JP2014032970A - Separator, and electrochemical device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator being excellent in liquid permeability while having a porous heat-resistant layer, and an electrochemical device using the separator.SOLUTION: The separator comprises a porous film 1, and a porous heat-resistant resin film 2 disposed on the porous film 1. An average pore diameter of a first region 2a of the heat-resistant resin film 2 including an interface with the porous film 1 is larger than that of a second region 2b of the heat-resistant resin film 2 farther from the interface than the first region.

Description

  The present invention relates to a separator and an electrochemical device using the separator.

In recent years, secondary batteries have been increased in energy density, and high heat resistance and highly reliable separators are required. From such a background, Japanese Unexamined Patent Application Publication No. 2006-289657 (Patent Document 1) proposes a multilayer separator in which a heat-resistant layer is provided on a resin porous film by coating. However, when the heat-resistant layer is directly applied to the resin porous film, the coating solution penetrates into the resin porous film, so that the electrolyte permeability in the separator may be reduced. In view of this, the technique disclosed in Japanese Patent Application Laid-Open No. 2001-23602 (Patent Document 2) discloses a method of filling a porous film with a liquid in advance and applying a heat-resistant layer to the porous film in that state. .

JP 2006-289657 A Japanese Patent Laid-Open No. 2001-23602

  However, in the method of filling a liquid disclosed in Japanese Patent Application Laid-Open No. 2001-23602, the liquid may remain slightly in the resin porous film, and a sufficient cleaning process is required. Further, depending on the conditions, the liquid permeability may deteriorate at the interface between the heat-resistant layer and the resin porous film, so a process that improves these problems is required.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a separator having an excellent liquid permeability while having a porous heat-resistant layer and an electrochemical device using the separator.

  The separator according to the present invention includes a porous film and a porous heat-resistant resin film provided on the porous film. The average pore diameter of the first region including the interface with the porous film in the heat resistant resin film is larger than the average pore diameter of the second region farther from the interface than the first region in the heat resistant resin film. Is also big.

  According to the present invention, since the average pore diameter of the first region including the interface is larger than that of the second region far from the interface, the electrolyte solution, ions, and the like can easily pass through the interface with the porous film in the heat resistant resin film. Become. In addition, since the second region having a smaller pore diameter than the first region is on the side away from the interface, it is easier to suppress a short circuit due to lithium metal precipitation than when the pore size is uniformly large. In addition, there is an advantage that the strength of the heat-resistant resin film is easily improved.

  Here, the thickness of the first region is preferably 4 to 95% of the thickness of the heat resistant resin film.

  Moreover, it is preferable that the average pore diameter of said 1st area | region is larger than the average pore diameter of the said porous film. Thereby, it becomes difficult for the heat resistant resin to block the pores of the porous film.

  The porous film is preferably a thermoplastic resin film.

  A separator having an excellent liquid permeability while having a porous heat-resistant layer and an electrochemical device using the separator are provided.

1A and 1B are schematic sectional views of a separator according to an embodiment of the present invention. 2A and 2B are schematic views sequentially showing steps of the method for manufacturing the separator of FIG. FIG. 3 is a schematic cross-sectional view of an electrochemical device using the separator of the present invention. FIG. 4 is a cross-sectional SEM photograph of the separator obtained in Example 1 near the heat-resistant resin film.

  With reference to drawings, the manufacturing method of the separator concerning this embodiment and the separator by this are explained.

  As shown in FIGS. 1A and 1B, the separator 18 according to this embodiment includes a porous film 1, a porous heat-resistant resin film 2 provided in contact with the porous film 1, and Have

  (Porous film)

  Examples of the form of the porous film 1 include, but are not limited to, a nonwoven fabric, a woven fabric, a paper, or a sheet. The material of the porous film 1 is preferably one having a shutdown characteristic, and examples thereof include a thermoplastic resin. In order to provide a shutdown function to the separator for a nonaqueous electrolyte secondary battery, the material of the porous film 1 is preferably a thermoplastic resin. The thermoplastic resin is more preferably a thermoplastic resin that can be softened at 80 to 180 ° C. to close the porous voids and does not dissolve in the electrolytic solution. Specific examples include polyolefin and thermoplastic polyurethane. Examples of the polyolefin include at least one selected from polyethylene such as low density polyethylene, high density polyethylene, and ultrahigh molecular weight polyethylene, and polypropylene.

