Synthetic resin microporous film and manufacturing method thereof, and separator for power storage device and power storage device

Abstract

The present invention provides a synthetic resin microporous film which has excellent permeability of lithium ions, can constitute high performance power storage devices, and is less likely to cause a short circuit between a positive electrode and a negative electrode as well as rapid decrease in discharge capacity due to a dendrite even when used in high power applications. The synthetic resin microporous film of the present invention is a synthetic resin microporous film comprising a synthetic resin, the synthetic resin microporous film being stretched, the synthetic resin microporous film having, in a cross section along a thickness direction and a stretching direction of the synthetic resin microporous film: a plurality of support portions extending in the thickness direction of the synthetic resin microporous film; a plurality of fibrils formed between the support portions; and the support portions having the number of branch structures of 150 or less per 100 μm.sup.2; and the synthetic resin microporous film being configured such that micropore portions are formed in areas surrounded by the support portions and the fibrils.

Claims

1. A synthetic resin microporous film comprising a synthetic resin, the synthetic resin microporous film being stretched, the synthetic resin microporous film having, in a cross section along a thickness direction and a stretching direction of the synthetic resin microporous film; a plurality of support portions extending in the thickness direction of the synthetic resin microporous film; a plurality of fibrils formed between the support portions; and the support portions having the number of branch structures of 150 or less per 100 μm.sup.2; the synthetic resin micro porous film being configured such that micropore portions are formed in areas surrounded by the support portions and the fibrils; and the synthetic resin microporous film having a degree of gas permeability of 30 sec/100 mL/16 μm or more and 80 sec/100 mL/16 μm or less; and a porosity is 40% or more and 70% or less.

2. The synthetic resin microporous film according to claim 1, wherein the synthetic resin includes an olefin-based resin.

3. A separator for a power storage device comprising the synthetic resin microporous film according to claim 1.

4. A power storage device comprising the separator for a power storage device according to claim 3.

5. A method of manufacturing the synthetic resin microporous film according to claim 1, comprising: an extrusion step of supplying a synthetic resin into an extruder for melting and kneading, and extruding the melted and kneaded synthetic resin from a T die attached to a tip of the extruder to obtain a synthetic resin film; an aging step of aging the synthetic resin film obtained in the extrusion step for 1 minute or more such that a surface temperature of the synthetic resin film is in a range from 30° C. less than a melting point of the synthetic resin to 1° C. less than the melting point of the synthetic resin; a stretching step consisting of a step of uniaxially stretching the synthetic resin film after the aging step at a strain rate of 10%/min or more and 250%/min or less and a stretching ratio of 1.5 to 2.8 times; and an annealing step of annealing the synthetic resin film after the stretching step.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an enlarged photograph of a cross section along a thickness direction and a stretching direction of a synthetic resin microporous film (homopolypropylene microporous film) manufactured in Example 1.

(2) FIG. 2 is a schematic view illustrating the concept of directions in judging whether or not it is a fibril.

(3) FIG. 3 is a schematic view illustrating a procedure for judging whether or not support portions are branched from each other.

(4) FIG. 4 is a schematic view illustrating a procedure for judging whether or not support portions are branched from each other.

(5) FIG. 5 is an enlarged photograph of a cross section along a thickness direction and a stretching direction of a synthetic resin microporous film (homopolypropylene microporous film) manufactured in Example 1.

(6) FIG. 6 is an enlarged photograph of a cross section along a thickness direction and a stretching direction of a synthetic resin microporous film (homopolypropylene microporous film) manufactured in Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

(7) Although examples of the present invention will foe described below, the present invention is not limited by these examples.

Examples 1 to 6, Comparative Example 1

(8) (Extrusion Step)

(9) A homopolypropylene having a weight-average molecular weight, number-averaged molecular weight, and melting point indicated in Table 1 was supplied into an extruder, melted and kneaded at a resin temperature indicated in Table 1, and extruded from a T die attached to the tip of the extruder into a film shape. Thereafter, the extruded product was cooled until the surface temperature thereof became 30° C. to obtain a long-length homopolypropylene film having a thickness of 30 μm and a width of 200 mm. It is noted that the film forming rate, extrusion amount, and draw ratio were as indicated in Table 1.

(10) (Aging Step)

(11) Next, the homopolypropylene film was aged for a time (aging time) indicated in Table 1 such that the surface temperature thereof became an aging temperature indicated in Table 1.

