Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-05-18
2001-06-19
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S082000
Reexamination Certificate
active
06248463
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to alkaline electrochemical cells having metallic anodes and air cathodes, commonly known as metal-air cells. More particularly, this invention relates to the structure of the cathode cans, and to the use of small air ports on the bottom of the cathode cans. The air ports have specifically limited open area to provide ingress of air containing cathodic oxygen, while restricting water vapor movement.
BACKGROUND OF THE INVENTION
The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 0.25 inch up to about 1.0 inch, and height ranging from less than 0.15 inch up to about 0.60 inch. The small size and the limited amount of electrochemically reactive material contained in these small metal-air cells result in considerable attention being directed to improving the efficiency and completeness of the power generating electrochemical reactions which occur therein.
Metal-air cells convert atmospheric oxygen to hydroxyl ions in the air cathode. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc.
More particularly, the desired reaction in a metal-air cell air cathode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions, the hydroxyl ions being able to migrate through the electrolyte toward the anode, where oxidation of zinc may occur, forming zinc oxide, and liberating electrons.
In most metal-air cells, air enters the cell through a port extending through the bottom of the cathode can. The port extends through the bottom of the cathode can, and may be immediately adjacent the cathode assembly, or may be separated from the cathode assembly by an air chamber or an air diffusion member.
In any of such arrangements, the port facilitates the movement of air through the port and into the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water at the cathode assembly as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms hydroxyl ions.
In normal operation, the reaction surface of the cathode assembly is laden with electrolyte, water being a major constituent of the electrolyte. Accordingly, the water at the reaction surface of the cathode assembly has a vapor pressure, and is subject to evaporation at the reaction surface. To the extent water does evaporate at the reaction surface, moisture content of the cell is reduced, along with a corresponding reduction in efficiency of the cell. Where moisture loss is excessive, the cell may fail before the electrochemical reaction materials have been chemically used up.
A second, and undesirable function facilitated by the port in the bottom of the cathode can is that moisture e.g. evaporated from the reaction surface of the cathode assembly can escape from the cell through the port, whereby the cell dries out, and correspondingly loses effectiveness. Thus, there is a relationship between the amount of oxygen that can be made available to the cell through conventional port configurations, and the amount of moisture loss associated with such port configurations.
It is an object of this invention to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having one or more air entry ports so structured and configured, both individually and relative to each other, that the port configuration provides an improved relationship between the amount of oxygen that is available to the cathode assembly and the amount of moisture lost from the cell through the port configuration.
It is another object to provide improved cathode can structure for a metal-air electrochemical cell, wherein the sum of the open area of the port configuration is reduced while maintaining the cell limiting current.
It is still another object to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having a plurality of ports, with the port configuration structured so that, in a metal-air cell made with the cathode can, oxygen is more uniformly distributed over the cathode assembly, while minimizing the combined open area of the ports through the cathode can, and thereby reducing the amount of moisture loss through the ports.
A further object is to provide improved metal-air electrochemical cells having an increase in the ratio of the limiting current of the cell to the combined area of gaseous ingress and egress available through the port configuration.
SUMMARY OF THE DISCLOSURE
Some of the objects are obtained in a first family of embodiments comprehending electrochemical cell apparatus including a cathode can for use in a metal-air electrochemical cell having an anode comprising metal, the cathode can having an outside diameter, and comprising a bottom, having an inner surface, an outer surface, and an average thickness between the inner surface and the outer surface; and a side wall extending upwardly from the bottom, the bottom having at least one port extending therethrough, between the inner surface and the outer surface, and defining an opening in the bottom, the port having a maximum dimension across the opening defined by the port, the maximum dimension being smaller than the average thickness of the bottom, preferably no greater than 90% of the average thickness of the bottom.
In preferred embodiments, the cathode can includes at least two ports, each having a maximum dimension smaller than the thickness of the bottom, the ports preferably being substantially evenly spaced with respect to each other.
Preferably, the electrochemical cell apparatus includes a cathode assembly in the cathode can, the cathode assembly being spaced from the bottom and having a reaction surface facing the bottom, and thereby defining a reservoir between the bottom and the cathode assembly. The reaction surface of the cathode assembly defines an effective reaction area, and gas entering the reservoir through the ports and traversing across the reservoir to the reaction surface expands somewhat as a plume from each port during such traverse, an outline of each such plume being represented by an imaginary closed figure on the reaction surface aligned with each respective port, the imaginary closed figures, when expanded uniformly to touch each other, but generally not overlap each other, representing a footprint on the cathode assembly corresponding with the spacing of the ports on the bottom, the spacing of the ports being such that the footprint covers at least 80% of the effective reaction area of the reaction surface of the cathode assembly.
It is preferred that each port comprise a substantially circular opening, such that the maximum dimension of each port comprises a diameter of the respective port, the ratio of the average of the diameters of the ports, to the outside diameter of the cathode can being no greater than 0.025/1, preferably no greater than 0.020/1, and more preferably 0.017/1 or less.
It is also preferred that each port have an area enclosed thereby, and that the ratio of the average of the enclosed areas of the ports, to the area of the electrochemical cell apparatus, as defined by the outside diameter of the cathode can, be no greater than 0.001/1, preferably no greater than 0.00075/1, and more preferably no greater than 0.0005/1.
A preferred feature of electrochemical cells made using the invention is that the ratio of the limiting current of the electrochemical cell, in milliamps, to the effective reaction area of the reaction surface in millimeters squared be at least 100/1, preferably at least 125/1, more preferably at least 150/1, and most preferably at least 220/1.
The advantages of the invention become more apparent as the diameter of the port, or ports, is reduced. Accordingly, it
Dopp Robert B.
Oltman John Edward
Kalafut Stephen
Rayovac Corporation
Wilhelm Thomas D.
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