Page 272 - Lindens Handbook of Batteries

11.10 pRIMARy BATTERIES

Recent research in developing a zinc powder for high-rate discharge applications has resulted
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in the patenting of a blended zinc powder. This blended powder contains selected portions of two
different particle size powder distributions. Advantageously, it allows battery manufacturers to maxi-
mize an alkaline cell’s performance while minimizing the cost of the zinc.
Anode Gel. The anode gel serves to suspend the zinc particles and to maintain them in contact
with one another. Starch or cellulosic derivatives, polyacrylates, and/or ethylene maleic anhydride
copolymers continue to be used as anode gelling agents. Common gelling agents can include sodium
carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer. Typically the selected gel
is well mixed with the zinc powder and any other additives prior to dispensing into the anode cavity
of the cell. As with the other cell components, these materials must also be of high purity in order
to minimize gassing. This is especially true of the carbonate, chloride, and iron levels. Depending
on the cell’s primary application, the volume fraction range of the gelling agent can vary. The lower
limit is based on maintaining good electronic conductivity in the anode, while the upper limit is
defined by limiting the accumulation of reaction products that could eventually passivate the undis-
charged zinc and hinder ionic diffusion within the anode. A recent patent has suggested the use of
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the crosslinked polymer polyvinylbenzyltri-methylammoniumhydroxide. It is claimed that the use
of this gelling agent allows better high-rate discharge performance.
Anode Collector. The anode collector used in the alkaline cell has typically been a high-purity
cartridge brass, but silicon bronzes have also been used. The collector in most current designs has a
pin or nail shape, but strip type collectors have been used in the past. The collector is part of the col-
lector assembly that also consists of the seal and cover. Once the collector is inserted into the anode
gel, its surface becomes rapidly coated with zinc, thus acting more like a zinc electrode than brass.
This provides good electronic contact with the zinc particles and suppresses gassing in the anode
due to any impurities in the brass that could be gassing promoters. In order to provide a rapid zinc
plating, the brass collector can undergo a special cleaning or surface coating. One such patented
method involves the electroplating of the collector wire with indium that forms an indium-plated
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wire with a thickness of about 0.1 to 10 microns. This coating reduces the amount of gassing
that may occur in the cell, especially in mercury-free alkaline cells, prior to the collector becoming
fully coated with zinc.
Separators. The separator insulates the cathode electronically from the anode. However, it must
be ionically conductive along with being chemically stable in the concentrated alkaline electrolyte
under both oxidizing and reducing conditions while also being strong, flexible, uniform, impurity-
free, and very absorptive. There are many ways to produce such a material, but the more frequently
used material is a nonwoven or felt-like material. The typical separator material can consist of cel-
lulose, vinyl polymers, polyolefins, or any combination. Depending on the battery manufacturer, the
separator can consist of two inserted cross strips or a preformed “convolute” separator basket. Other
types of separators that have not proven as successful include gelled, inorganic, and radiation-grafted
separators. A cellophane separator has also been used typically if there is a concern about zinc
dendrite growth through the separator. A recent patent claimed the use of a reinforced separator that
can withstand the forces applied during manufacture and contain any fragmented electrode particles
formed when the cell is dropped. 11
Containers, Seals, and Finish. The can or external container of the alkaline cell, unlike the carbon-
zinc can, does not take part in the discharge reaction. It is merely an inert container that provides
an external contact for the positive electrode. The can is typically made of mild steel that is thick
enough to maintain its shape during discharge as the cathode is known to expand and hydrogen gas
can form during storage or discharge, creating internal pressure. Over the years, the can thickness
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has been reduced in order to provide more internal space for the active battery materials. The can
is formed by the deep drawing of a steel strip.
The can materials must also be of high purity as the can does contact the cathode. Depending
on the cell construction, the interior contact can be the steel itself or it can be treated to improve