Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-06-11
2001-07-03
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218200, C429S223000, C029S623500, C148S513000
Reexamination Certificate
active
06255018
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydrogen-absorbing alloy electrode for use in an alkaline secondary battery such as a nickel-hydrogen secondary battery and, more particularly, to a method for enhancing the electrochemical activity of a hydrogen-absorbing alloy electrode.
BACKGROUND ART
Nickel-hydrogen secondary batteries have been known as one type of alkaline secondary batteries. The nickel-hydrogen secondary battery typically employs as its negative electrode a hydrogen-absorbing alloy electrode utilizing a hydrogen-absorbing alloy.
Examples of the hydrogen-absorbing alloy for the negative electrode include Mm-based hydrogen-absorbing alloys comprising a Misch metal which is a mixture of rare earth elements, and Laves-phase hydrogen-absorbing alloys.
However, these hydrogen-absorbing alloys are generally susceptible to natural oxidation so that an oxide film and the like are formed on the surfaces thereof. Where an alkaline secondary battery employs as its negative electrode a hydrogen-absorbing alloy electrode formed of such a hydrogen-absorbing alloy, hydrogen gas cannot sufficiently be absorbed by the electrode because the hydrogen-absorbing alloy has a low activity in an initial cycle. This presents problems such as a reduced battery capacity and an increased battery inner pressure in the initial cycle.
To this end, a variety of methods have been proposed to recover the electrochemical activity of the hydrogen-absorbing alloy as a negative electrode active substance. One of the methods is an acid treatment wherein a hydrogen-absorbing alloy is immersed in an acid treatment solution such as containing hydrochloric acid to remove an oxide film from the surface of the hydrogen-absorbing alloy (Japanese Unexamined Patent Publications No. 4-179055 (1992), No. 5-225975 (1993), No. 7-73878 (1995), No. 7-153460 (1995) and the like). The acid treatment ensures a relatively excellent oxide film removing effect, and operations for the treatment is easy. Therefore, the acid treatment is an effective method for the recovery of the electrochemical activity of the hydrogen-absorbing alloy.
Where a hydrogen-absorbing alloy is subjected to the aforesaid conventional acid treatment, active sites appear on the surface of the hydrogen-absorbing alloy. However, the active sites of the hydrogen-absorbing alloy are extremely susceptible to oxidization, so that re-oxidization occurs during a battery fabrication process. Therefore, the conventional acid treatment fails to satisfactorily enhance the activity of the hydrogen-absorbing alloy electrode in the initial cycle. Accordingly, an alkaline secondary battery employing such a hydrogen-absorbing alloy electrode suffers an increase in the inner pressure during initial charging, a low initial discharge capacity, a poor low-temperature discharge characteristic, and a poor high-rate discharge characteristic.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to solve the aforesaid problems associated with the hydrogen-absorbing alloy electrode.
To attain the aforesaid object, the inventors of the present invention have made intensive studies on an acid treatment for surface-treating a hydrogen-absorbing alloy with an acid treatment solution. As a result, the inventors have found that a highly active hydrogen-absorbing alloy can be obtained by promoting the pH rise of an acid treatment solution during an acid treatment of a hydrogen-absorbing alloy or by using an acid treatment solution containing metal ions and promoting the pH rise of the acid treatment solution during the acid treatment, thereby drastically improving the initial inner pressure characteristic and the electrochemical characteristics, such as an initial discharge characteristic, of a battery employing the hydrogen-absorbing alloy as its electrode substance. Thus, the inventors have achieved the present invention.
The following modes of the present invention are based on the same or similar ideas, and have common technical features. However, these modes of the present invention were not completed at a time, but respectively embodied by way of different embodiments. For clarity of explanation, these modes are classified into first, second, third and fourth inventive mode groups, which each include closely related modes, and an explanation will hereinafter be given to each of the inventive mode groups.
(1) First Inventive Mode Group
In accordance with the first mode group of the present invention, a process for producing a hydrogen-absorbing alloy electrode is provided which comprises an alloy activation treatment step wherein a hydrogen-absorbing alloy is surface-treated by immersing the hydrogen-absorbing alloy in an acid treatment solution containing metal ions and initially having a pH level of pH 0.5 to pH 3.0.
With this arrangement, an oxide film and the like on the surface of the alloy react with hydrogen ions so as to be dissolved in the acid treatment solution. As a result, the oxide film is removed, so that an isolated metal layer appears on the surface of the alloy. This reaction entails a pH rise of the solution and, in the course of the pH rise, the metal ions present in the acid treatment solution are deposited on the surface of the alloy in the solution thereby to increase the thickness of the isolated metal layer on the alloy surface. The metal layer deposited on the alloy surface serves as a protective film for hindering the alloy surface from being oxidized by air. Thus, the conductivity and reactivity of the hydrogen-absorbing alloy are enhanced thereby to improve the hydrogen-absorbing ability of the alloy. Therefore, when the hydrogen-absorbing alloy thus treated is used as an electrode active substance, the resulting hydrogen-absorbing alloy electrode has an excellent high-rate discharge characteristic and cycle characteristic.
The cycle characteristic of a battery is improved by the enhancement of the hydrogen-absorbing ability. This is due to the following fact. In general, an alkaline secondary battery employing a hydrogen-absorbing alloy electrode as its negative electrode has a sealed structure and, for safety, is constructed such that a safety valve operates when the pressure inside the battery becomes higher than a predetermined level. If the hydrogen-absorbing alloy electrode of the battery of such a construction has a poor hydrogen-absorbing ability, hydrogen gas is accumulated in the battery thereby to increase the inner pressure of the battery. As a result, the safety valve operates to release the gas accumulated in the battery. At this time, the electrolyte is scattered outside the battery, so that the shortage of the electrolyte occurs to reduce the battery performance. Conversely, if the hydrogen-absorbing alloy as the negative electrode active substance has a high activity, hydrogen gas is smoothly absorbed by the alloy, thereby preventing the increase in the inner pressure of the battery. Since the safety valve has no chance to operate, the battery performance is not deteriorated due to the shortage of the electrolyte. Therefore, the cycle lifetime is extended.
Usable as the metal ions are ions of a metal selected from the group consisting of nickel and cobalt. With these metal ions, an isolated metal layer having an excellent conductivity can be formed on the alloy surface. Thus, the high-rate discharge characteristic and the cycle characteristic can be further improved.
The acid treatment solution may further contain a substance having a pH buffer action for keeping the pH level of the acid treatment solution within a pH range between pH 4 and pH 6. The use of the pH buffering substance is preferred because the deposition of the metal can be increased to thicken the isolated metal layer by continuously keeping the pH level of the acid treatment solution within a pH range between pH 4 to pH 6. Usable as the pH buffering substance are, for example, aluminum ions.
(2) Second Inventive Mode Group
In accordance with one mode of the second inventive mode group, a process for producing a hydrogen-absorbing alloy el
Fujitani Shin
Fukuda Hiroshi
Higashiyama Nobuyuki
Imoto Teruhiko
Ise Tadashi
Armstrong Westerman Hattori McLeland & Naughton LLP
Edmondson L.
Ryan Patrick
Sanyo Electric Co,. Ltd.
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