JP2002184383A - Battery separator - Google Patents

Abstract

(57) [Problem] To provide an alkaline battery separator excellent in both characteristics of low resistance and self-discharge suppressing function. SOLUTION: An ultrahigh molecular weight polyolefin powder having a bulk density of 4.0 × 10 ² g / L or less and an average pore size of 100 μm
The following ultra-high molecular weight polyolefin powder is mixed, and the mixture is sintered at a temperature equal to or higher than its melting point and cut to a thickness of 5
A porous sheet having a thickness of 0 to 500 μm is formed, and after stretching, a hydrophilic treatment is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator, and more particularly, to a battery separator which is excellent in output characteristics and self-discharge suppressing function and which can be preferably used, for example, in power tools and batteries for driving (running) electric vehicles. About.

[0002]

2. Description of the Related Art In recent years, alkaline secondary batteries such as nickel-cadmium (nickel) batteries and nickel-metal hydride batteries have been used not only as small batteries for electric and electronic devices but also as batteries for driving (running) electric vehicles. Is also expected.

One of the characteristics required for a battery separator is that its electric resistance is low (hereinafter referred to as "low resistance"). If the electric resistance is too high, the output characteristics of the battery deteriorate. A battery for driving (running) an electric tool or an electric vehicle is required to have a particularly high output, and a battery separator used for such a large-capacity battery is required to have a particularly low resistance. In addition, a self-discharge suppressing function is also important. The term “self-discharge” refers to a phenomenon in which when a battery is left without being used, a discharge reaction occurs inside the battery and the battery capacity is reduced. If this is significant, economic losses and environmental damage will be significant. Therefore, it is necessary to add characteristics (self-discharge suppressing function) to the battery and the battery separator so as to minimize self-discharge.

[0004] In order to realize low resistance of the battery separator, there are measures such as reducing the film thickness and increasing the amount of impregnation with the electrolytic solution. However, in these cases, self-discharge may easily occur. Therefore, there is a strong demand for providing a battery separator having both excellent low resistance and self-discharge suppressing properties.

[0005] As a separator for an alkaline battery, a nonwoven fabric made of a polyamide-based synthetic fiber, a nonwoven fabric made of a polyolefin and the like have been generally used, but these have a problem in mechanical strength. On the other hand, ultra high molecular weight polyethylene (UHP)
E) It has been proposed to use a porous sheet as a battery separator (JP-A-8-77997).
The ultra-high molecular weight polyolefin porous sheet such as UHPE has an advantage that it has excellent mechanical strength and does not easily collapse against electrode expansion of an alkaline secondary battery. However, the ultrahigh molecular weight polyolefin porous sheet has a problem that it has both low resistance and self-discharge suppressing function.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a battery separator excellent in all of mechanical strength, low resistance and self-discharge suppressing function.

[0007]

In order to solve the above-mentioned problems, a battery separator of the present invention is a battery separator using an ultra-high molecular weight polyolefin porous sheet, and has an output control factor of 15 cm ^-1 or more. And an average pore diameter of 30 μm or less.

Since the battery separator of the present invention has the above-mentioned physical properties, it has excellent characteristics of both low resistance and self-discharge suppressing function, and uses an ultra-high molecular weight porous sheet. Also has excellent target strength. less than,
The relationship between the output control factor and the low resistance and the relationship between the average pore size and the self-discharge suppressing function will be described.

The electric resistance R (Ω) when the electrolyte is permeated into the battery separator made of a porous sheet is as follows: the single-sided area S (cm ² ) of the porous sheet, the volume impregnation rate Pd of the electrolyte, Specific resistance Rf (Ω · cm), sheet thickness t (c
m), the curvature ratio X of the sheet, and the volume porosity p, the present inventors have found that it is represented by the following formula (1).
Here, the volume porosity is the proportion of the pore volume in the total volume of the porous sheet, and the volume impregnation is the proportion of the volume occupied by the electrolyte in the pore volume, Is a parameter considered to represent the complexity of the shape of the holes in the porous sheet.

