Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
2002-04-08
2004-11-23
Yuan, Dah-Wei (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S142000, C429S144000, C429S145000, C429S247000, C429S248000, C429S254000
Reexamination Certificate
active
06821680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to battery separators, to processes for producing the battery separators, and to alkaline batteries using the battery separators. Such alkaline batteries, in particular, alkaline secondary batteries, are employed as batteries in, for example, electric vehicles and electric tools.
2. Description of the Related Art
Batteries generally include, as components, a positive electrode, a negative electrode, an electrolyte, a separator, and a battery jar. The positive and negative electrodes are separated by the separator from each other, are immersed in the electrolyte and are housed in the battery jar. As the separator, non-woven fabrics made of polyamide fibers, polypropylene fibers or acrylic fibers are generally employed.
In addition to the properties of the positive and negative electrodes and of electrolyte, the properties of the separator also significantly affect the characteristic of the battery, and various attempts have been made to improve the properties of such separators.
Of controls of such battery characteristics, demands have been made in recent years to sufficiently control self-discharge in nickel metal hydride batteries and other alkaline batteries, as these alkaline batteries exhibit a relatively large self-discharge.
The self-discharge is known to be caused by nitrate groups, and such nitrate groups are formed by the oxidation of ammonia contaminating a battery. The reduction of ammonia in a battery can reduce the amount of the nitrate groups to thereby suppress the self-discharge. However, ammonia is liable to contaminate a battery in a production process of electrodes and contamination of the battery with ammonia cannot be completely prevented.
Consequently, a technique has been proposed, which comprises grasping or trapping the contaminated ammonia in a battery by a separator before the ammonia is converted into a nitrate group. By this technique, free ammonia is reduced to thereby reduce the formation of nitrate groups.
For example, Japanese Unexamined Patent Publication No. 10-116600 discloses a separator made of a polyolefin fiber which is grafted with a vinyl monomer. In this technique (hereinafter referred to as “the conventional technique 1”), the resulting separator absorbs and holds ammonia and other nitrogen-containing components.
In the conventional technique 1, ammonia is trapped basically by carboxyl groups. However, the carboxyl groups are highly liable to be selectively combined with potassium ions that are present in large amounts in the electrolyte, and the carboxyl groups trap ammonia only in small amounts. In addition, such a separator which has been converted into hydrophilic by treatment with a vinyl monomer has a poor heat resistance. For example, constitutive acidic groups in the separator are eliminated from the separator at relatively high temperatures, for example, at 60° C., and a satisfactory ammonia trapping property cannot be significantly maintained.
As an another candidate for separators, a separator made of a sulfonated polypropylene is proposed in Proceedings of The 28th Battery Symposium in Japan (page 113, 1987) (hereinafter referred to as “the conventional technique 2”). This literature states that the use of such a non-woven fabric separator made of a sulfonated polypropylene exhibits a less self-discharge and a higher rate of holding a battery capacity than conventional polyamide separators.
However, the separator proposed in the conventional technique 2 is directed not to positively trapping ammonia but to suppress nitrogen impurities from releasing or eluting out of the separator. In fact, the results in an experiment stated in the above conventional technique 2 show that the proposed separator has an insufficient ammonia trapping property. Demands have been therefore made to improve the ammonia trapping property of a separator made of a sulfonated polypropylene.
A possible solution to improve the ammonia trapping property is introduction of large amounts of sulfonic groups into a matrix polymer. However, fibers made of polyolefin resins are highly resistant to acids, and if sulfonic groups are introduced into the fibers to a great extent, the introduced sulfonic groups are unevenly distributed. In other words, the resulting fibers have portions where large amounts of sulfonic groups are introduced and portions where relatively less amounts of sulfonic groups are introduced. The portions where large amounts of sulfonic groups are introduced will have a markedly deteriorated strength and may be collapsed and eliminated from the separator. Consequently, a separator having large amounts of sulfonic groups cannot be significantly obtained. Furthermore, such fibers are liable to be cut in portions having a deteriorated strength and therefore have a low fiber strength. The resulting separator using these fibers having a low strength may be cut in a winding process, in which the separator is wound up into a battery, or in other processing and assembly steps of the separator into the battery.
Regarding thermal stability, the sulfonated separator according to the conventional technique 2 has a higher thermal stability than the carboxylated separator according to the conventional technique 1. However, yet some amounts of sulfonic groups will be inevitably eliminated by heating, and such elimination of sulfonic groups is liable to occur particularly in highly sulfonated portions. In other words, uneven introduction of sulfonic groups in a separator will result in marked elimination of the sulfonic groups by heating, and a satisfactory ammonia trapping property cannot be stably maintained.
Japanese Unexamined Patent Publication No. 10-326607 (hereinafter referred to as “the conventional technique 3”) teaches that a sulfur concentration can be used as an index of a degree of sulfonation and describes a separator having a sulfur concentration of 7 mg/g in an example. This relatively high sulfur concentration is considered to be obtained by composing the fiber from a constitutive monofilament having a markedly small fineness of 0.01 to 0.1 denier (corresponding to 0.011 to 0.11 decitex) to thereby have a large surface area. Specifically, the fiber is to have large total amounts of sulfonic groups introduced therein by increasing the surface areas of constitutive fibers.
However, such a separator made of a polypropylene fiber having a monofilament fineness of 0.01 to 0.1 denier (0.011 to 0.11 decitex) should be inevitably obtained by a following process, as is described in the above publication. That is, a polypropylene fiber having a fineness of 0.01 to 0.1 denier (0.011 to 0.11 decitex) is obtained by sulfonating a material fibrous mixture comprising an island-in-sea type composite fiber and an olefinic binder fiber, where island-in-sea type composite fiber comprises a polyamide resin as a sea component and a polypropylene as an island component, and then dissolving and eliminating the polyamide resin. According to this production process, the polyamide resin in the center of the island-in-sea type composite fiber cannot be completely eliminated by a sulfonation treatment alone, and the residual polyamide will cause the self-discharge of the battery instead.
In addition, an oxygen gas is formed in the positive electrode at a terminal stage of battery charging, but the use of ultrafine fibers having a fineness of 0.01 to 0.1 denier (0.011 to 0.11 decitex) in a separator results in an extremely low gas permeability and the separator permeates only an insufficient amount of oxygen gas from the positive electrode into the negative electrode. The accumulated oxygen gas will cause the expansion of the battery, resulting in leakage of the electrolyte or further explosion of the battery. Such ultrafine fibers have a very low fiber strength and are liable to break to thereby create a risk of a short-circuit.
In the conventional technique 3, the sulfonic group introduction is optimized from the viewpoint of affinity for the electrolyte alone, but is not optimiz
Hamamoto Shiro
Takimoto Naohiko
Tanaka Toshio
Yamaguchi Hiroki
Yamashita Masahiro
Toyo Boseki Kabushiki Kaisha
Yuan Dah-Wei
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