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Recent Assessment of Metal-Air Systems for
Commercial and Military Applications
Alexander P. Karpinski and Arthur Dobley
Yardney Technical Products, Inc.
2000 South County Trail, East Greenwich, Rhode Island 02818-1530 USA
Tel: 401-471-6580, e-mails [email protected] and [email protected]
Abstract: It can be argued that the genesis for metal-air
started with the work of Leclanche' in the mid to late
1800’s while experimenting with MnO2 electrodes. In the
last 10 years metal-air has evolved to include a number of
alkaline earth metal anodes which electrochemical
equivalent energy densities are well beyond any traditional
primary or rechargeable battery. Although zinc-air and a
lesser extent aluminum-air are the dominant technologies
that serve a number of applications, lithium-air (i.e.,
primary or secondary) is gaining a lot of traction due to its
very high energy density and higher voltage potential.
In the past few years there have been some number of
advancements that are starting to mainstream these
electrochemistries on a variety of applications.
Keywords: metal-air; zinc-air; aluminum-air; lithium-air;
magnesium-air; air cathode; gas diffusion electrode;
oxygen; metal anode; electrolyte; air management
Introduction
Since the 1930s, Metal-air battery systems have been
commercialized for an array of unique applications. The
interest level for metal-air in these applications is
somewhat cyclical based on the newer competing systems
that solve some of the short comings associated with metalair battery technology. Although these shortcomings are
not fully resolved this system is still very attractive due to
its very high specific energy. Theoretical specific energies
are up to 13 kWh/kg (for the Li-oxygen cell). The metal
anodes are electrochemically coupled to oxygen in the air
through the air cathode. Oxygen gas, from the air, is
essentially an unlimited cathode reactant source.
Theoretically with oxygen as an unlimited cathode reactant,
the capacity of the battery is limited by the metal anode.
Anodes commonly investigated are aluminum, lithium,
magnesium and zinc. These would be paired with a specific
air cathode to give aluminum-air, lithium-air, magnesiumair, and zinc-air cells. In addition to their high specific
energies, the metal-air battery usually offers a flat discharge
voltage profile, environmental friendliness and long storage
life. Metal-air systems in hybrid battery systems have also
shown increased performance.
Soldiers require high specific energy battery systems to
power electronics, weapons, and equipment. Metal-air
batteries can power military devices while reducing the
weight to the soldier. Man portable batteries and reserve
power units exist with metal-air batteries.
Several programs at Yardney Technical Products focused
on development of rechargeable and non-rechargeable
batteries of various metal-air chemistries with high
specific energy. Yardney will report efforts about
developing metal-air battery systems. Different designs
were investigated for the metal-air systems. The designs
will be discussed along with the integration and
comparison of the systems. Yardney’s experience and
technologies in the aluminum-air, lithium-air, and zinc-air
power sources were adopted in the designs for the high
specific energy systems. These batteries have the potential
to power electronic equipment, back-up power for
telecommunications, vehicles, robots, sensors, munitions,
or any equipment where air or oxygen is present.
Survey of Metal-Air Couples
Of all battery systems the metal-air system (i.e.,
uncommonly called a semi-fuel cell) is very unique in that
it can be configured many different ways. Some low rate
designs utilize a jelled anode which is typically a mixture
of zinc and a caustic electrolyte coupled with a catalyzed
thin carbon cathode. Some designs can be wound into a
cylindrical celli for higher rate applications. Some designs
are simple prismatic cells where the electrodes are stacked
as parallel elements and use either a static or in a flowing
electrolyte system. Other designs use a hopper system
where zinc pellets are fed into an electrolyte stream and
pumped across the cathode. Finally the last system is a
“dry” system in which three foils consisting of zinc, a solid
electrolyte membrane and an air cathode in the form of a
tape that is fed into a common ply which results into a
semi-fuel cell for an electric vehicle application. All these
systems provide flexibility in packaging depending at the
application. Refer to Table 1 for a cross section of these
applications.
