Chemistry: electrical current producing apparatus – product – and – Sealed cell having gas prevention or elimation means – Prevention or elimination means is one of the cell...
Patent
1993-04-28
1995-02-28
Maples, John S.
Chemistry: electrical current producing apparatus, product, and
Sealed cell having gas prevention or elimation means
Prevention or elimination means is one of the cell...
429223, 420900, H01M 432, H01M 438
Patent
active
053936167
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a metal hydride electrode having excellent charging and discharging ability etc., a nickel electrode having a high energy density and including no cadmium, and a nickel-hydrogen battery which is inexpensive and free from pollution.
BACKGROUND ART
With a recent progress of portable electric equipment, demands for improving a capacity and enhancing an energy density are increasing against batteries serving as its power sources. In order to meet these demands, a nickel-metal hydride battery so-called "nickel-hydrogen battery" which utilizes MmNi.sub.5 system (Mm: misch metal comprising a mixture of rare earth elements such as La, Ce or Nd etc.) hydrogen absorbing alloy as its negative active material, makes its advent recently and is expected to be put in practical use.
In the negative electrode (metal hydride electrode) of nickel-hydrogen battery utilizing alkaline electrolyte, the reactions expressed by equations (1) and (2) take place at time of charging and discharging.
An electron is supplied from outside to cause a proton to be reduced to a hydrogen atom on a surface of alloy and absorbed in the hydrogen absorbing alloy when the charging is carried out. On the contrary, the absorbed hydrogen atom is ionized on-the surface of alloy to release the proton when the discharging is carried out.
In the charge/discharge reaction of the metal hydride electrode, as described above, the alloy surface on which the ionization reaction (or reverse reaction) of hydrogen atom expressed by the equation (1) takes place plays an important part.
However, when charging and discharging (oxidation and reduction) are repeated in the alkaline electrolyte, the MmNi.sub.5 system hydrogen absorbing alloy has such problems that surface corrosion of alloy progresses so as to inhibit the foregoing ionization reaction of hydrogen atom, increase a resistance between alloy particles (decrease an electronic conductivity) and gradually decrease the capacity to exhaust its service life.
In addition to the above problems, a sealed type nickel-hydrogen battery further includes such a problem that the hydrogen absorbing alloy of negative electrode is oxidized by oxygen gas generated from the positive electrode at time of over-charging, so that deterioration of alloy and shortening of service life are further accelerated.
In order to avoid the deterioration of alloy and shortening of service life due to the corrosion or oxidation of hydrogen absorbing alloy, there has so far been used a method for improving the corrosion resistance of the alloy itself by substituting a composition of MmNi.sub.5 system alloy i.e. a part of Ni with Al and Fe, Cu, Mn or Co etc. In order to further improve the corrosion resistance and oxidation resistance because of imperfection of this method, there have been proposed methods called as "microencapsulation method" in which corrosion resistant metal such as nickel or copper is coated on surfaces of hydrogen absorbing alloy (Published Patent Application (KOKAI) Nos. 61-64069 & 63-51051) and a method in which conductive agent of metallic powder or metal oxide is mixed in the hydrogen absorbing alloy in order to improve the electron conductivity between alloy particles, etc.
Although these methods have some effect to avoid the deterioration of alloy, a manufacturing cost is increased because they require troublesome processes such as the alkaline etching or the electroless plating and a capacity per unit weight of active material is decreased because a percentage of plating amount or amount of conductive agent is large which is unnecessary for the electrochemical capacity.
The electroless plating process for the microencapsulation of hydrogen absorbing alloy is composed of plural processes such as [1] Pretreatment of alloy, [2] Electroless plating, [3] Washing and [4] Drying etc. so that this process is troublesome and expensive in its manufacturing cost. In addition, heavy metal is included in waste liquid after the electroless plating so that pollution control facilities
REFERENCES:
patent: 4396576 (1983-08-01), Osumi et al.
patent: 4605603 (1986-08-01), Kanda et al.
Hasegawa Keiichi
Mori Hiroyuki
Oshitani Masahiko
Watada Masaharu
Maples John S.
Yuasa Corporation
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