Propylene-based resin microporous film, separator for battery, battery, and method for producing propylene-based resin microporous film

Abstract

The present invention provides a propylene-based resin microporous film which has excellent electrolyte solution retention property, and can provide a lithium ion battery in which a decrease in discharge capacity is highly reduced even after repeated charge and discharge. The propylene-based resin microporous film is a propylene-based resin microporous film having micropores, wherein a propylene-based resin having a weight average molecular weight of 250,000 to 500,000, a melting point of 160 to 170° C., and a pentad fraction of 96% or more is contained, the surface aperture ratio is 27 to 42%, the ratio of a surface aperture ratio to a porosity is 0.6 or less, and the degree of gas permeability is 50 to 400 s/100 mL.

Claims

1. A propylene-based resin microporous film having micropores, comprising a propylene-based resin having a weight average molecular weight of 250,000 to 500,000, a melting point of 160 to 170° C., and a pentad fraction of 96% or more, and having a surface aperture ratio of 27 to 42%, a ratio of a surface aperture ratio to a porosity of 0.6 or less, a degree of gas permeability of 50 to 400 s/100 mL, and an electrolyte solution retention amount of 0.90 g/cm.sup.3 to 1.5 g/cm.sup.3.

2. The propylene-based resin microporous film according to claim 1, wherein aperture edges of the micropores have a longest diameter of 100 nm to 1 μm and an average longer diameter of 10 to 500 nm.

3. A separator for a battery, comprising the propylene-based resin microporous film according to claim 1.

4. A battery, comprising the separator for a battery according to claim 3.

5. A propylene-based resin microporous film having micropores, comprising a propylene-based resin having a weight average molecular weight of 250,000 to 500,000, a melting point of 160 to 170° C., a pentad fraction of 96% or more and a molecular weight distribution (weight average molecular weight/number average molecular weight) of 7.5 to 12.0, and having a surface aperture ratio of 27 to 42%, a ratio of a surface aperture ratio to a porosity of 0.6 or less, and a degree of gas permeability of 50 to 400 s/100 mL.

Description

DESCRIPTION OF EMBODIMENTS

(1) Hereinafter, Examples of the present invention will be described. The present invention is not limited to Examples.

Examples 1 to 6 and Comparative Examples 1 to 4

(2) (Extrusion Step)

(3) A homopolypropylene having the weight average molecular weight, the number average molecular weight, the melting point, and the pentad fraction, shown in Table 1, was supplied to an extruder, melt-kneaded at a resin temperature of 200° C., and extruded through a T-die attached to the tip of the extruder into a film, to obtain an elongated homopolypropylene film. The homopolypropylene film was then cooled to a surface temperature of 30° C. The thickness of the homopolypropylene film was 30 μm and the width thereof was 200 mm. The extruded rate was 10 kg/hr., the film-forming rate was 22 m/min., and the draw ratio was 83.

(4) (Aging Step)

(5) The resulting elongated homopolypropylene film having a length of 50 m was wound around a cylindrical core having an outer diameter of 3 inches into a roll, to obtain a homopolypropylene film roll. The homopolypropylene film roll was allowed to stand in a hot blast furnace at a temperature under an atmosphere where the homopolypropylene film roll was placed of 150° C. over 24 hours, and the homopolypropylene film was aged. At this time, the overall temperature of the homopolypropylene film from the surface to the inside of the homopolypropylene film roll was the same as the temperature in the hot blast furnace.

(6) (First Stretching Step (A))

(7) Next, the homopolypropylene film was unwound from the aged homopolypropylene film roll, and cut into a strip shape of 300 mm in the extrusion direction (longitudinal direction) and 160 mm in the width direction. The cut homopolypropylene film was uniaxially stretched at a surface temperature of 23° C., a stretching rate of 50%/min, and a stretching ratio shown in a column of first stretching step (A) of Table 1, only in the extrusion direction using a uniaxial stretching device (“IMC-18C6” manufactured by Imoto Machinery Co., Ltd.).

(8) (First Stretching Step (B))

(9) Subsequently, the homopolypropylene film was uniaxially stretched at a surface temperature of 120° C., a stretching rate of 42%/min, a stretching ratio shown in a column of first stretching step (B) of Table 1, only in the extrusion direction using the uniaxial stretching device (“IMC-18C6” manufactured by Imoto Machinery Co., Ltd.).

(10) (First Annealing Step)

(11) After that, the homopolypropylene film was allowed to stand over 10 minutes at a surface temperature of 130° C. so that a tension was not applied to the homopolypropylene film, and thus annealed to obtain a homopropylene microporous film having a thickness of 25 μm. The shrinkage ratio of the homopolypropylene film in the first annealing step was 20%.

