Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
1999-10-06
2003-09-23
Kalafut, Stephen (Department: 1745)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S535000, C429S247000, C429S249000
Reexamination Certificate
active
06623809
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a battery separator and a manufacturing method thereof, and an alkali secondary battery having the separator incorporated therein, in particular to a nickel-metal hydride secondary battery. More particularly, the present invention relates to a battery separator and a manufacturing method thereof in which preferred hydrophilization treatment is applied, and thus, an alkali secondary battery having the separator incorporated therein causes its superior self-discharge properties; and a nickel-metal hydride secondary battery having the separator incorporated therein, its superior self-discharge properties, and superior charge and discharge cycle life performance.
2. Prior Art
In recent years, with advancement of cordless use, high performance, down-sizing, and reduced weight of various electronic devices such as hand-held telephone or hand-held type personal computers, there has been a growing need for a secondary battery with high-capacity being a power source of these devices.
As power sources of these electronic devices, alkali secondary batteries have been generally employed, and specifically, secondary nickel-cadmium batteries has been mainly used. With a growing demand for the above-mentioned high capacity, recently, secondary nickel-hydrogen batteries are widely used because they are compatible with voltages of the secondary nickel-cadmium batteries, and have higher capacity than the secondary nickel-cadmium battery.
However, there is a problem that, although such secondary nickel-hydrogen batteries have high capacity than the nickel-cadmium batteries, self-discharge is likely to occur if they are in charged state, and maintained under high-temperature environment.
Therefore, corresponding to the fact that use environment of the secondary nickel-hydrogen-batteries is diversified, and these batteries are increasingly used under severe conditions, it is of course that the secondary nickel-hydrogen batteries have high capacity, and it is strongly required that the batteries have improved self-discharge properties and charge and discharge cycle life performance.
In the meantime, an alkali secondary battery is manufactured as follows: In general, a paste consisting of an active substance and a bonding agent is applied to and filled in a collector to manufacture positive and negative electrodes, respectively; an electrode group (generator element) consisting of a separator intervened between the positive and negative electrodes is manufactured; the electrode group is housed in a battery case together with an alkali electrolyte; and then, the battery case is sealed.
For example, in the case of a nickel-metal hydride battery, in general, a separator is intervened between a nickel electrode (a positive electrode) for supporting powders of nickel compounds such as nickel hydroxides and a hydrogen absorbing alloy electrode (a negative electrode) for supporting powders of hydrogen absorbing alloys to form an electrode group; this electrode group is housed in a battery case compatible with a negative electrode terminal together with an alkali electrolyte, and the battery case is sealed.
In such nickel-metal hydride secondary battery, it is of course that the separator must have electric insulation properties, and it is required that the separator has good wettability with an filled alkali electrolyte, whereby it is required to provide electrolyte absorption properties for the alkali electrolyte.
From the foregoing, as a separator, a non-woven fabric consisting of polyamide fibers having good hydrophilicity is widely employed to ensure wettability with the alkali electrolyte.
However, this polyamide fibers are hydrolyzed in an alkali electrolyte, and impurities such as ionic nitrate, nitrous ion, or ammonia, for example, are produced. These impurities cause so-called shuttle reaction, and reduces nickel oxy-hydroxide produced on a nickel electrode during charging, thereby promoting self-discharge in the nickel electrode and degrading battery self-discharge properties.
From the foregoing, as the separator materials, there are attempted selection of fiber materials having oxidization proof superior to the polyamide fibers, for example, polyolefin fibers such as polypropylene fibers, or fluoro resin fibers such as PTFE fibers, and use of which hydrophilization treatment is applied thereto.
Specifically, for example, there is attempted use of a separator in which a non-woven fabric made of polyolefin fibers is surface-treated by an anionic, cationic, or non-ionic surface active agent.
However, the thus manufactured separator ,is not fixed with a hydrophilic group introduced by the surface active agent, which is chemically bonded with a surface of a separator material. Thus, there is a problem that, if battery charging and discharging are repeated, a surface active agent is left from the surface, hydrophilicity are lowered, resulting in lowered charge and discharge cycle life performance.
In addition, the following treatment is attempted: A treatment such as sulphonation treatment or acrylic acid graft treatment is carried out on the full surface of a non-woven fabric made of polyolefin fibers, whereby a hydrophilic active group is added from the outside, and hydrophilicity of the entire surface of the non-woven fabric is enhanced to improve self-discharge properties. Even if these treatments are carried out, self-discharge properties are not sufficiently improved. Moreover, in the case of these treatments, since waste treatment of used chemicals is required, there is a problem that separator manufacturing cost and battery manufacturing cost are increased.
Further, in the case of the nickel-metal hydride secondary battery, it is known that self-discharge is promoted by hydrogen being left from a hydrogen absorbing alloy used for a negative electrode. The mechanism is described as follows:
When a nickel-metal hydride secondary battery is stored under a high temperature, an equilibrium pressure of a hydrogen absorbing alloy for a negative electrode rises together with a temperature rise, and a quantity of hydrogen capable of being absorbed by the negative electrode decreases. As a result, hydrogen gas which cannot be accumulated by the negative electrode is discharged into a battery. This hydrogen permeates the separator, reaches a surface of a nickel electrode, reduces a nickel oxy-hydroxide being a charged product of the nickel electrode, and causes self-discharge.
Thus, requirements for improvement of self-discharge properties of an alkali secondary battery, in particular, a nickel-metal hydride secondary battery are that a separator to be incorporated is composed of materials without producing the aforementioned impurities, and that high hydrophilicity are imparted, thus making it possible to ensure wettability with an alkali electrolyte over a long period of time.
On the other hand, when electric conductivity between active substances (nickel hydroxides) in the aforementioned nickel electrode and between an active substance and a collector is increased, the utility of the active substance is increased. Also, since a firm electrically conductive matrix is formed at the surface of nickel hydroxide powder, self-reducibility of the nickel electrode per se is suppressed. In this manner, the charge and discharge cycle life performance of the secondary nickel-hydrogen batteries having the nickel electrode incorporated herein are improved.
Therefore, conventionally, the following method is carried out for the purpose of increasing the utility of the active substance and improving the charge and discharge cycle life performance.
For example, when a positive electrode synthetic agent to be filled in or applied to a nickel electrode is paste-prepared, cobalt compounds such as cobalt metals, cobalt hydroxides, cobalt tri-oxides, tri-cobalt tetra-oxides, or cobalt monoxides or a predetermined amount of particles of a mixture of these compounds are added as an electric conducting material, mixed powders mixed with po
Hata Katsuyuki
Tajima Michiko
Teraoka Hirohito
Tsukiashi Masahiko
Kalafut Stephen
Mercado Julian
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Toshiba Battery Co., Ltd.
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