Separator for power storage device and power storage device

Abstract

A separator for power storage devices includes a synthetic resin film having minute pore portions, the separator having an air resistance of 30 sec/100 mL/16 μm or more and 100 sec/100 mL/16 μm or less, and a first scattering peak in a stretching direction measured by small-angle X-ray scattering measurement (SAXS) present in a range where a scattering vector is 0.0030 nm.sup.−1 or more and 0.0080 nm.sup.−1 or less.

Claims

1. A separator for a power storage device, the separator comprising: a synthetic resin film having minute pore portions, wherein the separator has: an air resistance of 30 sec/100 mL/16 μm or more and 100 sec/100 mL/16 μm or less; a first scattering peak in a stretching direction measured by small-angle X-ray scattering measurement (SAXS) present in a range where a scattering vector is 0.0030 nm.sup.−1 or more and 0.0080 nm.sup.−1 or less; and a lamellar long period measured by SAXS of 25 nm or less.

2. The separator according to claim 1, wherein the separator is uniaxially stretched, and has a bubble point pore diameter rBP and an average flow diameter rAVE measured by a bubble point method, the bubble point pore diameter rBP and the average flow diameter rAVE satisfying:
100×(rBP−rAVE)/rAVE<40.

3. The separator according to claim 1, wherein the separator has a porosity of 45% or more and 65% or less, and a specific surface area of 20 m.sup.2/g or more and 60 m.sup.2/g or less.

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

5. The separator according to claim 4, wherein the olefin-based resin has a weight-average molecular weight of 30,000 or more and 500,000 or less and a melting point of 130° C. or higher and 170° C. or lower.

6. The separator according to claim 1, wherein the separator is uniaxially stretched.

7. A power storage device comprising the separator according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing an example of a pore diameter distribution curve measured in the measurement of a maximum pore diameter.

DESCRIPTION OF EMBODIMENTS

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

EXAMPLES

Examples 1 to 5, Comparative Examples 1 and 2

(3) (Extrusion Step)

(4) A homopolypropylene having a weight-average molecular weight Mw, number-averaged molecular weight Mn, molecular weight distribution (Mw/Mn), 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 film was cooled until the surface temperature thereof became 30° C. to obtain a long-length homopolypropylene film having a thickness of 18 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.

(5) (Aging Step)

(6) 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.

(7) (First Stretching Step)

(8) 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 stretch ratio indicated in Table 1 such that the surface temperature thereof became a temperature indicated in Table 1.

(9) (First Annealing Step)

(10) 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 the temperature indicated in Table 1. In this manner, the homopolypropylene film was annealed to obtain a long-length separator for power storage devices. The thickness of the separator for power storage devices was 16 μm. It is noted that the shrinkage rate of the homopolypropylene film in the annealing step was a value indicated in Table 1.

Example 6

(11) (Extrusion Step and Aging Step)

(12) A homopolypropylene film was manufactured in the same manner as in Example 1.

(13) (First Stretching Step)

(14) 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 stretch ratio indicated in Table 1 such that the surface temperature thereof became a temperature indicated in Table 1.

(15) (First Annealing Step)

(16) 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 the temperature indicated in Table 1. In this manner, the homopolypropylene film was annealed. The thickness of the homopolypropylene film was 17 μm. It is noted that the shrinkage rate of the homopolypropylene film in the first annealing step was a value indicated in Table 1.

(17) (Second Stretching Step)

(18) Next, using a uniaxially stretching apparatus, the homopolypropylene film having been subjected to the first annealing was uniaxially stretched only in the extrusion direction at a strain rate indicated in Table 1 and a stretch ratio indicated in Table 1 such that the surface temperature thereof became a temperature indicated in Table 1.

(19) (Second Annealing Step)

(20) 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 the temperature indicated in Table 1. In this manner, the homopolypropylene film was annealed to obtain a long-length synthetic resin microporous film. The thickness of the synthetic resin microporous film was 16 μm. It is noted that the shrinkage rate of the homopolypropylene film in the second annealing step was a value indicated in Table 1.