Examples 7 to 12 and Comparative Examples 5 to 9

(12) (Extrusion Step)

(13) A homopolypropylene having the weight average molecular weight, the number average molecular weight, the melting point, and the pentad fraction, shown in Table 2, was supplied to an extruder, melt-kneaded at a resin temperature of 200° C., and extruded through a T die attached to the tip of the extruder into a film, to obtain an elongated homopolypropylene film. The homopolypropylene film was then cooled to a surface temperature of 30° C. The thickness of the homopolypropylene film was 30 μm and the width thereof was 200 mm. The extruded rate was 10 kg/hr., the film-forming rate was 22 m/min., and the draw ratio was 83.

(14) (Aging Step)

(15) The resulting elongated homopolypropylene film having a length of 50 m was wound around a cylindrical core having an outer diameter of 3 inches into a roll, to obtain a homopolypropylene film roll. The homopolypropylene film roll was allowed to stand in a hot blast furnace at a temperature under an atmosphere where the homopolypropylene film roll was placed of 150° C. over 24 hours, and the homopolypropylene film was aged. At this time, the overall temperature of the homopolypropylene film from the surface to the inside of the homopolypropylene film roll was entirely the same as the temperature in the hot blast furnace.

(16) (First Stretching Step (A))

(17) Next, the homopolypropylene film was unwound from the aged homopolypropylene film roll, and cut into a strip shape of 300 mm in the extrusion direction (longitudinal direction) and 160 mm in the width direction. The cut homopolypropylene film was stretched at a surface temperature of 23° C., stretching rate of 50%/min, and a stretching ratio shown in a column of first stretching step (A) of Table 2, only in the extrusion direction using a uniaxial stretching device (“IMC-18C6” manufactured by Imoto Machinery Co., Ltd.).

(18) (First Stretching Step (B))

(19) Subsequently, the homopolypropylene film was stretched at a surface temperature of 120° C., a stretching rate of 42%/min, a stretching ratio shown in a column of first stretching step (B) of Table 2, only in the extrusion direction using the uniaxial stretching device (“IMC-18C6” manufactured by Imoto Machinery Co., Ltd.).

(20) (First Annealing Step)

(21) After that, the homopolypropylene film was allowed to stand over 10 minutes at a surface temperature of 130° C. so that a tension was not applied to the homopolypropylene film, and thus annealed. The shrinkage ratio of the homopolypropylene film in the first annealing step was 20%.

(22) (Second Stretching Step)

(23) Subsequently, the homopolypropylene film was hot-stretched at a surface temperature of 120° C., a stretching rate of 42%/min, and a stretching ratio of 1.2 times in the width direction (direction perpendicular to the extrusion direction) using the uniaxial stretching device (“IMC-18C6” manufactured by Imoto Machinery Co., Ltd.). As a result, a homopolypropylene film biaxially stretched was obtained.

(24) (Second Annealing Step)

(25) After that, the homopolypropylene film was allowed to stand over 10 minutes at a surface temperature of 130° C. so that a tension was not applied to the homopolypropylene film, and thus annealed to obtain a homopropylene microporous film having a thickness of 23 μm. The shrinkage ratio of the homopolypropylene film in the second annealing step was 20%.

(26) [Evaluation 1]

(27) The surface aperture ratio, the porosity, the degree of gas permeability, the longest diameter and the average longer diameter of aperture edges of the micropores, and the electrolyte solution retention amount of the homopolypropylene microporous films obtained in Examples and Comparative Examples were measured by the above-discussed procedures. The results are shown in Tables 1 and 2.

(28) [Evaluation 2]

(29) Further, the electrolyte solution permeability (in the extrusion direction and the width direction) of the homopolypropylene microporous films obtained in Examples and Comparative Examples were measured as described below. The results are shown in Tables 1 and 2.

(30) A homopolypropylene microporous film was first cut to obtain a specimen having a plane rectangle with a width of 10 mm and a length of 120 mm. At this time, the extrusion direction (longitudinal direction) of the homopolypropylene microporous film was set to the longitudinal direction of the specimen. Subsequently, an end portion of the specimen in the longitudinal direction was fixed to a stainless plate with an adhesive tape. After that, the specimen was disposed so that the longitudinal direction thereof was perpendicular to the water surface of the electrolyte solution, and the other end portion of the specimen in the longitudinal direction was sunk into the electrolyte solution over 10 minutes. At this time, the highest height (mm) at which the electrolyte solution rose in the longitudinal direction of the specimen from the water surface was measured. As the electrolyte solution, an electrolyte solution containing 1 mol/L of LiPF.sub.6 in an organic solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC (by volume)=1:1) was used. By the same procedure as described above, five specimens were prepared from the homopolypropylene microporous film. The highest heights (mm) at which the electrolyte solution rose in the longitudinal direction of the specimens from the water surface were measured, and the arithmetic average thereof was considered as the electrolyte solution permeability in the extrusion direction of the homopolypropylene microporous film.