  The thickness of the porous film 1 can be 3 to 30 μm, and further can be 5 to 20 μm. If the thickness is less than 3 μm, the shutdown function may be insufficient. If the thickness exceeds 30 μm, the thickness of the separator for a non-aqueous electrolyte battery including the heat-resistant porous layer is too large to achieve high electric capacity. It may be difficult to do.

  The average pore diameter measured by the mercury intrusion method of the porous film 1 can be 3 μm or less, can be 1 μm or less, or 0.5 μm or less. Further, there is no particular lower limit of the average pore diameter, but it can be 50 nm or more or 100 nm or more.

(Heat resistant resin film)
The heat resistant resin film 2 is a porous heat resistant resin layer. Examples of the heat resistant resin include polyimide, polyamideimide, aromatic polyamide (hereinafter sometimes referred to as aramid), polycarbonate, polyacetal, polysulfone, polyphenylsulfide, polyetheretherketone, aromatic polyester, polyethersulfone, poly At least one selected from the group consisting of etherimides can be used.

  Although the total thickness T2 of the heat resistant resin film 2 is not particularly limited, for example, it can be set to 0.5 to 20 μm, and can be set to 2 to 10 μm.

  In the present embodiment, the average pore diameter Da of the first region 2 a including the interface IF with the porous film 1 in the heat resistant resin film 2 is a second region farther from the interface IF than the first region 2 a in the heat resistant resin film 2. It is larger than the average pore diameter Db of 2b. That is, Da> Db.

  The thickness T2a of the first region 2a can be 4 to 95% with respect to the total thickness T2 of the heat-resistant resin film 2. Moreover, thickness T2a of the 1st area | region 2a can be 0.1-15 micrometers and 0.5-5 micrometers. The second region 2b does not necessarily include the surface of the heat resistant resin film 2 opposite to the interface IF. That is, the separator may have a structure of third region / second region 2b / first region 2a / porous film, and the average pore diameter Dc of the third region is smaller than that of the first region or second region. It can be determined regardless of the hole diameter. The thickness T2b of the second region 2b can be set to 3 to 90% with respect to the total thickness T2 of the heat resistant resin film 2. Or thickness T2b of the 1st field 2b can be 0.1-15 micrometers and 0.5-5 micrometers.

  The ratio (Da / Db) of the average pore diameter Da to the average pore diameter Db can be 2 to 30, for example, 5 to 16. The average pore diameter Da can be specifically set to 60 to 1000 nm, and the average pore diameter Db can be specifically set to 30 to 200 nm. Further, the average pore diameter Da can be larger than the average pore diameter of the porous film 1.

  The average pore diameter is obtained by obtaining an electron microscopic image of the vertical cross section of the separator 18, the square first region 2 a including the interface IF, and the square second region 2 b having the same size that is further away from the interface IF than the first region 2 a. Are extracted, binarized to extract holes, and the constant direction diameter of each hole is obtained and averaged by the number of holes.

  In addition, although the heat insulation resin film 2 may be only on one surface of the porous film 1 as shown in FIG. 1A, the heat insulation resin film 2 is porous as shown in FIG. It may be present on both sides of the film 1.

  According to the separator according to the present embodiment, the average pore diameter Da of the first region 2a including the interface IF is larger than the average pore diameter Db of the second region 2b far from the interface IF. At the interface IF with the quality film 1, electrolytes, ions, and the like can easily pass. In addition, since the second region 2b having a smaller average pore diameter than the first region 2a is located on the side away from the interface IF, the advantage of easily improving the strength of the heat-resistant resin film and a short circuit due to precipitation of lithium metal are caused. There is an advantage that it is easy to suppress.

(Manufacturing method of separator)
Next, an example of a method for manufacturing such a separator 18 will be described with reference to FIG. First, an undercoat liquid layer 3 is formed on the surface of the porous film 1 serving as a substrate, and a heat resistant resin solution layer 4 is formed on the undercoat liquid layer 3.

