JPH06101323B2 - Battery separator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement in storage characteristics of a battery using a non-woven fabric or woven fabric made of resin fiber as a separator, especially in self-discharge characteristics under high temperature atmosphere.

2. Description of the Related Art Recently, as the need for portable various electronic devices and their memory backup has increased, the demand for batteries as a power source for these devices has increased. However, as the applications of such batteries expand, the chances that the battery will be used under severer operating conditions such as a high temperature atmosphere will increase more than before, and in terms of battery characteristics, further improvement in storage characteristics such as self-discharge characteristics is desired. Has been reached. Ni- for this application
A Cd battery is often used, but this battery is a battery system that generally has a large self-discharge, and improvement of this characteristic has been a major issue. For example, normal Ni-
When the Cd battery was fully charged and then discharged in an atmosphere of 45 ° C for one month, the discharge capacity decreased to 1/3 or less of the initial value, which was a problem when used immediately after leaving.

However, recent research results have pointed out the improvement of separators as an important factor for reducing this self-discharge. That is, it has been found that the impurities due to the decomposition of the commonly used polyamide fiber separator are caused by the increase in self-discharge.

Therefore, the use of an olefin resin such as polypropylene has attracted attention as a separator material that has appropriate heat resistance and does not decompose even in high temperature and high concentration alkali.

However, since the polyolefin-based resin is poor in hydrophilicity and has a difficulty in liquid retention required as a battery separator, the following proposals for hydrophilic treatment have been made.

(1) A surfactant is attached to the surface of the resin.

(2) Plasma is irradiated to the resin to chemically adsorb —CHO groups and the like.

(3) A group having hydrophilicity such as acrylic acid is graft-polymerized.

(4) the resin was immersed in fuming sulfuric acid or concentrated sulfuric acid in the appropriate temperature and concentration conditions, sulfone group and (-SO ₃ H) is introduced into the resin.

Problems to be Solved by the Invention In the above method (1), the separator is exposed to oxygen gas generated in the positive electrode during overcharge in a high temperature and high concentration alkali, so that the surfactant causes the polyolefin resin material to And becomes an impurity and dissolves in the alkaline aqueous solution, which results in an increase in self-discharge. Similarly, in the methods (2) and (3), carboxylic acid ions and acrylate ions are eluted into the alkaline aqueous solution due to severe oxidation in the battery, and the ions increase self-discharge. On the other hand, the method (4) was extremely effective in reducing self-discharge because the impurities or impurity ions as described above were hardly eluted.

However, when the same quality polyolefin resin fiber or polyolefin resin fiber whose inside is easily oxidized is used, the condition range such as temperature, concentration and immersion time of fuming sulfuric acid or concentrated sulfuric acid for uniform sulfonation is narrow. However, it has been found that it causes various problems industrially. That is,
As a separator material, it is important that the entire resin fiber has a substantially uniform hydrophilicity, but since the unsaturated hydrocarbon part of the polyolefin resin that is easily sulfonated is unevenly present, when trying to sulfonate the entire resin However, there is a risk that the inside of the fiber is sulfonated or oxidized, and in some cases the fiber is carbonized at this portion, resulting in a decrease in the mechanical strength of the separator. Naturally, when the joint between resin fibers is carbonized, the strength of the separator is significantly reduced.

Means for Solving the Problems In order to solve this problem, the present invention uses a material whose interior is relatively sulfonated or oxidized as a structure of a polyolefin resin fiber, and is relatively easily sulfonated near the surface layer. The materials are arranged, and the internal materials are partially heat-sealed at the entangled portions of the fibers.

Effect According to the structure of this polyolefin resin fiber, sulfonation in the vicinity of the surface layer required for hydrophilicity is easy, and
In order to uniformly sulfonate, even if it is immersed in fuming sulfuric acid for a relatively long time, the polyolefin resin inside is hardly carbonized and the strength as a separator can be maintained. This also applies to the joint between the resin fibers. That is,
A wide range of sulfonation conditions can be adopted for sulfonation, which is industrially suitable. As a result, chemically stable sulfone groups are likely to be uniformly introduced into the polyolefin resin fiber, which contributes to imparting hydrophilicity and reducing self-discharge.

Embodiments Embodiments of the present invention will be described below with reference to FIGS. 1 to 3.

A polypropylene fiber having an average diameter of about 15 μm and an average length of about 40 mm is loosened and dispersed to form a nonwoven fabric. At this time, an organic solvent solution in which polyethylene is dispersed is evenly sprinkled to give the fibers a binding property. The resulting non-woven fabric is passed between heat rolls at 150 ° C to heat-bond each fiber to a final thickness of 0.2 mm and a porosity of about 60%, and the amount of polyethylene to polypropylene is about 20%. To do. In addition, since the solvent in which polyethylene is dispersed scatters during this manufacturing process, a non-woven fabric composed almost entirely of polyolefin resin is obtained, and the fibers constituting this have a structure of polypropylene inside and polyethylene near the surface layer. Become.

This non-woven fabric is immersed in fuming sulfuric acid at a temperature of 35 ° C. and a concentration of 20% for about 20 minutes, then washed with water and dried to obtain a hydrophilic separator having a sulfonated polyethylene portion. After that, it may be passed through between rolling rolls as necessary to adjust the thickness to a desired value.

FIG. 1 shows a schematic view of the obtained nonwoven fabric. In the figure, 1 is a resin fiber whose surface portion is made into a fulphon, and 2 is a space between the fibers. FIG. 2 shows a schematic cross-sectional view of the joint portion between the fibers shown in FIG. The shaded portion 3 is a polypropylene base material, which is covered with the sulfonated polyethylene of the shaded portion 5. Fibers parallel to the drawing and fibers perpendicular to the drawing are heat-sealed at the intersections of 6 and part of the polyethylene 4 is incorporated therein. In the figure, the black part shows the sulfonated part of the resin, and this part extends from the surface layer of polyethylene to part of polypropylene. Reference numeral 7 in the figure is a defective portion where polyethylene does not exist. Most of the sulfonated compound remains in the polyethylene layer because polypropylene is less likely to be sulfonated than polyethylene.

