Zinc-air secondary batteries with inorganic separator

Sulkes, M. J.

English

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ZlNO-AIR BATTERIES
by ¢J9- 1 028Martin Sulkes
Power Sources Division, Electronic Components Laboratory
UBAECOM, Fort Monmouth, New Jersey
Good results have been obtained to date with zinc as the anode in
primary zinc-alr or zinc oxygen batteries. Zinc electrodes in thicknesses
up to i/4", are operating at 60% or better of theoretical efficiency with
regularity. These batteries yield energy densities in the order of 80-150
/Ib.
Either noble or non-noble metal catalyzed air or oxygen electrodes may
be used for primary batteries. In general, Pt is used as the catalyst in
"refuelable" batteries, while carbon, nickel, silver, and silver-amalgam
are used for one-shot types. The general mode of operation for the one-
shot battery is to add water to activate. However, it is more desirable
to store the battery wet and activate by admitting air. For an air activated
battery the storageability of the zinc electrode as well as the wetting
characteristics of the air electrodes become important considerations.
Zinc electrodes must be heavily amalgamated to prevent self discharge
and the evolution of hydrogen. The recrystallization properties of zinc
and zinc oxide must be taken into account since crystal growth during long
term stand, particularS7 at elevated temperatures, can partially passivate
the zinc electrode. Therefore zinc electrode characteristics must be
determined and co_trolled before activated storage zinc-air batteries are
c_mpletely feasible.
In order to use the zinc-air system as a secondary battery one basic
characteristic of zinc must be overcome. This is the fact that zinc, unlike
other secondary battery electrodes such as Ni, Ag, Fe, and Cd, has a
relatively high solubility in alkaline electrolyte. With oxidized zinc
species being soluble. In addition, zinc tends to supersaturate the
electrolyte during anodization to a value approximately 2 times the
equilibrium value.
The high zinc so%ubility leads directly to such failure causing
mechanisms, as zinc electrode shape change, dendrite growth and separator
penetration, and the precipitation of zinc oxides in the air electrode.
"Shape change" _s the loss of geometric surface area due to sluffing
off and redistribution of zinc with cycling. This leads to the densification
of the zinc metal formed on charge at the surface of the electrode, thus
preventing full _tilization of the remaining zinc oxide. Shape change
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therefore results in a gradual loss of capacity and voltage with cycling
until such time as the limiting current density is exceeded and then capacity
rapidly falls below minimum levels.
Shape change tends tQ be more severe in secondary zinc-air cells than
in Ag-Zn. This occurs because in the air system the zinc electrode is
limiting on both charge and discharge. This causes the zinc to be fully
worked each cycle rather than only i/3 to i/2 utilized as is the case in
Ag-Zn. Therefore the shape maintenance effect of reserve zinc oxide is
not available in Zn-air batteries.
Steps that can be taken to minimize shape change include the addition
of binders. Particularly good results have been obtained with Teflon, as
developed by the Yardney Electric Company (see reports on Contract DAABO7-
67-C-0185). Minimization of the amount of amalgamation has also reduced
shape change, although some Hg is requir_.d to prevent self discharge. Hg
contents of the order of 0.5-i.0% appear optimum.
Another development from the Ag-Zn system that should prove useful in
reducing shape change is the use of zinc electrodes having a larger geometric
area than the positive electrodes. These oversized zinc electrodes appear
to be effective in reducing edge effects which are points of higher potential
and faster dissolution. Based on results obtained in silver-zinc cells
it would appear that shape change can be minimized sufficiently to provide
a life of more than lOO cycles, with BOO cycles a distinct possibility.
However, even if shape change is controlled the problem of separator
penetration by zinc dendrite growth must be overcome if adequate cycle life
is to be maintained. To date most of the conventional film type separators
tested have not proved adequate. These include cellophane, fibrous sausage
casing, poly (vinyl methyl ether/maleic anhydride) in a methyl cellulose base,
and radiation grafted polyethylene. These materials have all been subject
to oxidative attack by nascent oxygen or have high resistance which accentuates
shape change. However, inorganic separatol materials as developed by Douglas
and more recently by other companies have _-nownpromise as non-oxidative
materials that are capable of preventing zinc penetration. Over 50 deep
cycles have been obtained to date with these materials without failure.
Zinc dendrite growth may also be minimized by limiting overpotential
on charge. Studies by McBreen have indicated that 70 mV overpotential above
rest potential is sufficient to cause dendrite growth. Fortunately, zinc
is a highly active electrode and does not reach this level except at the end
of charge.
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One failure mode that is unique to the Zn-air system is zinc oxide
precipitation in and on the air electrode. The figure shows the solubility
of zinc as a function of KOH concentration. Precipitation 'can occur at the
end of charge when the electrolyte in the air electrode compartment is
dilute from the generation of water. However, it is more likely to occur
at the end of discharge when the electrolyte is very concentrated and large
amounts of soluble zlnd "are availaBleo The precipitation of zinc oxide in
the air electrode is more likely to occur if a random cycling regime with
long stand periods is followed rather than on rapid automatic cycling since
rapid cycling tends to minimize the swings in electrolyte concentration.
This leads to a critical current density for zinc-air batteries above which
zinc oxide precipitation occurs, changing the wetting characteristics of the
air electrode. This current density value may be considerably lower than the
polarization behavior would indicate. However, this critical value is
dependent On such factors as separator type, cycling conditions, and
temperature, etc.
The air electrode for secondary zinc-air batteries appears to require
the use of noble metal catalysts such as Pt, if auxiliary charging electrodes
are to be avoided. This is desirable since it is calculated that, for a
fixed volume, energy density will be reduced by approximately 1/3 if an
extra charging grid is used.
To summarize, Zn-air secondary batteries appear capable of energy
densities of 75-i00 watt-hours/ib and can operate for i00 or more cycles
using separators of the inorganic type. Much more work is required to
optimize the air-zinc system and to determine its l_miting use conditions.
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Zinc-air batteries

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