(21) [Evaluation]

(22) For the obtained synthetic resin microporous films, the air resistance, porosity, thickness, bubble point pore diameter rBP, average flow diameter rAVE, specific surface area, and maximum pore diameter were measured according to the above-described procedures. The results are shown in Table 1.

(23) For the obtained separators for power storage devices, the value of a scattering vector q at which a first scattering peak appeared was measured by small-angle X-ray scattering measurement (SAXS). The values are shown in Table 1.

(24) For the obtained separators for power storage devices, the lamellar long period was measured by small-angle X-ray scattering measurement (SAXS), and the results are shown in Table 1.

(25) For the obtained separators for power storage devices, the DC resistance, breakdown voltage, dendrite resistance, thermal shrinkage rate, and electrolytic solution impregnability were measured according to the following procedures, and the results are shown in Table 1.

(26) (DC Resistance)

(27) 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.

(28) <Production Method of Positive Electrode>

(29) 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 between 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.

(30) 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 (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 amount of the slurry solution applied was 1.6 g/cm.sup.3. The aluminum foil was pressed for cutting. Accordingly, a positive electrode was produced.

(31) <Production Method of Negative Electrode>

(32) 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 (#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 amount of the slurry solution applied was 2.0 g/cm.sup.3. The aluminum foil was pressed for cutting. Accordingly, a negative electrode was produced.

(33) <Measurement of DC Resistance>

(34) The positive electrode was punched into a circular shape having a diameter of 14 mm. The negative electrode was punched into a circular shape having a diameter of 15 mm. A small battery was constituted by impregnating the separator for power storage devices with an electrolytic solution while the separator for power storage devices was placed between the positive electrode and the negative electrode.

(35) The electrolytic solution used 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.

(36) 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.

(37) 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.1−I.sub.2)|

(38) (Breakdown Voltage)

(39) The breakdown voltage of the separator for power storage devices was measured according to the following procedure using a breakdown tester (manufactured by Yamazaki Sangyo K.K., trade name “HAT-300-100RHO”). More specifically, a test piece having a regular square shape in a plane view with a side 100 mm was first cut out from the separator for power storage devices. The test piece was subjected to a static elimination treatment. An upper electrode (made of SUS) was brought into contact with a mark with a diameter of about 25 mm formed in the center of the upper surface of the test piece while a lower electrode (made of SUS) was brought into contact with the lower surface of the test piece to measure a breakdown voltage. The measurement was performed under conditions where an atmospheric medium was air, a test temperature was 25° C., a relative humidity was 50%, and a temperature rise rate was 100 V/sec. Three test pieces were prepared, and an arithmetic mean value of breakdown voltages of the test pieces was defined as the breakdown voltage of the separator for power storage devices.

(40) (Dendrite Resistance)

(41) 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.

(42) <Production Method of Positive Electrode>

(43) 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 between 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.

(44) 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 92:4:4 (% 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: 15 μm) by a doctor blade method, and dried. The amount of the slurry solution applied was 2.9 g/cm.sup.3. Thereafter, the aluminum foil was pressed to produce a positive electrode.

(45) <Production Method of Negative Electrode>

(46) Natural graphite (average particle diameter 10 μm) as a negative electrode active material, acetylene black (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 95.7:0.5:3.8 (% by mass) to produce a mixture. This mixture was poured and mixed into N-methyl-2-pyrrolidone to produce a slurry solution. This slurry solution was applied onto a rolled copper foil (manufactured by Nippon Foil Mfg. Co., Ltd., thickness 10 μm) by a doctor blade method, and dried. The amount of the slurry solution applied was 1.5 g/cm.sup.3. Thereafter, the rolled copper foil was pressed to produce a negative electrode.

(47) <Measurement of Dendrite Resistance>

(48) The positive electrode was punched out into a circular shape having a diameter of 14 mm and the negative electrode was punched out into a circular shape having a diameter of 15 mm to prepare electrodes. A small battery was constituted by placing the separator for power storage devices between the positive electrode and the negative electrode and adding an electrolyte solution to the separator for power storage devices. 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. After the lapse of 6 months, it was confirmed whether a short circuit between the positive electrode and the negative electrode occurred in the small battery.