As the sulfonation progresses, the polyolefin is oxidized and carbonized to lower the mechanical strength of the nonwoven fabric. Therefore, in the same operation as in the example, a non-woven fabric was produced by changing only the dipping time in fuming sulfuric acid, and the width of the non-woven fabric having a width of 20 mm and a length of 35 mm was pulled in the lengthwise direction, and the load was measured until it was cut. Is shown in the following table. For comparison, the average diameter is about 15 μm.
The same table also shows the cutting load when a non-woven fabric composed only of polyethylene and polypropylene fibers was sulfonated under the same conditions. In addition, these measurement results are shown as an average value of five nonwoven fabrics.

Here, the cutting load is about 6 kg from the experience value in industrialization.
More than required. If this standard is adopted, a wide dipping time range of 40 minutes or less is allowed for the product of the present invention, whereas it is about 5 when polyethylene alone is sulfonated.
Within minutes, approx. When sulfonated polypropylene alone
A narrow range within 30 minutes is required. Further, when evaluated from the viewpoint of hydrophilicity, the product of the present invention had hydrophilicity that did not hinder normal charge / discharge at all when the immersion time was 10 minutes or more. On the other hand, when polypropylene was used alone, a soaking time of about 25 minutes or more was required. From these results, it is clear that the product of the present invention allows a wide dipping time of 10 to 40 minutes and is extremely convenient for industrial large-scale processing. The main reason for this is that when a polyolefin resin fiber having a double structure as in the present invention is subjected to a sulfonation treatment, a polyethylene layer having a lower hydrocarbon as a starting material, which is easily sulfonated, is easily sulfonated. However, it is considered that this is due to the fact that fuming sulfuric acid does not easily reach the polypropylene inside, and thus it is difficult to be easily sulfonated. Therefore, it is considered that the surface of the fiber is sulfonated almost uniformly by the immersion for a relatively short time, and the mechanical strength can be maintained because the deep part of the fiber is strong even after the immersion for a certain time. Further, it is considered that the polypropylene inside is heat-sealed at the joint between the fibers, which also contributes to the strength improvement.

From such a point of view, the same effect can be obtained even if the fiber obtained by combining the polypropylene and the copolymer of polypropylene-polyethylene and the fiber of the polypropylene-polyethylene copolymer and polyethylene is combined as in the present invention. It is clear.

The separator obtained in the example was used as a general-purpose AA-sized Ni-Cd.
The result of examining self-discharge when applied to a battery and stored at 45 ° C. after fully charged is shown in a of FIG. As a comparative example,
The result of a general-purpose battery of the same type using a polyamide-based separator is shown by b. The remaining capacity, which is a parameter on the vertical axis in the figure, means (100% -self-discharge amount%). From this result, the Ni-Cd battery using the separator according to the present invention has very little self-discharge, and it is still 70% even if left at 45 ° C for about 1 month.
It can be seen that the capacity is maintained.

In this example, only the non-woven fabric was described, but the same effect was observed for the woven fabric. Further, this separator can be applied not only to a battery using an alkaline aqueous solution but also to a battery using another aqueous solution and an organic electrolyte, and is excellent in chemical stability and lyophilicity. Therefore, it is effective in improving the storage stability and the electrolyte injection property. In addition to the solution such as fuming sulfuric acid, a gas of sulfur trioxide may be used for the sulfonation treatment.

EFFECTS OF THE INVENTION As is apparent from the above description, the sulfonated separator of the non-woven fabric or the woven fabric composed of the polyolefin resin fiber in which the surface layer is more easily sulfonated than the interior has a wide range of conditions in the sulfonation process. For example, a treatment time can be adopted and the entire fiber can be uniformly treated, which is industrially suitable, and a battery using this separator has an effect of remarkably improving self-discharge characteristics.

[Brief description of drawings]

FIG. 1 is a schematic view of a separator according to the present invention, FIG. 2 is an enlarged cross-sectional view of the sulfonated polyolefin resin fiber shown in FIG. 1, and FIG. 3 is a Ni-Cd using the same separator.
It is a figure which shows the self-discharge characteristic of a battery. 1 ... Sulfonated polyolefin resin fiber, 2
...... Space, 3 ... Polypropylene, 4 ... Polyethylene, 5 ... Sulfonated polyethylene resin part, 6 ...
… Bonded parts of fibers, 7 …… Defective parts of polyethylene.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoichi Ikeyama 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Inventor Kazutaka Iwasaki 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. (56) References JP 57-141862 (JP, A) JP 57-131233 (JP, A)

Claims (4)

[Claims] 1. A battery separator comprising a non-woven fabric or a woven fabric of a fiber whose main constituent is a polyolefin-based resin, wherein the fiber is composed of a polyolefin-based resin material whose inside and most of the surface layer are different from each other, A battery separator characterized in that the resin material of the surface layer is a material that is more easily sulfonated than that of the inside, and that the resin material of the surface layer is mainly sulfonated. 2. The battery separator according to claim 1, wherein the internal polyolefin resin is polypropylene or a copolymer of polypropylene and polyethylene. 3. The battery separator according to claim 1, wherein the polyolefin resin with a surface layer is a copolymer of polypropylene and polyethylene or polyethylene. 4. The contact portion between the polyolefin resin fibers is heat-sealed, and the polyolefin resin inside the fibers is also heat-sealed at this weld portion. The battery separator described.