R = (Rf × t × X ² ) / (p × Pd × S) (1)

Here, from the battery design conditions, Rf and S can be regarded as constants. If the amount of the electrolytic solution sealed in the battery is constant, Pd can be regarded as a constant. Therefore, if Rf / (Pd × S) = A, equation (1) can be replaced with the following equation (2).

R = A × (t × X ² / p) (2)

Here, assuming that the output control factor is Y,
Y is a physical property value represented by the following equation (3) from the sheet thickness t, the curvature ratio X, and the volume porosity p.

Y = p / (t × X ² ) (3)

From equations (2) and (3), it can be seen that the electrical resistance is inversely proportional to the output control factor Y. That is, a battery separator having a large output control factor is a battery separator excellent in low resistance. The output control factor of the alkaline battery separator of the present invention needs to be 15 cm ^-1 or more, and preferably 16 cm ^-1 or more. The upper limit of the output control factor is not limited, but is usually 200 cm ^-1.
It is as follows. The range of the output control factor is, for example, 15 to 20.
0 cm ^-1, preferably 16~200cm ^-1.

The volume porosity p of the porous sheet is determined based on the area S of one side of the porous sheet, the thickness t, and the pore volume V (cm).
^{From 3} ), it is obtained by the following equation (4).

P = V / (S × t) (4)

In the above equation (4), the pore volume V
Is the surface area S, thickness t and weight m of the porous sheet.
(G), and specific gravity r (g / c) of the sheet forming material.
m ³ ), it is obtained by the following equation (5).

V = (S × t) − (m / r) (5)

The curvature ratio can be obtained, for example, as follows. First, a glass cell having an effective diameter of 17 mmφ is prepared, and the internal space of the cell is divided into two using the porous sheet of the present invention. Next, an electrolyte solution having a known diffusion coefficient, for example, 2
A 5 ° C. aqueous KCl solution is charged, and the other is distilled water. If this cell is left as it is, ions move from the electrolyte solution side to the distilled water side, and try to equalize the ion concentrations of the liquids on both sides. The speed of the concentration change at this time, namely, the ion flux J (mol · cm ⁻² · sec ⁻¹ ), the diffusion coefficient of electrolyte ions D ₀ (cm ² · sec ⁻¹ ), the thickness t of the porous sheet, and The curvature ratio X is calculated from the volume porosity p of the porous sheet using the following equation (6). Here, the diffusion coefficient of the 25 ° C. KCl aqueous solution is D ₀
= 1.85 × 10 ⁻⁵ . The concentration of the KCl aqueous solution is, for example, 0.01 to 5.0 mol / L, and preferably 0.1 to 3.0 mol / L.

X = (D ₀ × p / (J × t)) ^1/2 (6)

The self-discharge of a battery is generally attributed to the fact that nitrogen-containing ions contained as impurities in the battery move around between the positive and negative electrodes to cause an oxidation-reduction reaction. In addition, it has been reported that there is a correlation between trapping of nitrogen-containing ions in an electrolytic solution by a battery separator and suppression of self-discharge (Electrochemical).
al Society Vol. 145 No. 3 p
844-847). The present inventors have made a series of studies on self-discharge suppression, and in the process, in order to make it easier for the battery separator to capture nitrogen-containing ions, the surface area inside the pores of the separator (hereinafter referred to as “pores”) The area)). Then, in order to increase the pore area, in the present invention, the average pore diameter is set to 30 μm or less. Assuming that the pore volume of the separator is constant, the pore area is proportional to the pore volume divided by the average pore diameter, and the pore area is smaller for the smaller average pore diameter.

As described above, the average pore diameter of the battery separator of the present invention needs to be 30 μm or less.
The lower limit of the average pore size is not particularly limited, but is usually 1 μm or more. The average pore diameter is, for example, 1 to 30 μm. In the present invention, when the battery separator is composed of the porous sheet alone, the average pore size is the average pore size of the porous sheet.

