Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-03-28
2002-10-29
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218200, C429S220000, C429S223000, C429S224000
Reexamination Certificate
active
06472101
ABSTRACT:
TECHNICAL FIELD
This invention relates to a nickel-metal hydride storage cell, and more particularly to a composition of a positive electrode active material and a composition of a negative electrode active material.
BACKGROUND ART
Nickel-metal hydride storage cells employ a hydrogen-absorbing alloy that is capable of reversibly absorbing and desorbing hydrogen for a negative electrode material. Such nickel-metal hydride storage cells have a larger energy density per unit volume than conventionally used storage cells such as a lead cell and a nickel-cadmium cell, and also exhibit a high degree of tolerance to overcharging and overdischarging as well as an excellent cycle characteristic by its nature. For these reasons, nickel-hydrogen cells have become widely utilized in an electric power supply for a variety of appliances such as portable appliances and electric motors.
Nickel hydroxide is used as a positive electrode active material in such nickel-metal hydride storage cells. In a charge reaction, nickel hydroxide is converted to nickel oxyhydroxide, and in a discharge reaction, nickel oxyhydroxide is converted back to nickel hydroxide. However, when charging is performed at high temperature, oxygen is generated as a side reaction at the same time of the charge reaction by nickel hydroxide, and the charge reaction, in which nickel hydroxide is converted to nickel oxyhydroxide, is thereby impeded. As a result, a utilization factor of nickel hydroxide is reduced, causing a degradation in a positive electrode capacity.
In view of this problem, as a method for suppressing such a side reaction, for example, Japanese Unexamined Patent Publication Nos. 5-28992, and 6-103973 disclose a technique in which an yttrium compound or the like is added to a nickel positive electrode. According to the technique disclosed therein, an yttrium compound or the like is adsorbed on the surface of nickel oxide, and serves to increase an overvoltage, which is a competitive reaction in charging at high temperature, leading to a sufficient charge reaction of nickel hydroxide to nickel oxyhydroxide. Hence, a utilization factor of the positive electrode active material at high temperature is improved.
However, in a nickel-metal hydride storage cell, when charged at high temperature, a side reaction of a dissociation of hydrogen takes place in the negative electrode as well as the positive electrode. This side reaction causes a reduction in a utilization factor of the negative electrode active material, resulting in a decrease in a negative electrode capacity. The dissociated hydrogen also causes a decrease in a positive electrode capacity. Accordingly, even if a utilization factor of the positive electrode active material is improved by employing the above-described technique, a cell capacity as a whole cannot be sufficiently improved, since the cell capacity is limited by the negative electrode in which the capacity is degraded, or the dissociated hydrogen is absorbed in the positive electrode, causing a decrease in a positive electrode capacity.
DISCLOSURE OF THE INVENTION
The present invention intended to solve the foregoing problems. It is an object of the present invention to provide a nickel-metal hydride storage cell in which both a utilization factor of a positive electrode active material and a utilization factor of a negative electrode active material are improved at high temperature, a high capacity is maintained at high temperature, and an excellent cycle characteristic is achieved. In order to accomplish this and other objects of the invention, the following configurations are provided in a group of the invention.
(1) First Group of the Invention.
The present invention provides a nickel-metal hydride storage cell comprising in a cell-case, a positive electrode comprising a positive electrode active material composed mainly of nickel hydroxide powder, a negative electrode comprising a negative electrode active material composed mainly of hydrogen-absorbing alloy powder, and a separator interposed between the positive and negative electrodes and impregnated with an electrolyte, the nickel-metal hydride storage cell characterized in that the negative electrode active material contains a copper compound, and the positive electrode active material contains at least one compound selected from the group consisting of bismuth compound, calcium compound, ytterbium compound, manganese compound, copper compound, scandium compound, and zirconium compound.
In accordance with the configuration set forth above, the compound contained in, the positive electrode, such as bismuth and the like, serves to accomplish a sufficient charge reaction of nickel hydroxide to nickel oxyhydroxide at high temperature. The copper compound contained in the negative electrode serves to suppress a decrease in a utilization factor of the negative electrode active material (hydrogen-absorbing alloy) at high temperature, and to prevent a degradation of the negative electrode capacity. Therefore, it is made possible to prevent such undesirable effects that a cell which is initially positive-electrode-limited changes into a negative-electrode-limited cell at high temperature, and that a cell capacity is limited by the negative electrode having a smaller capacity. In other words, with the above-described configuration, an increase of the positive electrode capacity entirely leads to an increase of a cell capacity (a capacity which can be outputted from a cell as a whole). Therefore, it is made possible to solve such a problem in prior art that a sufficient cell capacity cannot be achieved even if the positive electrode capacity is improved.
In the above-described configuration, the at least one compound may be an oxide or hydroxide of one of bismuth, calcium, ytterbium, manganese, copper, scandium, and zirconium.
In addition, the copper compound may be an oxide or hydroxide of copper.
In addition, the positive electrode active material may comprise an aggregate of coated particles each in which a coating layer comprising a sodium-containing cobalt compound is formed on a surface of a nickel hydroxide particle, and the positive electrode active material may be such that the oxide or hydroxide of one of bismuth, calcium, ytterbium, manganese, copper, scandium, and zirconium, is added to the aggregate of coated particles.
In the configuration of employing the coated particles each particle in which a coating layer comprising a sodium-containing cobalt compound is formed on a surface of on a nickel hydroxide particle, the cobalt compound exists on the surface of the nickel hydroxide particle, and thereby electrical conductivity in the active material is effectively improved. As a result, the addition of a cobalt compound is minimized, and a proportion of nickel hydroxide (active material) in the positive electrode active material is made sufficient. In other words, in this configuration, it is possible to suppress a degradation in a theoretical capacity caused by the addition of cobalt compound, and thus an increase of electrical conductivity (an increase in an active material utilization factor) leads to a significant increase in an actual capacity of the positive electrode.
In addition, in the positive electrode active material in which an oxide or a hydroxide of such an element as bismuth and the like is added to the aggregate of the coated particles, a compound such as an yttrium and the like serves to suppress a diffusion of the cobalt compound into the inside of the nickel hydroxide particle, thus preventing such an occurrence that a concentration of the cobalt compound on the surface of the nickel hydroxide decreases as charge-discharge cycles proceed. Hence, by these effects, it is made possible to significantly improve a cell capacity, which is a total of the positive electrode performance and the negative electrode performance, as well as a cycle characteristic of the cell.
(2) Second Group of the Invention
The present invention also provides a nickel-metal hydride storage cell comprising in a cell case, a positive electrod
Maeda Reizo
Matsuura Yoshinori
Nishio Koji
Nogami Mitsuzo
Shinyama Katsuhiko
Armstrong Westerman & Hattori, LLP
Chaney Carol
Sanyo Electric Co,. Ltd.
Yuan Dah-Wei
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