(31) The electrolyte solution permeability in the width direction of the homopolypropylene microporous film was measured in the same manner as described above except that when the homopolypropylene microporous film was cut into a plane rectangle with a width of 10 mm and a length of 120 mm to obtain a specimen, the width direction (direction perpendicular to the extrusion direction) of the homopolypropylene microporous film was set to the longitudinal direction of the specimen.

(32) [Evaluation 3]

(33) A lithium ion battery was produced using each of the homopolypropylene microporous films in Examples and Comparative Examples as a separator in accordance with the following procedure. The discharge capacity of the lithium ion battery was measured. The results are shown in Tables 1 and 2.

(34) (Production of Lithium Ion Battery)

(35) 92% by weight of LiMn.sub.2O.sub.4 (average particle diameter: 26 μm) as a positive electrode active material, 4% by weight of carbon black as a conductive auxiliary agent, and 4% by weight of poly(vinylidene fluoride) as a binder resin were mixed and stirred to prepare a composition for formation of a positive electrode. The composition for formation of a positive electrode was applied to a surface of an aluminum foil as a positive electrode collector using a coater, and then dried, to prepare a positive electrode active material layer. After that, the positive electrode collector having the positive electrode active material layer on the surface was punched to obtain a positive electrode. The positive electrode had a plane rectangle with a width of 30 mm and a length of 60 mm.

(36) Next, 91% by weight of graphite particles as a negative electrode active material, 5% by weight of carbon black as a conductive auxiliary agent, and 4% by weight of poly(vinylidene fluoride) as a binder resin were mixed and stirred to prepare a composition for formation of a negative electrode. As a negative electrode collector, an electrolytic copper foil of which a surface was roughened by an electrolysis method was prepared. The composition for formation of a negative electrode was applied to the roughened surface of the electrolytic copper foil using a coater, and then dried, to prepare a negative electrode active material layer. After that, the negative electrode collector having the negative electrode active material layer on the surface was punched to obtain a negative electrode. The negative electrode had a plane rectangle with a width of 30 mm and a length of 60 mm.

(37) The positive electrode, the homopolypropylene microporous film, and the negative electrode were overlaid so that the positive electrode active material layer and the negative electrode active material layer were opposite to each other with the homopolypropylene microporous film interposed therebetween, to form a layered body. A tab was disposed on each of the positive electrode and the negative electrode, and the layered body was dried under reduced pressure at 80° C. over 12 hours. The layered body dried under reduced pressure was put in an exterior case. Subsequently, an electrolyte solution was poured into the exterior case under an argon gas atmosphere, and the exterior case was sealed under reduced pressure, to produce a lithium ion battery. As the electrolyte solution, an electrolyte solution containing 1 mol/L of LiPF.sub.6 in a mixed solution obtained by mixing ethylene carbonate and ethylmethyl carbonate at 3:7 (by volume) was used.

(38) (Discharge Capacity)

(39) The lithium ion battery was placed in a constant temperature bath at 25° C., and charged and discharged as follows. The lithium ion battery was charged to a voltage of 4.1 V at a current corresponding to 0.2 C, and then discharged to a voltage of 2.7 V at a current corresponding to 1 C. The lithium ion battery was then charged to a voltage of 4.1 V at a current corresponding to 1 C, and discharged to a voltage of 2.7 V at a current corresponding to 1 C. After the charge and discharge, the initial discharge capacity A.sub.1 (mAh) of the lithium ion battery was measured. After that, the lithium ion battery was charged to a voltage of 4.1 V at a current corresponding to 1 C, and discharged to a voltage of 2.7 V at a current corresponding to 1 C. The charge and discharge under the above condition was considered as 1 cycle. 500 Cycles of charge and discharge were carried out under the same conditions. The discharge capacity A.sub.500 (mAh) of the lithium ion battery after 500 cycles of charge and discharge was measured. The retention ratio ((%)=A.sub.500/A.sub.1×100) of the discharge capacity to the initial discharge capacity A.sub.1 was calculated.