The measurement of the average pore diameter can be performed, for example, using a mercury porosimeter. As the mercury porosimeter, for example, an auto scan 33 (trade name, manufactured by Yuasa Corporation) or the like can be used. In this case, the pressure range is changed from vacuum to 230 MPa, and from the measured pressure and the amount of the injected mercury, a pore size distribution is determined assuming a cylindrical hole, and the mercury is injected to a volume impregnation rate of 50%. The measured pore size is defined as the average pore size.

In the battery separator of the present invention,
The ultrahigh molecular weight polyolefin is preferably UHPE because the strength of the porous sheet using the ultrahigh molecular weight polyolefin is excellent. For the same reason, the viscosity average molecular weight of the ultrahigh molecular weight polyolefin is preferably 500,000 to 16,000,000,
Particularly preferably, it is 600,000 to 10,000,000.

Next, the battery of the present invention is a battery having a separator interposed between the positive and negative electrodes, and the separator is the battery separator of the present invention. As described above, since the battery separator of the present invention has excellent physical properties, a battery using the same has excellent mechanical strength, output characteristics, and self-discharge suppressing function. The battery of the present invention
Alkaline batteries are preferred, and particularly preferred are large capacity alkaline secondary batteries used for electric vehicle driving (running) batteries. Similarly, the battery separator of the present invention is preferably used for an alkaline secondary battery, and particularly preferably used for a large-capacity alkaline secondary battery such as an electric vehicle driving (running) battery.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION The battery separator of the present invention can be manufactured, for example, as follows. First, an unstretched ultrahigh molecular weight polyolefin porous sheet is prepared. The thickness of the porous sheet is, for example, 10 to 500 μm.
m, preferably 50 to 400 μm. The porous sheet can be manufactured by sintering an ultrahigh molecular weight polyolefin powder and then forming the sintered body into a sheet, or by simultaneously forming the sintered body into a sheet. For example, a method of filling ultra-high molecular weight polyolefin powder into a shape retainer, sintering, cutting this sintered body into a sheet, uniformly dispersing the ultra-high molecular weight polyolefin powder in a solvent, and applying this to a heat-resistant sheet And sintering, or a method of sintering ultrahigh molecular weight polyolefin powder in a die by a screw extruder or the like and forming it into a sheet. The polyolefin powder is a polyolefin powder having a bulk density of 4.0 × 10 ² g / L or less and a polyolefin powder having an average particle diameter of 100 μm or less because a porous sheet having a small average pore diameter and a low curvature ratio can be obtained. Are preferably used in combination. The bulk density is particularly preferably 3.5 × 10 ² g / L or less. The lower limit of the bulk density is not particularly limited, but is usually 1.0 × 10 ² g /
L. The bulk density can be measured according to DIN 53 468. The average particle size is preferably 80 μm or less. Although the lower limit of the average particle diameter is not particularly limited, it is usually 20 μm. By mixing and using two kinds of polyolefin powders, a porous sheet having a small average pore diameter and a low curvature ratio can be obtained. But the reason is unknown. Weight mixing ratio (A /
B) is, for example, 0.1 to 9, preferably 0.25
~ 4. Among the various forming methods, the method using the shape retainer is preferable because the adjustment of the sheet thickness is relatively easy. This method is implemented, for example, as follows.

First, an ultrahigh molecular weight polyolefin powder is filled in a shape retainer. This is put in a pressure-resistant container, and after evacuating the air in the container, steam heated to a temperature higher than the melting point of the powder is introduced to heat and sinter the powder. The introduced steam is usually pressurized, and since the inside of the container is under a negative pressure, it easily penetrates between the powders and quickly transfers heat, so that heat sintering can be performed uniformly and in a short time.