(Heat resistant resin solution)
The heat resistant resin solution contains a heat resistant resin and a solvent that dissolves the heat resistant resin.

  Although a solvent will not be specifically limited if a heat resistant resin is soluble, It is preferable to use an organic polar solvent. For example, when the heat-resistant resin is polyamide-imide, specific examples of the solvent include cyclohexanone, γ-butyrolactone, ethylene carbonate, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, dimethyl sulfone, and hexamethylphosphoramide. Tetramethylurea and the like, and particularly preferred are dimethylformamide, dimethylacetamide, N-methylpyrrolidone and the like.

  Although the density | concentration of the heat resistant resin in a heat resistant resin solution is not specifically limited, It can be 5-20 wt%.

  The heat resistant resin solution alone does not dissolve the heat resistant resin, but may contain another solvent that is compatible with the solvent that dissolves the heat resistant resin, for example, glycol. Examples of the glycol include alkylene glycols such as ethylene glycol, propylene glycol, tetramethylene glycol, neopentyl glycol, and hexanediol, and polyalkylene glycols such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. Although the present invention can be carried out without containing glycol, the amount of glycol added to the undercoat liquid is preferably 1 to 20% by weight based on the solvent that dissolves the heat-resistant resin. In particular, in the case of a polyamideimide resin, a concentration of about 5 to 15% by weight is easy to apply.

  The thickness of the layer 4 of the heat resistant resin solution is not particularly limited, but can be 3 to 50 μm.

(Undercoat liquid)
The undercoat liquid mainly contains a solvent that is compatible with the solvent of the heat-resistant resin solution. As such a solvent, for example, a good solvent capable of dissolving a heat-resistant resin can be used. Examples of such a solvent include polar organic solvents such as N, N′-dimethylformamide, N, N′-dimethylacetamide, N-methyl 2-pyrrolidone, tetramethylurea, dimethyl sulfoxide, cresol, and o-chlorophenol. Can be mentioned.

The undercoat liquid does not dissolve the heat-resistant resin by itself, but can contain a solvent that is compatible with the good solvent. In particular, it is preferable to contain a glycol, and in particular, an alkylene glycol such as ethylene glycol, propylene glycol, tetramethylene glycol, neopentyl glycol, hexanediol, or the like, or a polyalkylene glycol such as polyethylene glycol, polypropylene glycol, or polytetramethylene glycol. Is more preferable. Although it can be carried out without including a poor solvent such as glycol, the amount of glycol added to the undercoat liquid can be 10 to 50% with respect to the solvent that dissolves the heat-resistant resin.
The undercoat liquid may be composed only of a poor solvent for the heat resistant resin as long as it contains a solvent compatible with the solvent for the heat resistant resin solution.

The method for applying the undercoat liquid and the heat-resistant resin solution on the porous film is not particularly limited. For example, the undercoat liquid layer 3 can be formed on the upper surface of the porous film by injecting the undercoat liquid from the lower surface of the porous film. In addition, using a roll coater, an air knife coater, a blade coater, a rod coater, a bar coater, a comma coater, a gravure coater, a silk screen coater, a die coater, or a micro gravure coater, the layer 3 of the undercoat liquid is applied to the surface of the porous film. It can also be formed.
Moreover, the layer 4 of the heat resistant resin solution can be formed on the layer 3 of the undercoat liquid by using the above coater or the like. Alternatively, the undercoat liquid layer 3 and the heat-resistant resin solution layer 4 may be simultaneously formed on the porous film using a coater having a two-stage nozzle.

  The thicknesses of the undercoat liquid layer 3 and the heat-resistant resin solution layer 4 are not particularly limited, but may be appropriately set according to the thickness of the heat-resistant resin film to be obtained, the thickness ratio of the first region, and the like.

(Extraction process)
After the film 5 having the undercoat liquid layer 3 and the heat resistant resin solution layer 4 is formed on the porous film 1, as shown in FIG. 2B, the film 5 of the laminate 18 ′ is formed. Is brought into contact with the poor solvent S to solidify the heat-resistant resin from the film 5 to obtain the heat-resistant resin film 2. The poor solvent S is a poor solvent for the heat-resistant resin and can diffuse into the film 5. As the poor solvent S, for example, a solvent containing water, alcohol, or ketone as a main component can be used. For example, examples of the alcohol include methanol, ethanol, propyl alcohol, ethylene glycol, propylene glycol, diethylene glycol, and polyethylene glycol. Examples of ketones include acetone and methyl ethyl ketone.

