminated porous film in which a heat-resistant layer containing an inorganic filler and a binder is laminated on one side of a polyolefin-based porous film with different surface roughness on the front and back,
A laminated porous film in which the heat-resistant layer is laminated on a surface of the polyolefin-based porous film having a small surface roughness.   A non-aqueous electrolyte secondary battery comprising the laminated porous film according to claim 1 as a separator.