Page 314 - Lindens Handbook of Batteries

13.20 PrImArY BATTErIES

too fine in pore size to allow liquid to pass from inside the cell to the outside. In most commercial
zinc/air batteries, this layer is made up of expanded PTFE film.
The next layer is the conductive mix of carbon and catalysts that are wetted with electrolyte. This
creates the conditions for oxygen to be held at activated sites and thus made available for the electro-
chemical reaction which produces hydroxide ions in the electrolyte, and water is consumed from the
electrolyte. To support this reaction, there needs to be a source of electrons. They come in from the
external circuit, driven by the potential of the zinc in the anode. The electronic path for the current is
through the current collector, usually a wire mesh screen or expanded metal layer impressed into the
cathode active material. In the case of most button or coin cells, this is a disk blanked from a larger
sheet which fits snugly into the inside diameter of the cathode can.
Air enters the cathode through vent holes that are punched into the flat bottom of the can. The
access of air is controlled by a combination of size and number of the vent holes, as well as the
degree to which the porosity of the PTFE sheet adjacent to the cathode material has been occluded
or compressed during manufacture of that subassembly. Depending on the intended use of the cell,
and the electrical current needed to meet that use, the parameters of vent size, their number, and the
cathode rate capability might all be increased or decreased to suit the application.
On the anode side of the cathode structure are the separator/barrier layers, wettable to the electro-
lyte, but capable of preventing zinc or zinc oxide from directly contacting the cathode. If this were
to happen, a direct electron path would be established, and the cell would self-discharge. The bar-
rier is most commonly a microporous layer of polymer film that does not break down in the caustic
environment of the cell and remains a good ionic conductor throughout discharge. The separator is
most commonly a nonwoven cellulosic that is highly absorbent to electrolyte and helps to prevent
zinc dendrite shorting. If the separator/barrier system interferes with the conductivity of the cell, then
the rate capability, the capacity of the cell, or both will be adversely affected.
The negative top of the button cell contains all of the zinc and the majority of the electrolyte. Only
a small portion of the electrolyte wets into the cathode, stopping at the hydrophobic layer. In order
to allow for the growth of the anode, it is not possible to initially fill the entire top. Zinc metal has a
density of approximately 7.14 g/cc, whereas the density of ZnO is about 5.47 g/cc. Zinc going to zinc
oxide gains in mass by a factor of 1.25, with a corresponding volume increase due to the decrease in
density of 1.63 g/cc. Space for that growth must be accounted for in the initial fill of anode material
into the cell. Under equilibrium conditions, the electrolyte mass does not change during discharge,
and occupies essentially the same volume at the end of discharge as it did when first assembled.
High-purity zinc is the active constituent of the anode. It is normally a distribution of finally divid-
ed, atomized particles. In some instances, it is alloyed to reduce the tendency for catalytic corrosion
of the metal. Historically, as in virtually all button cells, it was also amalgamated with a small amount
of mercury typically less than 25 mg/cell to reduce hydrogen overpotential, largely due to the segre-
gation of trace metals to the grain boundaries of the zinc during the solidification that takes place as
the zinc is atomized. mercury would preferentially concentrate at the grain boundaries. Insoluble in the
zinc oxide, the mercury would further concentrate into the remaining zinc, and eventually be released
as beads of liquid metal suspended in the discharged anode. The impending introduction in 2011 of
zero mercury zinc/air cells will eliminate the use of mercury in most commercial products.
The electrolyte used in zinc/air cells is normally a caustic solution of KOH at a concentration
of about 30%. The electrolyte is highly conductive and wets through the cathode structure readily,
providing excellent ionic access to both zinc and cathode active sites. At this concentration, the elec-
trolyte is at equilibrium with water vapor at roughly 50% rH for ambient temperatures. Performance
of the zinc/air system will be compromised if the rate of ingress or egress of water is significant.
Gain of water will dilute the electrolyte slightly, but more significant is the increase in the volume
of the anode and the reduction of the space that was initially left for the discharge product. The gain
in water does not have a large effect on rate capability, but capacity of the battery will be cut short
as the internal materials push the cathode against the inside bottom of the cell and reduce the effec-
tive cathode active area. When the environmental conditions promote drying out of the cell, there
is a more immediate detrimental effect on the battery’s electrical performance. Good ionic contact
within the cell may be affected as gaps develop at the electrode interface and sufficient water is not
available in the cathode to support the destruction of peroxide and subsequent hydroxyl formation.