  The poor solvent may be a good solvent capable of dissolving the heat-resistant resin alone, for example, an amide solvent such as N, N′-dimethylformamide, N, N′-dimethylacetamide, N-methyl-2-pyrrolidone, etc. May contain a small amount.

  By passing through such an extraction process, the heat-resistant resin film 2 having an average pore diameter distribution in the thickness direction as described above can be formed.

  By adjusting the temperature of the poor solvent, the composition and thickness of the undercoat liquid layer 3 and the heat-resistant resin solution 4, the pore diameter can be adjusted and the porosity can be adjusted.

  And what is necessary is just to perform washing | cleaning, drying, etc. for the obtained separator as needed. There is no particular limitation on the drying method, and examples thereof include a hot air drying method in which hot air is applied to the film. In this way, the separator is completed.

(Electrochemical device)
Subsequently, a lithium ion secondary battery will be described as an example of an electrochemical device using the above separator.

  FIG. 3 is a schematic cross-sectional view of an example of the lithium ion secondary battery 100 using the separator 18 described above.

  The lithium ion secondary battery 100 is mainly impregnated in the laminate 30, the case 50 that accommodates the laminate 30 in a sealed state, the pair of leads 60 and 62 connected to the laminate 30, and the laminate 30. It has an electrolyte.

  The laminated body 30 is configured such that a pair of the positive electrode 10 and the negative electrode 20 are opposed to each other with the separator 18 interposed therebetween. The positive electrode 10 is obtained by providing a positive electrode active material layer 14 on a plate-like (film-like) positive electrode current collector 12. The negative electrode 20 is obtained by providing a negative electrode active material layer 24 on a plate-like (film-like) negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.

  Hereinafter, the positive electrode 10 and the negative electrode 20 are collectively referred to as electrodes 10 and 20, and the positive electrode current collector 12 and the negative electrode current collector 22 are collectively referred to as current collectors 12 and 22, and the positive electrode active material layer 14 and the negative electrode The active material layers 24 are collectively referred to as active material layers 14 and 24.

(Positive electrode 10)
The positive electrode current collector 12 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

  The positive electrode active material layer 14 includes an active material according to the present embodiment, a binder, and a conductive material in an amount as necessary.

As the positive electrode active material, for example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), layered lithium manganate (LiMnO ₂ ), or LiMn _x Ni _y Co, which is a composite oxide containing a plurality of transition metals. Layered compounds such as _z O ₂ (x, y and z satisfy the formula x + y + z = 1, 0 ≦ y <1, 0 ≦ z <1, 0 ≦ x <1), and one or more kinds of these compounds Substituted transition metal element, lithium manganate (Li _{1 + x} Mn _2−x O ₄ (where x is a number from 0 to 0.33)), Li _{1 + x} Mn _2−xy M _y O ₄ (however, , M includes at least one metal selected from the group consisting of Ni, Co, Cr, Cu, Fe, Al, and Mg, x represents a number from 0 to 0.33, and y is from 0 to 1.0. Number and x and y satisfy the formula of 2-xy> 0), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ , LiMn _2−x M _x O ₂ (where M is Co, Ni, Fe, Cr, Including at least one metal selected from the group consisting of Zn and Ta, x represents a number of 0.01 to 0.1, Li ₂ Mn ₃ MO ₈ (where M is Fe, Co, Ni, At least one metal selected from the group consisting of Cu and Zn), copper-lithium oxide (Li ₂ CuO ₂ ), iron-lithium oxide (LiFe ₃ O ₄ ), LiFePO ₄ , LiV ₃ O ₈ , Examples include vanadium oxides such as V ₂ O ₅ and Cu ₂ V ₂ O ₇ , disulfide compounds, and Fe ₂ (MoO ₄ ) ₃ .