(12) (Stretching Step)

(13) Next, using a uniaxially stretching apparatus, the aged homopolypropylene film was uniaxially stretched only in the extrusion direction at a strain rate indicated in Table 1 and a stretching ratio indicated in Table 1 such that the surface temperature thereof became a temperature indicated in Table 1.

(14) (Annealing Step)

(15) Thereafter, the homopolypropylene film was supplied into a hot air furnace, and traveled for 1 minute while tension was not applied to the homopolypropylene film, such that the surface temperature of the homopolypropylene film became 130° C. In this manner, the homopolypropylene film was annealed to obtain a long-length homopropylene microporous film having a thickness of 25 μm. It is noted that the shrinkage rate of the homopolypropylene film in the annealing step was a value indicated in Table 1.

(16) [Evaluation]

(17) For the obtained homopolypropylene microporous film, the number of branch structures of the support portions per 100 μm.sup.2 was measured according to the above-described procedure. The results are shown in Table 1.

(18) For the obtained homopolypropylene microporous film, the degree of gas permeability, shrinkage rate (90° C. and 120° C.), porosity, and thickness were measured. The results are shown in Table 1.

(19) For the obtained homopolypropylene microporous film, the DC resistance and dendrite resistance were measured. The results are shown in Table 1.

(20) For the homopolypropylene microporous film manufactured in Example 1, an enlarged photograph at a magnification of 10,000 times of a cross section along a thickness direction and a stretching direction is shown in FIG. 5.

(21) For the homopolypropylene microporous film manufactured in Comparative Example 1, an enlarged photograph at a magnification of 10,000 times of a cross section along a thickness direction and a stretching direction is shown in FIG. 6.

(22) (Shrinkage Rate)

(23) The shrinkage rate at 90° C. and 120° C. of homopolypropylene was measured according to the following procedure. A test piece was prepared by cutting out the homopolypropylene microporous film at room temperature into a square of 12 cm×12 cm such that one side became parallel to the MD direction (extrusion direction). A straight line having a length of 10 cm was drawn parallel to the MD direction (extrusion direction) on the center section of the test piece. While the test piece was inserted between two pieces of blue plate float glass having a planar rectangular shape with a 15 cm side and having a thickness of 2 mm for stretching the wrinkles of the test piece, the length of the straight line was read to the 1/10 μm place at room temperature (25° C.) using a two-dimensional length measuring machine (trade name “CW-2515N” manufactured by Chien Wei Precise Technology Co., Ltd.). The read length of the straight line was defined as an initial length L.sub.3. Next, the test, piece was stored in a constant temperature bath (trade name “OF-450B” manufactured by AS One Corporation) having been set to become 90° C. or 120° C. for one week, and thereafter removed. The length of the straight line of the test, piece after heating was read to the 1/10 μm place at room temperature (25° C.) using a two-dimensional length measuring machine (trade name “CW-2515N” manufactured by Chien Wei Precise Technology Co., Ltd.). The read length of the straight line was defined as a length after heating L.sub.4. According to the following formula, the shrinkage rate at 90° C. or 120° C. was calculated.
Shrinkage rate (%)=100×[(initial length L.sub.3)—(length after heating L.sub.4)]/(initial length L.sub.3)

(24) (DC Resistance)

(25) A positive electrode and a negative electrode were prepared according to the following procedure to produce a small battery. The DC resistance of the obtained small battery was measured.

(26) <Production Method of Positive Electrode>

(27) In an Ishikawa grinding mortar, Li.sub.2CO.sub.3 and a coprecipitated hydroxide represented by Ni.sub.0.5Co.sub.0.2Mn.sub.0.3(OH).sub.2 were mixed such that the molar ratio of Li and the whole transition metal became 1.08:1. Thereafter, the mixture was subjected to a heat treatment in the air atmosphere at 950° C. for 20 hours, and thereafter pulverized. Accordingly, Li.sub.1.04Ni.sub.0.5CO.sub.0.2Mn.sub.0.3O.sub.2 having an average secondary particle diameter of about 12 μm was obtained as a positive electrode active material.

