Laminated porous film and non-aqueous electrolyte secondary battery

Abstract

The present invention provides a laminated porous film and a non-aqueous electrolyte secondary battery. The laminated porous film is a laminated porous film in which a heat-resistant layer comprising a binder resin and a filler is laminated on one or both of the surfaces of a porous film substrate mainly comprising a polyolefin, wherein a part occupied by at least one out of the binder resin and the filler is formed in the porous film substrate so as to touch the heat-resistant layer, and the total thickness of the occupied part is not less than 1% and not more than 20% of the overall thickness of the porous film substrate. The non-aqueous electrolyte secondary battery comprises the laminated porous film according as a separator.

Claims

1. A laminated porous film in which a heat-resistant layer comprising a binder resin and a filler is laminated on one or both of the surfaces of a porous film substrate mainly comprising a polyolefin, wherein a part occupied by at least one out of the binder resin and the filler is formed in the porous film substrate so as to touch the heat-resistant layer, and the total thickness of the occupied part is not less than 1% and not more than 20% of the overall thickness of the porous film substrate, the shape retention ratio upon heating of the laminated porous film heated at 150 C. for one hour, expressed by the smaller value of the MD direction or the TD direction, is 95% or more.

2. The laminated porous film according to claim 1, the resistivity measured at 145 C. is 7800 or more.

3. The laminated porous film according to claim 1, the thickness of the part occupied by at least one out of the binder resin and the filler is 0.4 m or more and 1.4 m or less.

4. The laminated porous film according to claim 1, wherein the thickness of a part substantially unoccupied by any binder resin or any filler in the porous film substrate is not less than 7 m.

5. The laminated porous film according to claim 1, wherein the binder resin is at least one resin selected from the group consisting of polyolefins, fluorine-containing resins, fluorine-containing rubbers, styrene-butadiene copolymers and hydrogenated products thereof, methacrylate copolymers, acrylonitrile-acrylate copolymers, styrene-acrylate copolymers, ethylene-propylene rubbers, polyvinyl acetate, resins having a melting point or a glass transition temperature of 180 C. or higher and water-soluble polymers.

6. A coating slurry for producing a laminated porous film used for a separator for a non-aqueous electrolyte secondary battery, wherein a heat-resistant layer comprising a binder resin and a filler is laminated on one or both of the surfaces of a porous film substrate mainly comprising a polyolefin, the coating slurry comprising the binder resin, the filler and a solvent to form the heat-resistant layer and having a contact angle with a polyethylene sheet (a hard polyethylene sheet of 1-mm thick grade produced by Kyoei Jushi Corporation) of 60 or more.

Description

EXAMPLES

(1) The present invention is described in more detail by way of examples, but the invention is not limited to the examples unless its gist is modified.

(2) In examples and comparative examples, physical properties of the separators were measured by the following methods (1) through (9).

(3) (1) Thickness Measurement (Unit: m)

(4) The thickness of a laminated porous film and the thickness of an A layer before the preparation of a laminated porous film were measured in accordance with JIS standard (K7130-1992).

(5) (2) Weight Per Area (Unit: g/m.sup.2)

(6) A film was cut into a square measuring 10 cm long on each side and then the weight W (g) thereof was measured. The weight per area was calculated by the following formula. The weight per area of the B layer was calculated by subtracting the weight per area of the porous film substrate (A layer) from the weight per area of the laminated porous film.
Weight per area (g/m.sup.2)=W/(0.10.1)
(3) Porosity (Unit: % by Volume)

(7) A film was cut into a square measuring 10 cm long on each side, and then the weight W (g) and the thickness D (cm) thereof were measured. The weights of the materials contained in the sample were calculated, the weight of each material Wi (g) was divided by the true specific gravity to calculate the volume of each material, and then the porosity (% by volume) was calculated from the following formula.
Porosity (% by volume)=100[{(W1/true specific gravity 1)+(W2/true specific gravity 2)+ . . . +(Wn/true specific gravity n)}/(1010D)]100
(4) Air Permeability (Unit: sec/100 cc)

(8) The air permeability of a film was measured using a Gurley densometer equipped with a digital timer manufactured by Toyo Seiki Seisaku-sho Ltd. on the basis of JIS P 8117.

(9) (5) Measurement of Shutdown (SD) Performance

(10) A cell for the measurement of shutdown was prepared by impregnating a 17.5 mm laminated porous film with an electrolytic solution, sandwiching the film between two SUS electrodes, and then fixing the film with a clip. A solution prepared by dissolving 1 mol/L of LiBF.sub.4 in a mixed solvent of 50% by volume of ethylene carbonate and 50% by volume of diethyl carbonate was used as the electrolytic solution. Terminals of an impedance analyzer were connected to the electrodes of the assembled cell, and the resistivity at 1 kHz was measured while raising the temperature at a rate of 15 C./minute in an oven. Thus, the resistivity measured at 145 C. was considered as the shutdown performance of the laminated porous film.