(Conductive aid)
Examples of the conductive assistant include metals such as nickel, aluminum, copper, and silver, and conductive carbon materials. Examples of the conductive carbon material include carbon blacks such as acetylene black and ketjen black, and carbon fibers such as graphite and carbon nanofibers. As the conductive assistant, carbon black is particularly preferable. In addition, it is not necessary to contain a conductive support agent.

(Binder)
Examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trichloroethylene (CTFE) copolymer [P (VDF-CTFE)], fluoride. Fluorine series such as vinylidene-hexafluoropropylene fluororubber, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene fluororubber [P (VDF-TFE-HFP)], vinylidene fluoride-tetrafluoroethylene-perfluoroalkyl vinyl ether fluororubber, etc. Polymers are preferred. As the vinylidene fluoride-based polymer, those in which vinylidene fluoride is 50% by weight or more, particularly 70% by weight or more are preferable, and in particular, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene (HFP), A copolymer of vinylidene fluoride and ethylene trifluoride chloride [P (VDF-CTFE)] is preferred.

  The negative electrode current collector 22 may be a conductive plate material, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

  As the negative electrode active material, for example, lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers, can be occluded and released. One kind or a mixture of two or more kinds of carbon-based materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb, In and alloys thereof, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material. Then, a negative electrode mixture obtained by appropriately adding a conductive additive (carbon material such as carbon black) or a binder such as PVDF to these negative electrode active materials is finished into a molded body using the current collector as a core material. A negative electrode mixture layer can be formed on the current collector surface.

(Electrolytes)
The electrolyte is contained inside the laminate 30. The electrolyte is not particularly limited, and, for example, in the present embodiment, an electrolyte solution containing a lithium salt (electrolyte aqueous solution, electrolyte solution using an organic solvent) can be used. However, the electrolyte aqueous solution is preferably an electrolyte solution (non-aqueous electrolyte solution) using an organic solvent because the electrochemical decomposition voltage is low, and the withstand voltage during charging is limited to a low level. As the electrolyte solution, a lithium salt dissolved in a non-aqueous solvent (organic solvent) is preferably used. For example, as the non-aqueous solvent, methyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, 1,2-dimethoxyethane, 1,3 -The organic solvent which consists only of 1 type, such as dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethyl ether, or these 2 or more types of mixed solvents is mentioned.

Examples of the lithium _{salt, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 SO 2 ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like.

  The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / L, and more preferably 0.9 to 1.25 mol / L.

  In addition, non-aqueous electrolytes include esters with double bonds such as vinylene carbonate; sulfur-containing organic compounds such as propane sultone; fluorine-containing fluorine such as fluorobenzene for the purpose of improving charge / discharge cycle characteristics and load characteristics of the battery. It is preferable to add additives such as aromatic compounds.

  In the present embodiment, the electrolyte may be a gel electrolyte obtained by adding a gelling agent in addition to liquid. Further, instead of the electrolyte solution, a solid electrolyte (a solid polymer electrolyte or an electrolyte made of an ion conductive inorganic material) may be contained.

  The case 50 seals the laminate 30 and the electrolyte solution therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the electrochemical device 100 from the outside. For example, as the case 50, as shown in FIG. 2, a metal laminate film in which a metal foil 52 is coated with a polymer film 54 from both sides can be used. For example, an aluminum foil can be used as the metal foil 52 and a film such as polypropylene can be used as the polymer film 54. For example, the material of the outer polymer film 54 is preferably a polymer having a high melting point such as polyethylene terephthalate (PET) or polyamide, and the material of the inner polymer film 54 is polyethylene (PE) or polypropylene (PP). preferable.

  The leads 60 and 62 are made of a conductive material such as aluminum.

  The material of the case is not particularly limited, and examples thereof include a cylindrical steel can (such as a rectangular tube or a cylinder) and an aluminum can that are used in a conventionally known lithium ion secondary battery. In addition, a laminate film obtained by vapor-depositing a metal on a resin film can be used for the exterior body.

  As mentioned above, although the separator of this invention, its manufacturing method, and an example of the lithium ion secondary battery using a separator were demonstrated, this invention is not limited to the said embodiment. The separator etc. of this invention can be suitably set according to a use etc.