(49) For three small batteries, a determination was made as to whether or not a short circuit between the positive electrode and the negative electrode had occurred according to the above-described procedure, and an evaluation was made on the basis of the number of batteries in which a short circuit occurred.

(50) A . . . The number of batteries in which a short circuit had occurred was 0.

(51) B . . . The number of batteries in which a short circuit had occurred was 1.

(52) C . . . The number of batteries in which a short circuit had occurred was 2 or 3.

(53) (Thermal Shrinkage Rate)

(54) A test piece having a regular square shape in a plane view with a side 12 cm was cut out from the separator for power storage devices at 23° C. Any one of the edges of the test piece was adjusted to correspond to the extrusion direction (MD). Straight lines L and L each having a length of 10 cm were drawn in the center of the test piece so as to be orthogonal to each other. The straight lines L and L were adjusted to be parallel to the edges of the test piece. The test piece was inserted between two plates of float glass each having a regular square shape in a plane view with a side 15 cm and a thickness of 2 mm to smooth out wrinkles of the test piece. In such a state, the length of each of the straight lines L and L drawn on the test piece was read at 23° C. to 1/10 of a micrometer using a two-dimensional length measuring machine (CW 2515N manufactured by Chien Wei Precise Technology Co., Ltd.). An arithmetic mean value of lengths of the two straight lines L and L was defined as the initial length L.sub.0 of the straight line L. Then, the test piece was removed from between the float glass plates. The test piece was placed in a thermostatic chamber (manufactured by AS ONE Corporation, trade name “OF-450B”) set at 105° C., allowed to stand for 1 hour, and then removed. The test piece was inserted between two plates of float glass each having a regular square shape in a plane view with a side 15 cm and a thickness of 2 mm to smooth out wrinkles of the test piece. In such a state, the length of each of the straight lines L and L drawn on the test piece was read at 23° C. to 1/10 of a micrometer using a two-dimensional length measuring machine (CW 2515N manufactured by Chien Wei Precise Technology Co., Ltd.). An arithmetic mean value of lengths of the two straight lines L and L was defined as the length after heating L.sub.1 of the straight line L. According to the following formula, a thermal shrinkage rate was calculated.
Thermal shrinkage rate (%)=100×(initial length L.sub.0−length after heating L.sub.1)/initial length L.sub.0

(55) (Electrolytic Solution Impregnability)

(56) A white sheet of paper (manufactured by KOKUYO Co., Ltd., trade name “KB paper KB-39N”) was put on a smooth glass plate horizontally placed, and the separator for power storage devices was superimposed onto the white sheet of paper. Then, 100 μL of an electrolytic solution was dropped onto the upper surface of the separator for power storage devices using a micropipette so as to spread in a rough circle. The electrolytic solution used was one commercially available from Tomiyama Pure Chemical Industries, Ltd. under the trade name of “LIPASTE-3E7MEC/PF1” (composition: EC (ethylene carbonate):MEC (methyl ethyl carbonate)=3:7 (volume ratio)/LiPF.sub.6 (lithium hexafluorophosphate) 1.0 M). After the electrolytic solution was dropped onto the upper surface of the separator for power storage devices, the time until a stain of the electrolytic solution seeping through the separator for power storage devices into the white sheet of paper had the same size (area) as the electrolytic solution spreading over the upper surface of the separator for power storage devices was measured and defined as electrolytic solution impregnation time.