(28) The positive electrode active material obtained as described above, acetylene black (trade name “HS-100” manufactured by Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive auxiliary, and polyvinylidene fluoride (trade name “#7208” manufactured by Kureha Corporation) as a binder were mixed at a ratio of 91:4.5:4.5 (% by mass). This mixture was poured and mixed into N-methyl-2-pyrrolidone to produce a slurry solution. This slurry solution was applied onto an aluminum foil (manufactured by Toyo Tokai Aluminium Hanbai K.K., thickness: 20 μm) by a doctor blade method, and dried. The mixture applying amount was 1.6 g/cm.sup.3. The aluminum foil was pressed tor cutting. Accordingly, a positive electrode was produced.

(29) <Production Method of Negative Electrode>

(30) Lithium titanate (trade name “XA-105” manufactured by Ishihara Sangyo Kaisha, Ltd., median diameter: 6.7 μm), acetylene black (trade name “HS-100” manufactured by Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive auxiliary, and polyvinylidene fluoride (trade name “#7208” manufactured by Kureha Corporation) as a binder were mixed at a ratio of 90:2:8 (% by mass). This mixture was poured and mixed into N-methyl-2-pyrrolidone to produce a slurry solution. This slurry solution was applied onto an aluminum foil (manufactured by Toyo Tokai Aluminium Hanbai K.K., thickness: 20 μm) by a doctor blade method, and dried. The mixture applying amount was 2.0 g/cm.sup.2. The aluminum foil was pressed for cutting. Accordingly, a negative electrode was produced.

(31) <Measurement of DC Resistance>

(32) The positive electrode and the negative electrode were punched into a circular shape having a diameter of 14 mm and 15 mm, respectively. A small battery was constituted by impregnating the synthetic resin microporous film with an electrolytic solution while the synthetic resin microporous film was placed between the positive electrode and the negative electrode.

(33) The used electrolytic solution was obtained by dissolving lithium hexafluorophosphate (LiPF.sub.6) in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 3:7 to become a 1 M solution.

(34) The small battery was charged at a current density of 0.20 mA/cm.sup.2 to a previously determined upper limit voltage. The small battery was discharged at a current density of 0.20 mA/cm.sup.2 to a previously determined lower limit voltage. The upper limit voltage was 2.7 V, and the lower limit voltage was 2.0 V. The discharge capacity obtained in the first cycle was defined as the initial capacity of the battery. Thereafter, the battery was charged to 30% of the initial capacity. Then, a voltage (E.sub.1) when the battery was discharged at 60 mA (I.sub.1) for 10 seconds and a voltage (E.sub.2) when the battery was discharged at 144 mA (I.sub.2) for 10 seconds were measured.

(35) The measured values were used to calculate a DC resistance value (Rx) at 30° C. according to the following formula.
Rx=|(E.sub.1-E.sub.2)/discharge current (I.sub.1I.sub.2)|

(36) (Dendrite Resistance)

(37) After a positive electrode and a negative electrode were prepared according to the following condition, a small battery was produced. The dendrite resistance of the obtained small battery was evaluated. The dendrite resistance was evaluated according to the following procedure. Three small batteries were prepared under an identical condition. As a result of the following evaluation, when all batteries did not have a short circuit, it was rated as A. When one had a short circuit, it was rated as B. When two or more had a short circuit, it was rated as C.

(38) <Production Method of Positive Electrode>

(39) In an Ishikawa grinding mortar, Li.sub.2CO.sub.3 and a coprecipitated hydroxide represented by Ni.sub.0.33Co.sub.0.33Mn.sub.0.33 (OH).sub.2 were mixed such that the molar ratio of Li and the whole transition metal became 1.08:1. Thereafter, the mixture was subjected to a heat treatment in the air atmosphere at 950° C. for 20 hours, and thereafter pulverized. Accordingly, Li.sub.1.04Ni.sub.0.33Co.sub.0.33Mn.sub.0.33O.sub.2 having an average secondary particle diameter of about 12 μm was obtained as a positive electrode active material.

(40) The positive electrode active material obtained as described above, acetylene black (HS-100 manufactured by Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive auxiliary, and polyvinylidene fluoride (#7208 manufactured by Kureha Corporation) as a binder were mixed at a ratio of 92:4:4 (% by mass). This mixture was poured and mixed into N-methyl-2-pyrrolidone to produce a slurry solution. This slurry was applied onto an aluminum foil (manufactured by Toyo Tokai Aluminium Hanbai K.K., thickness: 15 μm) by a doctor blade method, and dried. The mixture applying amount was 2.9 g/cm.sup.3. Thereafter, the aluminum foil was pressed to produce a positive electrode.