(40) TABLE-US-00001 TABLE 1 EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE 1 2 3 4 5 HOMOPOLY- WEIGHT AVERAGE 427000 371000 371000 427000 390000 PROPYLENE MOLECULAR WEIGHT (Mw) NUMBER AVERAGE 45100 43200 43200 45100 48800 MOLECULAR WEIGHT (Mn) MOLECULAR WEIGHT 9.5 8.6 8.6 9.5 8.0 DISTRIBUTION (Mw/Mn) MELTING POINT (° C.) 165 165 165 165 165 PENTAD FRACTION (%) 96 98 97 96 96 FIRST STRETCHING STRETCHING RATIO (TIME) 1.2 1.2 1.3 1.2 1.2 STEP (A) FIRST STRETCHING STRETCHING RATIO (TIME) 2.0 2.0 2.2 1.7 2.0 STEP (B) HOMOPOLY- SURFACE APERTURE 30 31 32 27 28 PROPYLENE RATIO (%) MICROPOROUS POROSITY (%) 53 52 54 46 48 FILM RATIO (SURFACE APERTURE 0.57 0.59 0.59 0.59 0.58 RATIO/POROSITY) DEGREE OF GAS 160 164 100 400 190 PERMEABILITY (s/100 mL) LONGEST DIAMETER (nm) 430 440 530 350 380 AVERAGE LONGER 220 230 320 190 200 DIAMETER (nm) ELECTROLYTE SOLUTION 0.98 1.10 1.05 0.98 0.98 RETENTION AMOUNT (g/cm.sup.3) ELECTROLYTE SOLUTION 29 29 30 28 28 PERMEABILITY (mm) IN EXTRUSION DIRECTION ELECTROLYTE SOLUTION 49 48 50 47 48 PERMEABILITY (mm) IN WIDTH DIRECTION LITHIUM ION INITIAL DISCHARGE 60 60 59 59 60 BATTERY CAPACITY A.sub.1 (mAh) DISCHARGE CAPACITY A.sub.500 50 51 51 49 50 (mAh) AFTER 500 CYCLES RETENTION RATIO (%) OF 83 85 86 83 83 DISCHARGE CAPACITY AFTER 500 CYCLES EXAMPLE COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE 6 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 HOMOPOLY- WEIGHT AVERAGE 400000 376000 375000 330000 428000 PROPYLENE MOLECULAR WEIGHT (Mw) NUMBER AVERAGE 39400 57300 62300 49900 101000 MOLECULAR WEIGHT (Mn) MOLECULAR WEIGHT 10.2 6.6 6.0 6.6 4.3 DISTRIBUTION (Mw/Mn) MELTING POINT (° C.) 164 165 165 161 165 PENTAD FRACTION (%) 97 92 94 93 92 FIRST STRETCHING STRETCHING RATIO (TIME) 1.2 1.2 1.2 1.2 1.2 STEP (A) FIRST STRETCHING STRETCHING RATIO (TIME) 2.0 2.0 2.0 2.0 2.0 STEP (B) HOMOPOLY- SURFACE APERTURE 31 26 26 26 13 PROPYLENE RATIO (%) MICROPOROUS POROSITY (%) 53 41 42 54 45 FILM RATIO (SURFACE APERTURE 0.58 0.63 0.62 0.48 0.29 RATIO/POROSITY) DEGREE OF GAS 150 210 190 200 600 PERMEABILITY (s/100 mL) LONGEST DIAMETER (nm) 400 350 550 1020 300 AVERAGE LONGER 220 210 270 550 140 DIAMETER (nm) ELECTROLYTE SOLUTION 1.00 0.67 0.60 0.91 0.52 RETENTION AMOUNT (g/cm.sup.3) ELECTROLYTE SOLUTION 29 25 29 29 20 PERMEABILITY (mm) IN EXTRUSION DIRECTION ELECTROLYTE SOLUTION 50 42 40 50 34 PERMEABILITY (mm) IN WIDTH DIRECTION LITHIUM ION INITIAL DISCHARGE 60 58 59 60 60 BATTERY CAPACITY A.sub.1 (mAh) DISCHARGE CAPACITY A.sub.500 51 44 44 45 42 (mAh) AFTER 500 CYCLES RETENTION RATIO (%) OF DISCHARGE CAPACITY 85 76 75 75 70 AFTER 500 CYCLES