The porous body obtained by the above method is cut with a lathe or the like to obtain the unstretched ultrahigh molecular weight polyolefin porous sheet, which is further stretched to obtain a target battery separator. The stretching treatment is usually performed by applying tension while heating the porous sheet to a temperature equal to or lower than its melting point. A stretching machine can be appropriately used such as a tenter or a roll, and the stretching method may be uniaxial stretching or biaxial stretching. The stretching temperature is, for example, 70 to 180 ° C., preferably 80
１６０160 ° C. The stretching ratio is, for example, 1.05 to 50 times, based on the area of the porous sheet before stretching,
Preferably it is 1.1 to 20 times.

In this way, the battery separator of the present invention can be manufactured, but the present invention is not limited to this manufacturing method.
It may be manufactured by another manufacturing method.

Since the battery separator obtained by the above method is hydrophobic, it is preferable to perform a hydrophilic treatment when the battery separator is used for an alkaline battery. The method of the hydrophilic treatment includes, for example, a surfactant impregnation treatment, a graft polymerization treatment of a hydrophilic monomer, a sulfonation treatment, a plasma treatment, and the like.
There is no particular limitation.

The structure of the battery of the present invention is not particularly limited except that the battery separator of the present invention is used, and the battery can be manufactured by a usual method.

[0033]

Next, examples will be described together with comparative examples. In addition, about various physical-property values, it measured by each said method. The sheet thickness was measured with a dial gauge.

(Example 1) First, a cylindrical shape retainer for filling with UHPE powder was prepared. This shape retainer is a metal cylindrical outer mold having a large number of holes and a polytetrafluoroethylene porous film stuck on the inside thereof, and a fixed mold disposed at the bottom of the outer mold and fixing the outer mold. Consists of Next, UHPE powder a (viscosity average molecular weight 4,000,000, average particle size 130μm, bulk density 2.4
× 10 ² g / L, melting point 140 ° C) 2.0 kg and UHPE
Powder b (viscosity average molecular weight 4.5 million, average particle size 60 μm,
(Bulk density 4.2 × 10 ² g / L, melting point 140 ° C.) 1.0
A mixture of kg was prepared. The mixed powder is filled in the shape retainer, placed in a metal heat-resistant and pressure-resistant container (including a steam introduction pipe and an opening / closing valve thereof), and the atmospheric pressure inside the heat-resistant and pressure-resistant container is reduced to 1 by a vacuum pump. .
3 kPa. The time required at this time was 30 minutes. After stopping the vacuum pump, the valve was opened, steam (temperature: 145 ° C., pressure: 0.4 MPa) was introduced, and the mixture was heated and sintered for 1 hour and allowed to cool to obtain a cylindrical UHPE porous body.
This porous body is cut by a cutting lathe and unstretched UHPE
A porous sheet was obtained. The obtained porous sheet was stretched 3.0 times at 130 ° C. by a uniaxial stretching method to obtain an intended battery separator. Further, this battery separator was immersed in a surfactant (butyl naphthalene sulfonic acid) aqueous solution (5% by volume) for 10 minutes, and then immersed in an aqueous solution of 80%.
A hydrophilic treatment was performed by drying at 15 ° C. for 15 minutes.
The sheet has a thickness of 125 μm and a volume porosity of 60% by volume.

Example 2 The procedure of Example 1 was repeated except that a mixed powder of 1.5 kg of the UHPE powder a and 1.5 kg of the UHPE powder b was used instead of the mixed powder of the first embodiment. Cutting was performed to obtain an unstretched UHPE porous sheet. The obtained porous sheet was subjected to a stretching treatment in the same manner as in Example 1 to obtain a battery separator, and then subjected to the same hydrophilic treatment as in Example 1. The thickness of the sheet is 165 μm and the volume porosity is 59% by volume.

