Zinc-based electrode particle form

Details Zinc-based electrode particle form Zinc-based electrode particle form

2000-08-30

2002-10-29

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S218100, C252S182100

Reexamination Certificate

active

06472103

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to improvements in electrochemical cells, particularly cells having negative electrodes comprising zinc-based particles, such as in alkaline batteries.
An electrochemical cell (i.e., a galvanic cell or battery) has the following basic components: a negative electrode (sometimes called an anode), a positive electrode (sometimes called a cathode), and an ion-conductive solution (sometimes called an electrolyte) providing a path for the transfer of charged ions between the two electrodes when they are connected through an external load.
Some alkaline cells have anodes with zinc as an active element, and cathodes with manganese dioxide (MnO
2
) as an active element. Anodes do not have to be solid; in fact, conventional alkaline cells have a gelled zinc anode mixture. The mixture contains individual zinc-based particles suspended in a thickened liquid or gel containing a gelling agent, an alkaline electrolyte such as potassium hydroxide (KOH), and minor amounts of other additives, such as indium or bismuth (gassing inhibitors for reducing the undesirable tendency for hydrogen gas to build up inside the cell). The zinc-based particles are characterized by a specific size range, commonly indicated by the standard mesh size through which the particles pass. Typically, average anode particle sizes fall in the range of about −50/+200 mesh, indicating particles that pass through a 50 mesh screen and do not pass through a 200 mesh screen (the larger the screen number, the smaller the aperture size of the screen).
Common gelling agents used in anodes include carboxymethycellulose, polyacrylic acid (e.g., Carbopol 940™ from B. F. Goodrich in Brecksville, Ohio, or POLYGEL-4P™ from 3V in Bergamo, Italy), sodium polyacrylate (e.g., CL-15™ from Allied Colloids in Yorkshire, England), and salts. Non-limiting examples of gassing inhibitors include inorganic additives such as indium, bismuth, tin and lead and organic inhibitors such as phosphate esters and anionic and non-ionic surfactants. See U.S. Pat. Nos. 5,283,139, 5,168,018, 4,939,048, 4,500,614, 3,963,520, 4,963,447, 4,455,358, and 4,195,120 for examples of various anode mixtures.
The gel anode is typically separated from the cathode by a separator, such as a thin layer of non-woven material or paper, that prohibits electronic conduction between the anode and the cathode but allows ions to pass between them.
Alkaline Zn/MnO
2
cells have been commercially available for over 30 years, during which time their performance characteristics have been incrementally optimized by the industry in an attempt to provide the “longest lasting” battery (i.e., one with the greatest overall capacity, measured in ampere-hours) within the volume constraints imposed by the international size standards (e.g., AAAA, AAA, AA, C, D cylindrical and 9 volt prismatic sizes). The volume within such standard cells, into which the active materials are packed, is more or less fixed. The amount of energy available from any given cell size (which is a function of the total amount of the active elements in the cell) has a theoretical upper limit which is defined by the internal cell volume and the practical densities of the active components that are employed.
In addition to trying to produce the “longest-lasting” battery, battery manufacturers are also trying to increase the maximum instantaneous rate of electrical current that can be generated from a battery under a given load without the battery voltage dropping below a minimum value. The motivation for increasing this “maximum discharge rate” capability includes the ongoing development of electronic products, such as cellular phones, which require high currents from small packages. Some of these new devices automatically test the voltage levels of their batteries, and therefore may cause the premature disposal of batteries which have remaining overall capacity, if the sensed voltage dips excessively during a period of high current draw.
When a high current is being drawn from a battery, the voltage of the battery may drop due to zinc-based particle surface “passivation” or anode polarization which can indicate a localized lack of sufficient hydroxide ions to sustain the chemical reaction of the cell. It is believed that a certain amount of porosity is necessary for the free supply of OH- ions coming from the electrolyte and the free disposal of Zn(OH)
4
−
, Zn(OH)
2
or ZnO reaction products back into the electrolyte. If the zinc-based particles are too densely crowded, or if their surfaces are inaccessible due to accumulation of reaction products, the reaction cannot keep up with the rate of current draw. Batteries with densely packed zinc-based particles in their anodes may perform acceptably with very stable voltage levels while supplying low continuous currents, but drop to very low, unacceptable voltages when a high current is drawn due to zinc crowding (sometimes referred to as “choking” or being “electrolyte starved”).
In addition, too little electrolyte can starve the overall chemical reaction of the cell or cause the battery to “dry out”, as water from the electrolyte is continuously consumed during discharge. The net reaction inside the cell is:
Zn+2MnO
2
+H
2
O→ZnO+2MnOOH.
Thus, competing with the desire to pack as much zinc-based material as possible into the available anode volume to increase overall capacity for “long life” is the need to provide a sufficient amount of electrolyte to avoid “choking” during periods of high discharge rate.
SUMMARY OF THE INVENTION
The invention is based upon the discovery that including very small zinc-based particles (i.e., fines or dust) among the zinc-based particles of the anode of an alkaline electrochemical cell can provide good cell performance characteristics, especially those characteristics related to high discharge rate performance.
As used herein, “fines” are particles small enough to pass through a standard 200 mesh screen in a normal sieving operation (i.e., with the sieve shaken by hand). “Dust” consists of particles small enough to pass through a standard 325 mesh screen in a normal sieving operation.
A zinc-based particle can be formed of, for example, zinc or a zinc alloy. Materials that can be alloyed with zinc to form zinc-based particles include gassing inhibitors, such as indium and/or bismuth. Generally, a zinc-based particle formed of a zinc alloy will be mostly zinc. A zinc-based particle can be spun or air blown.
The zinc-based particles can include a plating material, such as indium and/or bismuth.
As used herein, a “zinc-based particle” refers to a singular or primary particle of zinc-based material rather than an agglomeration of more than one particle of zinc-based material. An anode can contain primary particles of zinc-based material and/or agglomerates of primary particles of zinc-based material.
According to one aspect of the invention, a negative electrode for an electrochemical cell contains zinc-based particles suspended in a fluid medium, with at least about 1 percent, by weight, of the zinc-based particles being of −200 mesh size or smaller. Even higher weight percentages (e.g., 6 percent, 10 percent, 25 percent, 50 percent, 80 percent, 90 percent or 100 percent) of zinc-based fines can be preferable.
In some embodiments, the zinc-based particles also include at least about 25 percent, by weight, (e.g., at least about 50 percent, 75 percent, 90 percent or 99 percent) of particles between about 20 and 200 mesh size or larger.
In certain embodiments, it is preferable that a substantial percentage (e.g., 10, 45, 80, 90 or 100 weight percent) of the zinc-based particles are dust (of −325 mesh size or smaller, as defined above). However, in other embodiments, less than 10 weight percent of the zinc-based particles may be of −325 mesh size or smaller (e.g., about 1 weight percent to about 10 weight percent, such as about 6 weight percent).
The negative electrode may include a surfactant. The fluid medium preferably includes both a

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