(41) <Production Method of Negative Electrode>

(42) Natural graphite (average particle diameter 10 μm) as a negative electrode active material, acetylene black (trade name “HS-100” manufactured by Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive auxiliary, and polyvinyldene fluoride (trade name “#7208” manufactured by Kureha Corporation) as a binder were mixed at a ratio of 95.7:0.5:3.8 (% by mass). To this mixture, N-methyl-2-pyrrolidone was further poured and mixed. Accordingly, a slurry solution was produced. The resulting slurry was applied onto a rolled copper foil (manufactured by UACJ Foil Corporation, thickness 10 μm) by a doctor blade method, and dried. The mixture applying amount was 1.5 g/cm.sup.3. Thereafter, the rolled copper foil was pressed to produce a negative electrode.

(43) <Measurement of Dendrite Resistance>

(44) The positive electrode and the negative electrode were punched out into a circular shape having a diameter of 14 mm and 15 mm, respectively, to produce electrodes. A small battery was constituted by impregnating the homopolypropylene microporous film with an electrolytic solution while the homopolypropylene microporous film was placed between the positive electrode and the negative electrode. It is noted that the used electrolytic solution was obtained by dissolving lithium hexafluorophosphate (LiPF.sub.6) in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 3:7 to become a 1 M solution. The small battery was charged at a current density of 0.2 mA/cm.sup.2 to a previously determined upper limit voltage of 4.6 V. The small battery was placed in a blast oven at 60° C., and the voltage change was observed for 6 months. Whether or not a short circuit occurred due to a dendrite was judged as follows. That is, when the voltage change of the small battery was −Δ0.5 V/min or more, it was judged that an internal short circuit occurred due to the generation of a dendrite.

(45) TABLE-US-00001 TABLE 1 Comparative Example Example 1 2 3 4 5 6 1 Homopolypropylene Weight-Average 413,000 413,000 413,000 413,000 371,000 427,000 413,000 Molecular Weight Mw Number-Average 44,300 44,300 44,300 44,300 43,200 45,100 44,300 Molecular Weight Mn Molecular Weight 9.3 9.3 9.3 9.3 8.6 9.5 9.3 Distribution (Mw/Mn) Melting Point (° C.) 163 163 163 163 165 165 163 Extrusion Step Resin Temperature (° C.) 220 220 220 220 220 220 220 Film Forming Rate (m/min) 22 22 18 32 22 22 22 Extrusion Amount 12 12 12 12 12 12 12 (Kg/hour) Draw Ratio 70 70 55 70 70 70 70 Aging Step Aging Temperature (° C.) 147 147 148 147 147 147 147 Aging Time (minutes) 10 10 12 10 10 10 10 Stretching Step Surface Temperature (° C.) 140 140 140 140 140 140 140 Stretching Ratio (times) 2.5 2.5 2.5 2.5 2.5 2.5 3.2 Strain Rate (%/min) 80 240 240 40 240 240 206 Annealing Step Shrinkage Rate (%) 14 14 14 14 14 14 7 Homopolypropylene Number of Branch 32 15 83 17 48 66 221 Microporous Film Structures (/100 mm.sup.2) Degree of Gas 48 63 80 37 58 72 96 Permeability (sec/100 mL/16 μm) Shrinkage Rate  90° C. 1.0 1.2 1.2 0.9 1.0 2.0 5.0 120° C. 9.4 5.8 5.2 9.6 8.7 12.3 15.2 Porosity (%) 56 56 54 56 56 57 62 Thickness (μm) 16 16 20 16 16 16 16 Evaluation DC Resistance (Ω) 1.68 1.73 1.76 1.71 1.72 1.75 1.82 Dendrite Resistance B A A B A A C

INDUSTRIAL APPLICABILITY

(46) The synthetic resin microporous film of the present invention can smoothly and uniformly transmit ions such as lithium ions, sodium ions, calcium ions, and magnesium ions. Therefore, the synthetic resin microporous film is suitably used as a separator for power storage devices.

CROSS-REFERENCE TO RELATED APPLICATION

(47) The present application claims the priority under Japanese Patent Application No. 2017-22339 filed on Feb. 9, 2017, the disclosure of which is hereby incorporated in its entirety by reference.

REFERENCE SIGNS LIST

(48) 1 support portion

(49) 2 fibril

(50) 3 micropore portion

(51) A synthetic resin microporous film