(11) (6) Measurement of Thickness Change of Porous Film Substrate (A Layer) Caused by Application

(12) A laminated porous film was immersed in water and thereby the heat-resistant layer (B layer) was washed away with water completely. Subsequently, without drying, the thickness of the porous film substrate (A layer) was measured by the same method as in the thickness measurement (1), and then the change in thickness of the A layer before and after coating was evaluated using the following formula.
Thickness change (m) of A layer=(thickness of A layer after removal of B layer)(thickness of A layer before application of B layer)
(7) Thickness of Heat-Resistant Layer (B Layer)

(13) The thickness of a B layer was calculated by the following formula.
Thickness (m) of B layer=(overall thickness of laminated porous film)(thickness of A layer after removal of B layer)
(8) Contact Angle Measurement

(14) One drop (2 L) of a coating slurry was dropped into a sample, and a contact angle was measured in 10 to 30 seconds after the dropping. This contact angle measurement was repeated 5 times in total and the average thereof was used as the contact angle of the sample. A contact angle meter (Model CA-X, manufactured by Kyowa Interface Science Co., Ltd.) was used for the measurement of a contact angle.

(15) The standard polyethylene sheet used was a hard polyethylene sheet of 1-mm thick grade (produced by Kyoei Jushi Corporation) available from KOKUGO Co., Ltd.

(16) (9) Evaluation of Thickness of Each Part in Laminated Porous Film

(17) Overall thickness of A layer: L

(18) Thickness of part occupied by at least one out of binder resin and filler relative to the interface between A layer and B layer: 11 (on one side), 12 (on the other side)

(19) Total thickness of part occupied by at least one out of binder resin and filler: L1

(20) Thickness of part substantially unoccupied by any binder resin or any filler: L2

(21) A laminated porous film was electronically stained with ruthenium tetroxide and then an epoxy resin was filled into the pores of the laminated porous film. After the epoxy resin cured, cross-section processing was conducted with FIB and the cross-section formed was observed with a SEM at an acceleration voltage of 2 kV and a magnification of 5000. Thus, L, 11, and 12 were evaluated.

(22) In the case of lamination on both sides, the sum total of 11 and 12 was taken as L1. The difference between L and L1 was taken as the thickness L2 of the part substantially unoccupied by any binder resin or any filler.

Example 1

(23) (1) Preparation of Coating Slurry

(24) The coating slurry of Example 1 was prepared in the following procedures. First, carboxymethylcellulose sodium (CMC, Cellogen 3H produced by Dai-Ichi Kogyo Seiyaku Co., Ltd.) was dissolved in a 20% by weight aqueous ethanol solution as a solvent, yielding a CMC solution (CMC concentration: 0.70% by weight vs. CMC solution). Subsequently, 3500 parts by weight alumina (AKP3000, produced by Sumitomo Chemical Co., Ltd.) was added and mixed with 100 parts by weight, in a CMC equivalent, of the CMC solution, followed by treatment with a Gaulin Homogenizer under high pressure dispersion conditions (60 MPa) repeated three times. Thus, a coating slurry 1 was prepared. The contact angle of the coating slurry 1 with a polyethylene sheet was 64. The composition of the coating slurry 1 is shown in Table 1.

(25) (2) Preparation of Porous Film Substrate

(26) A polyolefin resin composition was prepared by adding 70% by weight of an ultrahigh-molecular-weight polyethylene powder (340M, produced by Mitsui Chemicals, Inc.) and 30% by weight of a polyethylene wax with a weight average molecular weight of 1000 (FNP-0115, produced by Nippon Seiro Co., Ltd.) as well as, per 100 parts by weight of the ultrahigh-molecular-weight polyethylene and the polyethylene wax in total, 0.4% by weight of an antioxidant (Irg 1010, produced by Ciba Specialty Chemicals), 0.1% by weight of an antioxidant (P168, produced by Ciba Specialty Chemicals), and 1.3% by weight of sodium stearate, further adding calcium carbonate with an average pore diameter of 0.1 m (produced by Maruo Calcium Co., Ltd.) so as to occupy 38% by volume based on the overall volume, mixing these ingredients in the form of powder with a Henschel mixer, followed by melt-kneading with a twin screw kneading machine. The polyolefin resin composition was rolled into a sheet with a pair of rolls having a surface temperature of 150 C. Calcium carbonate was removed by immersing the sheet into an aqueous hydrochloric acid solution (hydrochloric acid: 4 mol/L, nonionic surfactant: 0.5% by weight) and then the sheet was stretched in TD, affording a porous film substrate A1. Properties of A1 are shown in Table 2.

