Magnesium-air batteries with anodic solutions having pH range of 6 to 

Knapp, H. R.

English

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)PIc-m 209/ii
MAGNES II_-AIR BATTERIES
by
Howard R. Knapp
Power Sources Division, Electronic Components Laboratory
UBAECOM, Fort Monmouth, New Jersey _ N 6% m I 0 _ _ _
Among the anodic materials considered for use in metal-air batteries,
magnesium ranks second only to zinc in characteristics deemed suitable for
use in portable power supplies. From a theoretical point of view, magnesium
would appear to rate higher than zinc, having both a higher thermodynamic
potential and a higher calculated coulombic capacity per unit weight (Mg-lO00
A.H./ib vs Zn-372 A.H./lb). However, practical considerations in past studies
have shown that magnesium does not produce its theoretical potential in
•. aqueous solutions and because of undesirable, and to some extent, uncontrollable
. side reactions does not provide its theoretical coulombic capacity either.
For optimum use as a battery anode, magnesium should be used in solutions
having a pH range from 6 _o 9- Below pH 6, hydrogen formation becomes quite
rapid and chemical corrosion consumes the major portion of the anode to the
detriment of the electrochemical coulombic efficiency. Above a pH of 9,
magnesium metal passivates through oxide or hydroxide film formation which
acts as an insulating layer and electrochemical aQtivity is inhibited severely.
There is, of course, no difficulty in obtaining solutions falling
within the proper range of pH. General Electric Company pursued the use
of sodium chloride solutions and achieved some measure of success at
:. temperatures above freezing. The optimized solution of _odium chloride
freezes at approximately 25°F. Prototype batteries were built and operated.
Current levels were not as high as hoped for, and voltage transients pro-
;, duced some difficulties in achieving good voltage regulation h_ the battery.
Excess water requirements also caused greater battery volume and weight than
was originally estimated. These difficulties along with some component
. designs will require further investigation which could not be made under the
, :;_ time stresses of the original program. This work did point out the feasibility
_: of a high rate, high energy density magnesium-air battery producing 50 watt-
hours per lb.
'":* In research studies for UBAECO_, Yardney Electric Company, after a
limited investigation of several salts, settled on use of a 3:2 eutectic of
4 N sodlum-lithium perchlorate solution which was found advantageous in
operation to -15°F. At -15°F, approximately 33% of room temperature capacity
is obtained. Room temperature capacity approaches only 65% coulombic
efficiency of the magnesium anode because of the corrosion resulting from
chemical interaction of magnesium and water. At 125°F this undesirable
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PIC-BAT 209/ii
side reaction increases and only 40% of the nominal capacity is obtained.
Figure i shows the effect of temperature on the cell voltage in the magnesium-
air system using mixed perchlorate electrolyte.
Current densities with the magnesium-air system are not as high as
with the zinc system. Because of the resistlvities of the electrolyte and
the air cathode, and the use of sheet magnesium rather than the hi_hl_
porous structure anode (as used in the zinc system) about 25-30 ma/cmo is
optimum current density for efficient use in the magnesium-air system. Cell
voltages of 1.24-1.28 volts are obtained at current densities of 12 ma/cm2
(80 ma/in2). Figure 2 shows tht effect of current density on the cell
voltage in the magnesium-air system using perchlorate electrolytes.
In battery structures, the need for precise and proper designs is
much more stringent in magnesium-air batteries than in zinc systems. Volume
considerations especially must be viewed judiciously. The interelectrode
spacing requirement, for example, was found to be 3 times the thickness of
the magnesium sheet, i.e., the required interelectrode distance was 170 mils
for a 55-60 rail thick magnesium anode. The large volume requirement is
the result of the need for a much greater quantity of water in the magnesium
system than for the zinc system. There is at least a threefold reason for
the excess water requirement; to accommodate the voluminous reaction product
(Mg(OH)_ evolving frcm both electrochemical activity and from the un-
controllable chemical interaction of magnesium and water, to allow for the
inherent water vapor evaporation through the cathode walls and, of course,
to provide an ionic medium for cell reaction. It is mainly this excess
of water that reduces the energy density of the magnesium-air system.
Electrolyte in the magnesium system amounts to 80-85% of the weight of the
active materials whereas in the zinc system only 30-35% is required for the
electrolyte.
Despite the above difficulties these studies show that magnesium-air
batteries can be produced having energy densities on the order of 50-55 watt-
: hours per lb.
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Magnesium-air batteries

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