Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-11-29
2003-04-15
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S235000, C429S236000, C429S144000, C428S615000, C428S680000
Reexamination Certificate
active
06548210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nickel electrodes for alkaline secondary battery wherein a porous sintered nickel substrate is loaded with a nickel hydroxide-based active material, and alkaline secondary batteries employing the same as the positive electrode therefor. The invention is directed to the improvement of the nickel electrode for alkaline secondary battery for suppression of self discharge associated with oxygen evolution during storage of the charged alkaline secondary battery under high temperature conditions and for increase in the high-current discharge capacity of the electrode.
2. Description of the Related Art
The conventional alkaline secondary batteries such as nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries and the like, have employed sintered nickel electrodes or non-sintered nickel electrodes as the positive electrode therefor.
The non-sintered nickel electrode is fabricated by directly loading a nickel hydroxide-based active material paste into a porous conductive body, such as a nickel substrate foam. Although this electrode is easy to fabricate, there is a disadvantage of poor charge/discharge characteristics at high current.
On the other hand, the sintered nickel electrode employs a porous sintered nickel substrate obtained by sintering and is fabricated by chemically impregnating the porous sintered nickel substrate with a salt as the active material. The sintered nickel substrate presents higher conductivity. In addition, the electrode is excellent in the charge/discharge characteristics at high current because of good adhesion of the active material to the porous sintered nickel substrate. On this account, the alkaline secondary batteries with the sintered nickel electrodes are favorably used in electric power tools requiring high current discharge.
Unfortunately, the sintered nickel electrode has a lower loading ratio of the active material than the non-sintered nickel electrode and therefore, must be improved in the utilization of the active material therefor. In addition, repeated charges/discharges of the alkaline secondary battery with the sintered nickel electrode result in brittleness of the sintered nickel substrate. Thus, the sintered nickel electrode is susceptible to improvement in the charge/discharge cycle characteristics.
In this connection, proposals have been made in the art as follows. For instance, Japanese Unexamined Patent Publication No.1(1989)-200555 discloses a process aimed at the increase in the conductivity of the active material for improved utilization thereof, the process comprising the steps of laying a cobalt hydroxide layer on a surface of the active material loaded into the porous sintered substrate, and oxidizing the cobalt hydroxide layer by heat treatment in the presence of oxygen and an alkaline solution. Further, Japanese Unexamined Patent Publication No.63(1985)-216268 discloses a process aimed at the suppression of corrosion of the sintered nickel substrate during the loading of the active material and the improvement in the charge/discharge cycle characteristics of the alkaline secondary battery, the process comprising the steps of laying a cobalt hydroxide layer on a surface of a porous sintered nickel substrate, heating the substrate in the presence of oxygen and an alkaline solution, and then loading the nickel hydroxide-based active material into the sintered nickel substrate.
Unfortunately, where the sintered nickel electrode of Japanese Unexamined Patent Publication No.1-200555 is used as the positive electrode of the alkaline secondary battery, the alkaline secondary battery still suffers the occurrence of self discharge due to the oxygen evolution in the sintered nickel electrode when the charged battery is stored at high temperatures of about 50° C. over an extended period of time. Thus, the alkaline secondary battery is reduced in capacity.
Where the sintered nickel electrode of Japanese Unexamined Patent Publication No.63-216268(Japanese Examined Patent Publication No.5(1993)-50099) is used as the positive electrode of the alkaline secondary battery, as well, the oxygen evolution occurs in the alkaline secondary battery charged at high temperatures of about 50° C. before the positive electrode is charged to full. As a result, the battery is decreased in charge efficiency.
Further, Japanese Unexamined Patent Publication No.48(1973)-50233 has proposed a sintered nickel electrode employing a positive-electrode active material incorporating yttrium hydroxide for improvement in the utilization thereof under high temperature conditions. Alternatively, Japanese unexamined Patent Publication No.5(1993)-28992 discloses an alkaline secondary battery employing a nickel oxide-based active material with a compound, such as yttrium, indium, antimony and the like, added thereto for accomplishing improvement in the utilization of the active material under high temperature conditions.
In those batteries of the above official gazettes, however, the compounds such as of yttrium or the like, are simply added to the active materials and thud, the active materials or the sintered nickel substrates are not sufficiently covered with the compounds such as of yttrium of the like. This detrimentally allows for contact between the electrolyte and the active material and/or the sintered nickel substrate. Hence, there still exists the problem of the oxygen evolution in the nickel electrode under high temperature conditions and of the underutilization of the active material.
In the previous PCT application (PCT/JP99/00720), the present inventors have proposed a nickel electrode for alkaline secondary battery wherein a coating layer is laid on a surface portion of the nickel hydroxide-based active material loaded into the sintered nickel substrate, and is based on a hydroxide of at least one element selected from the group consisting of calcium, strontium, scandium, yttrium, lanthanide and bismuth, as well as a nickel electrode for alkaline secondary battery wherein an intermediate layer is interposed between the sintered nickel substrate and the above active material, and is based on a hydroxide of at least one element selected from the group consisting of calcium, strontium, scandium, yttrium, lanthanide and bismuth.
Where such a nickel electrode for alkaline secondary battery is used as the positive electrode for alkaline secondary battery, the self discharge due to the oxygen evolution in the nickel electrode is suppressed during the long term storage of the charged alkaline secondary battery under high temperature conditions. Thus are provided the alkaline secondary batteries excellent in high temperature storability.
Recently, however, there is an additional desire for a further increased discharge capacity at high current in order to cope with the aforementioned favorable use of the alkaline secondary batteries in the electric power tools.
SUMMARY OF THE INVENTION
An object of the invention is to improve the nickel electrode for alkaline secondary battery comprising a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, for suppression of the self discharge of the alkaline secondary battery employing this nickel electrode as the positive electrode and for enhancement of the storability of the battery stored under high temperature conditions.
Another object of the invention is to improve the alkaline secondary battery employing the above nickel electrode as the positive electrode in the high density current charge/discharge characteristics (high-rate characteristics).
According to a first aspect of the invention, a nickel electrode for alkaline secondary battery including a porous sintered nickel substrate loaded with a nickel hydroxide-based active material, the nickel electrode comprises a first coating layer of cobalt compound laid on a surface portion of the active material loaded into the sintered nickel substrate; and a second coating layer laid on the first coating layer and based on a compound of at lea
Harada Yasuyuki
Maeda Reizo
Matsuura Yoshinori
Nohma Toshiyuki
Shinyama Katsuhiko
Alejandro R
Sanyo Electric Co,. Ltd.
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