(27) (3) Contact Angle Evaluation

(28) The contact angle of the porous film substrate A1 (untreated) obtained in (2) with the coating slurry 1 was 80. Subsequently, surface treatment was conducted by subjecting the surface of the porous film substrate A1 to corona discharge treatment at an output of 100 W/(m.sup.2/minute). The contact angle of the porous film substrate A1 after the surface treatment with the coating slurry 1 was 40.

(29) (4) Preparation of Laminated Porous Film

(30) A laminated porous film was prepared by applying the above-mentioned coating slurry 1 sequentially to both surfaces of the surface-treated porous film substrate A1 as a substrate with a gravure coating machine, and then drying the slurry. Physical properties of the porous film substrate A1, the heat-resistant layer, and the laminated porous film are shown in Tables 2 and 3. L, 11, 12, L1, L2, and the ratios among them determined from a cross-section SEM image of the laminated porous film are shown in Table 4. As shown in Table 4, thicknesses 11 and 12 were the same.

(31) (5) Heat Resistance Evaluation

(32) A resulting laminated porous film was cut into 8 cm8 cm. The laminated porous film on which a square of 6 cm6 cm had been drawn was placed in an oven of 150 C. and heated for one hour while being sandwiched between paper sheets. The shape retention ratio upon heating in the MD direction (i.e., the longitudinal direction at the time of sheet production) and the TD direction (i.e., the width direction at the time of sheet production) was calculated by measuring the distance between the lines of the film after heating. Thus, the shape retention ratio was found to be 99% in both MD and TD, so that the laminated porous film was found to be high in heat resistance.

Example 2

(33) (1) Preparation of Laminated Porous Film

(34) A commercially available porous film of polyethylene was used as a porous film substrate A2. Properties of A2 are shown in Table 2. The above-mentioned coating slurry 1 was used as a coating slurry. The contact angle of A2 (untreated) with the coating slurry 1 was 85.

(35) Subsequently, surface treatment was conducted by subjecting the surface of the porous film substrate A2 to corona discharge treatment at an output of 100 W/(m.sup.2/minute). The contact angle of the porous film substrate A2 after the surface treatment with the coating slurry 1 was 43.

(36) In addition, a laminated porous film was prepared by applying the above-mentioned coating slurry 1 sequentially to both surfaces of the surface-treated porous film substrate A2 as a substrate with a gravure coating machine, and then drying the slurry. Physical properties of the porous film substrate A2, the heat-resistant layer, and the laminated porous film are shown in Tables 2 and 3. L, 11, 12, L1, L2, and the ratios among them determined from a cross-section SEM image of the laminated porous film are shown in Table 4. As shown in Table 4, thicknesses 11 and 12 were the same.

(37) (2) Heat Resistance Evaluation

(38) The shape retention ratio upon heating of the resulting laminated porous film was calculated by the same operations as in Example 1. Thus, the shape retention ratio was found to be 99% in both MD and TD, so that the laminated porous film was found to be high in heat resistance.

Example 3

(39) (1) Preparation of Laminated Porous Film

(40) A coating slurry 2 was prepared by carrying out the same operations as for the coating slurry 1 except that isopropanol (IPA) was used instead of ethanol. The contact angle of the coating slurry 2 with a polyethylene sheet was 51. The composition of the coating slurry 2 is shown in Table 1.

(41) A commercially available porous film made of a polyolefin having a three-layer structure (polypropylene layer/polyethylene layer/polypropylene layer) was used as a porous film substrate A3. Properties of A3 are shown in Table 2. The above-mentioned coating slurry 2 was used as a coating slurry. The contact angle of A3 (untreated) with the coating slurry 2 was 63.

(42) Subsequently, surface treatment was conducted by subjecting the surface of the porous film substrate A3 to corona discharge treatment at an output of 36 W/(m.sup.2/minute). The contact angle of the porous film substrate A3 after the surface treatment with the coating slurry 2 was 34.

(43) In addition, a laminated porous film was prepared by applying the above-mentioned coating slurry 2 sequentially to both surfaces of the surface-treated porous film substrate A3 as a substrate with a gravure coating machine, and then drying the slurry. Physical properties of the porous film substrate A3, the heat-resistant layer, and the laminated porous film are shown in Tables 2 and 3. L, 11, 12, L1, L2, and the ratios among them determined from a cross-section SEM image of the laminated porous film are provided in Table 4. As shown in Table 4, thicknesses 11 and 12 were the same.

(44) (2) Heat Resistance Evaluation

(45) The shape retention ratio upon heating of the resulting laminated porous film was calculated by the same operations as in Example 1. Thus, the shape retention ratio was found to be 99% in both MD and TD, so that the laminated porous film was found to be high in heat resistance.