(Comparative Example 1) UHPE powder c (viscosity average molecular weight 10 million, average particle diameter 150 μm, bulk density 4.0 × 10 ² g / L, melting point 140 ° C.) instead of the mixed powder of Example 1 Heat sintering and cutting were performed in the same manner as in Example 1 except that 0.0 kg was used alone.
A PE porous sheet was obtained. Example 1 was repeated except that the stretching ratio was changed to 1.7 times with respect to the obtained porous sheet.
Stretching is performed in the same manner as above to obtain a battery separator,
Thereafter, the same hydrophilic treatment as in Example 1 was performed. The thickness of the sheet is 177 μm, and the volume porosity is 61% by volume.

(Comparative Example 2) Heat sintering and cutting were performed in the same manner as in Example 1 except that 3.0 kg of the UHPE powder a was used alone instead of the mixed powder of Example 1, and unstretched UHPE porous material was used. A functional sheet was obtained. The obtained porous sheet was subjected to a stretching treatment in the same manner as in Example 1 to obtain a battery separator, and then subjected to the same hydrophilic treatment as in Example 1. The thickness of the sheet is 180 μm and the volume porosity is 59% by volume.

(Comparative Example 3) Heat sintering and cutting were performed in the same manner as in Example 1 except that 3.0 kg of the UHPE powder b was used alone instead of the mixed powder of Example 1, and the unstretched UHPE porous material was used. A functional sheet was obtained. The obtained porous sheet was subjected to a stretching treatment in the same manner as in Example 1 to obtain a battery separator, and then subjected to the same hydrophilic treatment as in Example 1. The sheet has a thickness of 139 μm and a volume porosity of 53% by volume.

The thicknesses and average pore diameters of the battery separators of Examples 1 and 2 and Comparative Examples 1, 2 and 3 obtained as described above were measured. , Curvature ratio and output control factor were calculated. The results are shown in Table 1 below.

(Table 1) Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Film thickness (μm) 125 165 177 180 139 Volume porosity (%) 60 59 61 59 53 Curvature 1.5 1.5 1.0 2.6 2.8 Output control factor 22 16 34 4.8 5.0 (cm ^-1 ) Average pore size (μm) 26 28 65 25 27

As can be seen from Table 1, the battery separator of the example had a large output control factor and a sufficiently small average pore diameter. On the other hand, in the battery separators of Comparative Examples 1 to 3 using UHPE powder alone, the average pore diameter was too large or the output control factor was too small.

[0042]

As described above, the battery separator of the present invention is a battery separator excellent in both characteristics of low resistance and self-discharge suppressing function, and has excellent mechanical strength. Therefore, the battery separator of the present invention can contribute to a higher output of the battery, and can be particularly preferably used for a power tool or a battery for driving (running) an electric vehicle requiring a high output.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Keisuke Kii 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (72) Inventor Masakatsu Urari 1-1-1, Shimohozumi, Ibaraki-shi, Osaka No. 2 Inside Nitto Denko Corporation (72) Inventor Yutaka Kishi 1-1-2 Shimohozumi, Ibaraki City, Osaka Prefecture Inside Nitto Denko Corporation (72) Takashi Yamamura 1-1-2 Shimohozumi, Ibaraki City, Osaka Prefecture Nitto Denko Co., Ltd. ) Inventor Toshi Uda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 5H021 EE04 HH00 HH03 HH07 </ S> </ s> </ s>

Claims (4)

[Claims] 1. A battery separator using an ultrahigh molecular weight polyolefin porous sheet, wherein an output control factor is 1
A battery separator having a size of 5 cm ^-1 or more and an average pore size of 30 μm or less. 2. The battery separator according to claim 1, wherein the ultrahigh molecular weight polyolefin is ultrahigh molecular weight polyethylene. 3. The battery separator according to claim 1, wherein the ultrahigh molecular weight polyolefin has a viscosity average molecular weight in a range of 500,000 to 16,000,000. 4. A battery in which a separator is interposed between the positive and negative electrodes, wherein the separator is the battery separator according to any one of claims 1 to 3.