  For example, the separator of the present invention can be used for electrochemical elements other than lithium ion secondary batteries. Electrochemical elements include secondary batteries other than lithium ion secondary batteries such as metal lithium secondary batteries (those using the active material of the present invention as the cathode and metal lithium as the anode), and electric batteries such as lithium capacitors. Examples include chemical capacitors. These electrochemical elements can be used for power sources such as self-propelled micromachines and IC cards, and distributed power sources arranged on or in a printed circuit board.

Example 1
(Preparation of heat-resistant resin solution)
Hybrid mixer containing 50% by weight of a polyamideimide solution (HPC-5000-30 manufactured by Hitachi Chemical Co., Ltd.), 15% by weight of polyethylene glycol (PEG4000 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and 35% by weight of N-methyl-2-pyrrolidone For 1 h.

(Preparation of undercoat liquid)
70% by weight of polyethylene glycol (PEG 400 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), 15% by weight of N-methyl-2-pyrrolidone, and 15% by weight of water were mixed for 1 hour with a hybrid mixer.

(Preparation of porous film)
A 20 μm thick polyethylene porous film (average pore diameter of 0.1 μm) was prepared as a porous film serving as a substrate.

(Application of liquid)
An undercoat liquid was injected into the polyethylene porous film from the back side thereof to form a 4 μm undercoat liquid layer on the upper surface of the polyethylene porous film. Thereafter, a 20 μm heat-resistant resin solution layer was formed on the undercoat layer.

  Thereafter, the liquid layer was immediately washed in a water bath (temperature: 40 ° C.) to deposit a polyamideimide resin, and a yellowish cloudy film (heat resistant resin film) was formed. The porous film on which the heat-resistant resin film was formed was immersed in a hot water bath for 5 minutes, and then immersed in ion-exchanged water and washed. After washing with ion exchanged water for 12 hours or more, the wet membrane was taken out of the water, and the water was blown off with air. Then, the whole was put into a heat oven and the membrane was dried while reducing the pressure at 60 ° C., and a polyethylene porous film having a thickness of 20 μm and a polyamideimide porous film having a thickness of 4 μm formed only on one surface (heat resistant) Resin film) was obtained.

(Examples 2 and 3)
The thickness of the layer of the undercoat liquid to be formed was the same as that of Example 1 except that the thickness was 6 μm and 8 μm. The thickness of the obtained polyamideimide porous membrane was 4, 5 μm, respectively.

(Comparative Example 1)
The same procedure as in Example 1 was performed except that no undercoat liquid layer was formed. The resulting polyamideimide porous membrane had a thickness of 4 μm.

Example 4
An undercoat layer was formed on both sides, a heat resistant resin solution was formed on both sides, and a polyamideimide porous membrane of 4.5 μm was finally obtained on both sides of the polyethylene porous film as in Example 1. did.

(Comparative Example 2)
The same procedure as in Example 4 was performed except that neither undercoat layer was formed on both sides. Each of the obtained polyamideimide porous membranes had a thickness of 3 μm.
Table 1 shows the manufacturing conditions, the thickness of the obtained heat-resistant resin film, the presence or absence of peeling of the heat-resistant resin film, the permeability of the separator, and the peel test of the heat-resistant resin film.

(Evaluation)
(1) Evaluation of adhesion of heat resistant resin film: It was visually confirmed whether the heat resistant resin film was peeled off from the porous film.

(2) Air permeability evaluation: A test piece of 100 × 100 mm was collected from the obtained separator, and the air permeability was measured according to JIS L8117.

(3) Peel test: A cross-cut tape peel test was performed on the heat-resistant resin film surface in accordance with JIS D0202-1988. Cellophane tape (“CT24”, manufactured by Nichiban Co., Ltd.) was adhered to the heat-resistant resin film with the belly of the finger, and then the cellophane tape was peeled from the separator. Judgment is represented by the number of squares in which the heat-resistant resin film has not peeled out of 100 squares, that is, the non-peeling ratio. The case where it did not peel at all was represented as 100/100. Interfacial peeling means peeling from the interface between the heat-resistant resin film and the porous film, and brittle fracture means peeling of the heat-resistant resin film and a part of the porous film from the remainder of the porous film. This indicates that the porous film has been brittlely broken.

