Page 310 - Lindens Handbook of Batteries

13.16 PrImArY BATTErIES

SECTion B
ZinC-air Batteries

13.6 GENERAL CHARACTERISTICS OF BUTTON CELL
ZINC/AIR BATTERIES

A zinc/air battery is remarkable because it contains so little oxygen when it is first made. Unlike
other electrochemical systems that we know as batteries, zinc/air batteries rely on the oxygen avail-
able from the environment to replenish the supply that is used during discharge. The thin layer of
cathode contains just enough oxygen to support the next instant of discharge, and that sets up a gradi-
ent of oxygen concentration—depleted at the site of use, and promoting the diffusion of more oxygen
into the cell, anticipating the next instant of use. While the cathode is constantly being refreshed, it
acts catalytically and is essentially unchanged throughout the life of the battery. The anode is zinc
powder dispersed in an alkaline electrolyte. During discharge, the zinc gives up electrons, and even-
tually zinc oxide is formed, building up amid the undischarged zinc. The capacity of a zinc/air cell
is dictated by the amount of zinc that is available for discharge. On a volumetric basis, zinc/air cells
offer one of the highest energy densities of any commercially available electrochemical system.
Is the zinc/air cell a battery or is it a fuel cell? Of course, the answer is both. The zinc makes
a one-way trip to becoming zinc oxide during discharge, and capacity is limited by the material
contained within the container when the cell is manufactured. On the other side, the zinc/air system
enjoys the fact that oxygen enters the cell only as it is needed. It can do this because the cell is open
to the environment. Unfortunately, this is the reason that zinc/air suffers from some inherent limita-
tions. Since both water vapor and oxygen have similar molecular size, it is hard to allow diffusion
of one without having roughly equivalent diffusion of the other. Gain or loss of water vapor leads to
the most immediate limitations we find in the use of zinc/air cells. If the zinc/air battery is designed
to deliver power quickly, and operate in a stable continuous mode, then it will respond quickly to
changes in the external environment from which the oxygen comes. The alkaline electrolyte needs
to be of a high molarity in order to support high currents, but at this concentration of caustic the
electrolyte readily picks up water in high humidity conditions and will lose it in low humidity. When
water is lost, the rate capability of the cell is impaired, as is its overall life. A gain of water engorges
the cell, and capacity is cut short due to a loss of the available volume for zinc oxide being formed
during discharge. Cells may bulge, and electrolyte may be expressed through the vent holes when
internal pressure forces it through the cathode structure.
Before a zinc/air cell is needed for use, it must be protected from the external environment. It still
needs a little oxygen in order to maintain a good open circuit voltage. The adhesive tab covering the
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vent holes prevents most of the oxygen in the outside air from freely entering the cell. However, a
small amount of air, roughly comparable to a current of a few nanoamperes, is needed to offset the
slow oxidation of the zinc that is always occurring in a zinc/air cell. This is the result of side reactions
that are minimized by excluding contaminants to the greatest extent possible, but never eliminated
completely. If this parasitic oxidation of the zinc is excessive, then the tabbed voltage will drop
because insufficient oxygen is able to replenish the active sites on the cathode. Table 13.4 lists the
major strengths and weaknesses of zinc/air batteries.
The use of zinc/air batteries has been explored in telecommunications, military, and industrial
applications. Backup power and portable power systems are examples of the attempts to apply a high-
rate zinc/air system for use over a limited period of time. Certain commercial and medical uses that
are not intermittent applications have also used zinc/air as a power source. The best use of zinc/air
chemistry is in persistent and repetitive applications. miniature hearing instruments have found zinc/
air batteries to be the ideal power source for almost 30 years, at this writing. Capacity, as compared
with the predecessor mercury oxide cells, was almost twice at the outset, and now stands at over three
times the comparable mercury cells. The desire for cosmetically discrete hearing instruments caused