(41) TABLE-US-00002 TABLE 2 EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE EXAMPLE 7 8 9 10 11 12 HOMOPOLY- WEIGHT AVERAGE 427000 371000 371000 427000 390000 400000 PROPYLENE MOLECULAR WEIGHT (Mw) NUMBER AVERAGE 45100 43200 43200 45100 48800 39400 MOLECULAR WEIGHT (Mn) MOLECULAR WEIGHT 9.5 8.6 8.6 9.5 8.0 10.2 DISTRIBUTION (Mw/Mn) MELTING POINT (° C.) 165 165 165 165 165 164 PENTAD FRACTION (%) 96 98 97 96 96 97 FIRST STRETCHING RATIO (TIME) 1.2 1.2 1.3 1.2 1.2 1.2 STRETCHING STEP (A) FIRST STRETCHING RATIO (TIME) 2.0 2.0 2.2 1.7 2.0 2.0 STRETCHING STEP (B) HOMOPOLY- SURFACE APERTURE 28 28 30 27 28 29 PROPYLENE RATIO (%) MICROPOROUS POROSITY (%) 55 54 56 54 54 55 FILM RATIO (SURFACE APERTURE 0.51 0.52 0.53 0.5 0.52 0.53 RATIO/POROSITY) DEGREE OF GAS 100 110 60 200 120 110 PERMEABILITY (s/100 mL) LONGEST DIAMETER (nm) 420 435 480 330 360 370 AVERAGE LONGER 210 220 290 180 190 200 DIAMETER (nm) ELECTROLYTE SOLUTION 1.17 1.20 1.21 1.15 1.20 1.22 RETENTION AMOUNT (g/cm.sup.3) ELECTROLYTE SOLUTION 37 36 37 35 35 36 PERMEABILITY (mm) IN EXTRUSION DIRECTION ELECTROLYTE SOLUTION 50 50 50 48 50 51 PERMEABILITY (mm) IN WIDTH DIRECTION LITHIUM ION INITIAL DISCHARGE 60 60 60 60 60 60 BATTERY CAPACITY A.sub.1 (mAh) DISCHARGE CAPACITY A.sub.500 52 53 53 51 52 52 (mAh) AFTER 500 CYCLES RETENTION RATIO (%) OF 87 88 88 85 87 87 DISCHARGE CAPACITY AFTER 500 CYCLES COMPAR- COMPAR- COMPAR- COMPAR- COMPAR- ATIVE ATIVE ATIVE ATIVE ATIVE EXAMPLE 5 EXAMPLE 6 EXAMPLE 7 EXAMPLE 8 EXAMPLE 9 HOMOPOLY- WEIGHT AVERAGE 375000 330000 428000 146000 587000 PROPYLENE MOLECULAR WEIGHT (Mw) NUMBER AVERAGE 62300 49900 101000 51100 76600 MOLECULAR WEIGHT (Mn) MOLECULAR WEIGHT 6 6.6 4.3 2.9 7.7 DISTRIBUTION (Mw/Mn) MELTING POINT (° C.) 165 161 165 163 162 PENTAD FRACTION (%) 94 93 92 96 96 FIRST STRETCHING RATIO (TIME) 1.2 1.2 1.2 1.2 1.2 STRETCHING STEP (A) FIRST STRETCHING RATIO (TIME) 2.0 2.0 2.0 2.0 2.0 STRETCHING STEP (B) HOMOPOLY- SURFACE APERTURE 26 25 14 9 9 PROPYLENE RATIO (%) MICROPOROUS POROSITY (%) 43 55 46 32 30 FILM RATIO (SURFACE APERTURE 0.6 0.45 0.3 0.28 0.3 RATIO/POROSITY) DEGREE OF GAS 160 180 540 1800 2100 PERMEABILITY (s/100 mL) LONGEST DIAMETER (nm) 510 990 290 420 200 AVERAGE LONGER 240 520 130 110 85 DIAMETER (nm) ELECTROLYTE SOLUTION 0.73 0.98 0.60 0.32 0.25 RETENTION AMOUNT (g/cm.sup.3) ELECTROLYTE SOLUTION 30 32 23 12 11 PERMEABILITY (mm) IN EXTRUSION DIRECTION ELECTROLYTE SOLUTION 42 48 35 13 13 PERMEABILITY (mm) IN WIDTH DIRECTION LITHIUM ION INITIAL DISCHARGE 59 60 60 52 50 BATTERY CAPACITY A.sub.1 (mAh) DISCHARGE CAPACITY A.sub.500 45 46 43 5 5 (mAh) AFTER 500 CYCLES RETENTION RATIO (%) OF 76 77 72 9.6 10 DISCHARGE CAPACITY AFTER 500 CYCLES

INDUSTRIAL APPLICABILITY

(42) The propylene-based resin microporous film of the present invention has excellent electrolyte solution retention property. Therefore, the propylene-based resin microporous film is suitably used as a separator for a battery. The propylene-based resin microporous film can provide a battery in which a decrease in discharge capacity due to degradation of an electrolyte solution is highly reduced even after repeated charge and discharge.