(4) Measurement of pore diameter FIG. 4 shows a cross-sectional SEM photograph of the separator obtained in Example 1. In each Example, the boundary of the porosity was confirmed in the middle of the thickness direction, and the pore diameter Da of the first region 2a was larger than the pore diameter Db of the second region 2b. On the other hand, in the comparative example, such a boundary was not seen, and the pore diameter was uniform in the thickness direction.
From the SEM photograph of the cross section of the separator, a region (first region) having a width of 1 μm × thickness of 1 μm including the interface with the porous film 1 in the heat resistant resin film 2 is extracted, binarized, and perpendicular to the film surface. The diameter was taken at 50 points, and the average value was obtained as the average pore diameter of the first region 2a. Moreover, the area | region of width 1 micrometer x thickness 1 micrometer including the surface on the opposite side to the porous film 1 in the heat resistant resin film 2 was extracted, and it was set as the average pore diameter of the 2nd area | region 2b after that similarly.
In Examples 1, 2, 3, and 4, the ratio of the first region 2a having a large pore diameter to the total thickness of the polyamideimide porous membrane is about 62%, 71%, 79%, and 55%, respectively. Met.

(5) Battery characteristics evaluation:
Battery creation "Positive electrode"
LiMn ₂ O ₄ as a positive electrode active material, carbon black as a conductive auxiliary agent, and P (VDF-CTFE) as a binder were prepared. These were mixed so that the positive electrode active material: conducting aid: binder = 90: 5: 5 by weight ratio. The obtained mixture and NMP (N-methyl-2-pyrrolidone) solvent were mixed at a weight ratio of 1: 1.3 and dispersed at room temperature to prepare a cathode slurry. The obtained slurry for cathode was formed into a coating film by a doctor blade method and dried to prepare a cathode.

"Negative electrode"
An easily graphitizable carbon material was prepared as a negative electrode active material, carbon black was prepared as a conductive additive, and P (VDF-CTFE) was prepared as a binder. These were mixed so that the negative electrode active material: conducting aid: binder = 90: 5: 5 by weight ratio. The obtained mixture and NMP (N-methyl-2-pyrrolidone) solvent were mixed at a weight ratio of 1: 1 and dispersed at room temperature to prepare an anode slurry. The obtained anode slurry was formed into a coating film by the doctor blade method and dried to prepare an anode.

"Electrolyte"
An electrolyte was prepared by dissolving LiPF ₆ in a non-aqueous solvent with EC (ethylene carbonate) / DEC (diethyl carbonate) = 30/70 (weight ratio) to a concentration of 1 mol / cm ³ .

"battery"
The above positive electrode (diameter 14 mm) and negative electrode (diameter 15 mm) are laminated via the separator (diameter 16 mm) produced in the example, sealed in a container together with the electrolyte, and a button battery (2032 type) having a capacity of 4.2 mAh. )

(5a) Cell initial impedance The initial impedance of the battery was measured with a Solartron 1260 chemical impedance analyzer.

(5b) Rate evaluation The lithium ion secondary battery was subjected to constant current and constant voltage charging under conditions of a maximum voltage of 4.2 V, a current density of 0.068 mA / cm ² , and a final current density of 0.034 mA / cm ² . After that, the capacity when discharged at the final voltage of 2.75 V and the current density of 0.068, 0.136, and 0.341 mA / cm ² is 0.1 C, 0.2 C, and 0.5 C capacity, respectively. Asked. The capacity is described as a relative value with the discharge capacity of 0.1 C of Example 1 being 100.
The conditions and results are shown in Table 1.

Claims (4)

A porous film, and a porous heat-resistant resin film provided on the porous film,
The average pore diameter of the first region including the interface with the porous film in the heat resistant resin film is larger than the average pore diameter of the second region farther from the interface than the first region in the heat resistant resin film. big,
The thickness of said 1st area | region is a separator which is 55 to 79% of the thickness of the said heat resistant resin film.   The separator according to claim 1, wherein the ratio (Da / Db) of the average pore diameter Da of the first region to the average pore diameter Db of the second region is 2-30.   The separator according to claim 1 or 2, wherein an average pore diameter of the first region is larger than an average pore diameter of the porous film. The separator according to claim 1, wherein the porous film is a thermoplastic resin film.