Comparative Example 1

(46) (1) Preparation of Coating Slurry

(47) A coating slurry 3 was prepared by carrying out the same operations as those for the coating slurry 1 except that the concentration of the aqueous ethanol solution was adjusted to 30% by weight in the operations of (1) preparation of coating slurry of the above-mentioned Example 1. The contact angle of the coating slurry 3 with a polyethylene sheet was 55. The composition of the coating slurry 3 is shown in Table 1.

(48) (2) Preparation of Laminated Porous Film

(49) A laminated porous film was prepared by applying the above-mentioned coating slurry 3 sequentially to both surfaces of the porous film substrate A1 as a substrate with a gravure coating machine, and then drying the slurry. Physical properties of the porous film substrate A1, the heat-resistant layer, and the laminated porous film are shown in Tables 2 and 3.

(50) L, 11, 12, L1, L2, and the ratios among them determined from a cross-section SEM image of the laminated porous film are provided in Table 4. Since the presence of the binder resin was observed throughout the A layer, the values of 11 and 12 are not provided.

Comparative Example 2

(51) The preparation of a laminated porous film was attempted by carrying out the operations as those in (4) preparation of laminated porous film of the above-mentioned Example 1 except that the corona discharge treatment was not carried out and the above-mentioned coating slurry 1 was applied onto both sides of the porous film substrate A1 sequentially and then dried. However, the coating slurry was repelled on the surfaces of the porous film substrate in applying the coating slurry 1 to the surfaces of A1, so that a uniform laminated porous film was not obtained.

(52) TABLE-US-00001 TABLE 1 Binder Filler Dispersion conditions resin (part by Liquid composition Number of Dispersing (part by weight) (% by weight) Contact angle Dispersing passes pressure weight) Alumina Binder () with PE Sample machine (pass) (MPa) CMC AKP3000 resin Water Alcohol sheet Coating gaulin 3 60 100 3500 0.7 80 20 64 slurry 1 Coating gaulin 3 60 100 3500 0.7 80 20 51 slurry 2 Coating gaulin 3 60 100 3500 0.7 70 30 55 slurry 3 Alcohol: ethanol in coating slurries 1 and 3, isopropanol in coating slurry 2. PE: polyethylene

(53) TABLE-US-00002 TABLE 2 Porous film substrate (A layer) Change in thickness Contact angle () Air before and After Weight Porosity permeability SD after Surface Coated Surface Thickness per area % by Gurley performance application Sample No. Material treatment surface Untreated treatment m g/m.sup.2 volume sec/100 cc m Example 1 A1 PE Present Both 80 40 18.1 7.0 59 88 8100 0.0 sides Example 2 A2 PE Present Both 85 43 17.3 10.0 38 524 85000 0.0 sides Example 3 A3 * Present Both 63 34 20.2 11.5 37 507 99000 0.0 sides Comparative A1 PE Absent Both 65 18.1 7.0 59 88 8100 1.2 Example 1 sides Comparative A1 PE Absent 65 18.1 7.0 59 88 8100 Example 2 PE: polyethylene *: three layers of polypropylene/polyethylene/polypropylene

(54) TABLE-US-00003 TABLE 3 Heat-resistant layer (B layer) Properties of laminated porous film Basis weight Overall Weight per Overall film weight per Coating Thickness area thickness area Air permeability Gurley SD performance Sample slurry m g/m.sup.2 m g/m.sup.2 sec/100 cc Example 1 Coating 8.2 11.7 26.3 18.7 120 7800 slurry 1 Example 2 Coating 6.6 8.9 23.9 18.9 699 10200 slurry 1 Example 3 Coating 6.4 7.8 26.6 19.3 526 40200 slurry 2 Comparative Coating 7.7 12.1 24.6 19.1 145 130 Example 1 slurry 3 Comparative Coating Example 2 slurry 1

(55) TABLE-US-00004 TABLE 4 Thickness (m) Proportion (%) L 11 12 L1 L2 11/L 12/L L1/L Example 1 17.9 1.4 1.4 2.8 15.1 7.8 7.8 15.6 Example 2 17.4 0.5 0.5 1 16.4 2.9 2.9 5.7 Example 3 20.0 0.4 0.4 0.8 19.2 2.0 2.0 4.0 Comparative 16.7 16.7 0.0 100.0 Example 1

INDUSTRIAL APPLICABILITY

(56) According to the present invention, a laminated porous film is provided which is superior in ion permeability (air permeability) and shutdown property and also in shape retention under heating and which is suitable as a separator of a non-aqueous electrolyte secondary battery.

(57) According to the present invention, a laminated porous film superior in thermal stability and ion permeability (air permeability) is provided. The present invention is very useful industrially because a non-aqueous electrolyte secondary battery including a laminated porous film as a separator allows the separator to prevent the positive electrode and the negative electrode from coming into contact directly with each other even if the battery generates heat and the non-aqueous electrolyte secondary battery is rendered safer by the preservation of insulating properties due to rapid closure of pores of the porous film substrate mainly made of a polyolefin.