(57) TABLE-US-00001 TABLE 1 Examples 1 2 3 4 Homopolypropylene Weight-Average Molecular Weight Mw 413,000 413,000 413,000 413,000 Number-Average Molecular Weight Mn 44,300 44,300 44,300 44,300 Molecular Weight Distribution (Mw/Mn) 9.3 9.3 9.3 9.3 Melting Point (° C.) 163 163 163 163 Extrusion Step Resin Temperature (° C.) 220 220 220 220 Film Forming Rate (m/min) 22 22 22 22 Extrusion Amount (kg/hr) 12 12 12 12 Draw Ratio 70 70 70 70 Aging Step Aging Temperature (° C.) 147 147 147 147 Aging Time (min) 10 10 10 10 First Stretching Surface Temperature (° C.) 140 140 140 140 Step Stretch Ratio (times) 2.5 2.2 2.7 2.5 Strain Rate (%/min) 40 45 122 110 First Annealing Surface Temperature (° C) 130 130 130 130 Step Shrinkage Rate (%) 10 14 14 14 Second Stretching Surface Temperature (° C.) — — — — Step Stretch Ratio (times) — — — — Strain Rate (%/min) — — — — Second Annealing Surface Temperature (° C.) — — — — Step Shrinkage Rate (%) — — — — Separator For Power Air Resistance (sec/100 mL/16 μm) 35 78 55 58 Storage Device Porosity (%) 58 45 57 55 Thickness (μm) 16 16 16 16 Scattering Vector q (nm.sup.−1) 0.0038 0.0071 0.0056 0.0064 Lamellar Long Period (nm) 25.0 — 24.6 24.1 Bubble Point Pore Diameter rBP (μm) 0.063 0.049 0.064 0.063 Average Flow Diameter rAVE (μm) 0.063 0.048 0.048 0.048 100 × (rBP − rAVE)/rAVE 0 2 34 32 Specific Surface Area (m.sup.2/g) 27.0 — 42.0 51.0 Maximum Pore Diameter (μm) 0.38 — 0.27 0.26 Evaluation DC Resistance (Ω) 1.62 1.75 1.72 1.73 Breakdown Voltage (kv) 1.1 1.5 1.2 1.3 Dendrite Resistance B A A A Thermal Shrinkage Rate (%) 1.8 — 2.8 2.6 Electrolytic Solution 13 — 15 16 Impregnation Time (sec) Examples Comparative Examples 5 6 1 2 Homopolypropylene Weight-Average Molecular Weight Mw 413,000 413,000 413,000 413,000 Number-Average Molecular Weight Mn 44,300 44,300 44,300 44,300 Molecular Weight Distribution (Mw/Mn) 9.3 9.3 9.3 9.3 Melting Point (° C.) 163 163 163 163 Extrusion Step Resin Temperature (° C.) 220 220 220 220 Film Forming Rate (m/min) 22 22 22 18 Extrusion Amount (kg/hr) 12 12 12 12 Draw Ratio 70 70 70 55 Aging Step Aging Temperature (° C.) 147 147 147 148 Aging Time (min) 10 10 12 12 First Stretching Surface Temperature (° C.) 140 140 140 140 Step Stretch Ratio (times) 2.7 1.6 3.2 2.7 Strain Rate (%/rain) 45 70 209 260 First Annealing Surface Temperature (° C) 130 130 130 130 Step Shrinkage Rate (%) 10 7 10 14 Second Stretching Surface Temperature (° C.) — 140 — — Step Stretch Ratio (times) — 1.7 — — Strain Rate (%/min) — 35 — — Second Annealing Surface Temperature (° C.) — 130 — — Step Shrinkage Rate (%) — 7 — — Separator For Power Air Resistance (sec/100 mL/16 μm) 32 52 96 124 Storage Device Porosity (%) 60 55 61 53 Thickness (μm) 16 16 16 16 Scattering Vector q (nm.sup.−1) — — 0.0124 0.0115 Lamellar Long Period (nm) 24.6 — 28.1 24.6 Bubble Point Pore Diameter rBP (μm) — 0.035 0.049 0.051 Average Flow Diameter rAVE (μm) — 0.032 0.034 0.036 100 × (rBP − rAVE)/rAVE — 9 44 42 Specific Surface Area (m.sup.2/g) 25.4 — 79.0 45.0 Maximum Pore Diameter (μm) 0.39 — 0.29 0.14 Evaluation DC Resistance (Ω) 1.59 1.70 1.82 1.85 Breakdown Voltage (kv) — 1.4 1.5 1.4 Dendrite Resistance — A C A Thermal Shrinkage Rate (%) 2.0 — 4.0 3.7 Electrolytic Solution 13 — 112 125 Impregnation Time (sec)

INDUSTRIAL APPLICABILITY

(58) The separator for power storage devices of the present invention has excellent permeability of ions such as lithium ions, sodium ions, calcium ions, and magnesium ions, and can substantially, effectively suppress the generation of a dendrite. Therefore, the separator for power storage devices is suitably used as a separator for power storage devices.