Table 1. Current Applications
Application
Electric
Vehicle
Electric
Vehicle
Electric
Vehicle
Electric
Vehicle
Electric
Vehicle
Portable
military
Portable
military
Grid Storage
Grid Storage
Hearing Aid
Organization
Couple
Electric Fuel
Zn-air
Phinergy
Evionyx
Al-air &
Zn-air
Ni Zn &
Zn-air
System
Static;
cartridges
Flowing
Electrolyte
Alupower/
Yardney
Al-Air
Flowing
Electrolyte
KIST*
Mg-Air
Static
Arotech
Zn-air
Static
Al-air
Static
Alupower/
Yardney
EOS
Technology
Powair
Duracell,
Energizer
Cegasa
(Spain)
Automatic
Power
Portable
Meters
Navigational
aides/telecom
Oxygen
Systech Illinois
sensors
Zn Hybrid Static and
Cathode
flowing
Zn-air
Flow battery
Pasted
Zn-air
anode
Static
Zn-air
Prismatic
Static
Zn-air
Prismatic
Pb-air
Static
Comments
hybrid power
systems
Range
Extender
All electric
hybrid
Range
Extender
Primary
Power
BA-8180/U &
BA-8140/U
2.9kWh
manportable
5 kW
modules
Button Cell
Electrochemi
cal sensor
*Korea Institute of Science and Technology
There are a number of batteries that utilize oxygen in the air
as the cathodic reactant. Table 2ii provides a comparison of
specific energy, energy density, and voltage for a range of
common metals. Of these, four metals have been
commercialized. These include aluminum, lithium,
magnesium, and zinc.
Table 2. Comparisons of Metals
Theoretical
Typical
Electrochemical
Theoretical*
Specific Energy
Cell
Equivalent (Ah/g)
Cell Voltage
(kWh/kg)
Voltage
Be
5.95
17.9
3.0
Li
3.86
13.0
3.4
2.4
Al
2.98
8.1
2.71
1.3
Mg
2.20
6.8
3.09
1.3
Ca
1.34
4.6
3.4
2.0
Cdiii
0.48
0.45
1.28
0.78
Fe
0.96
1.8
1.3
10
Pb
0.26
0.45
2.81
0.05
Ni
0.91
1.1
1.20
Si
3.92
8.5
2.16
1.15**
Zn
0.82
1.1
1.62
1.2
*Voltage with oxygen cathode and pH 14 electrolyte
Metal
**1-ethyl-3-methylimidazolium chloride and hydrogen
fluoride (EMI∙2.3HFF) RTILiv
The metal-air battery cells that use oxygen as the fuel and
can be paired with a variety of anodes. This adds a unique
flexibility to the system whereas there are a number of
anodes that make use of oxygen as the cathodic reactant.
First off, the oxygen can come in many forms such as the
air we breathe, dissolved oxygen in the seawater, chlorate
candles, hydrogen peroxide, or bottled oxygen.
Heat Generation of metal air cellsv was calculated from the
thermoneutral voltage and a typical operating voltage at a
current density of 40 mA/cm2 (with Li-air ~ 1mA/cm2).
Refer to Table 3 which summarizes the ratio of heat
generation to electrical power. This provides a measure to
the inefficiency of the system whereas active thermal
management may be required to prevent shutdown of the
battery during operation. At higher rates the inefficiency
becomes greater.
Table 3. Heat Generation Ratio
Couple
Aluminum-Air
Magnesium-Air
Zinc-Air
Lithium-Air
Thermoneutral
Voltage
2.93
3.11
1.80
3.1
Heat Generation to
Electrical Power
1.15
1.78
0.46
0.24
Lithium-Air: Lithium is the lightest of the metals, has the
highest potential at over 3 volts, and a very high theoretical
specific energy. There are two main types of Li-air
chemistries being aqueous and non-aqueous. The aqueous
cells can exhibit high power densities, but suffer from
safety problems. For the non-aqueous cells the practical
energy densities and especially power densities remain
relatively low. The lithium-air systems is currently being
developed as aqueous and non-aqueous systems
(sometimes both) and a trade-off exists between
performance and safety. There are two main discharge cell
reactions:
4Li + O2  2Li2O
2Li + O2  Li2O2
Zinc-Air: Considered one of the safest commercially
available metal-air systems the primary zinc-air battery is
the most widely used in terms of sales and volume. The
dominating market is hearing aids but include many other
relevant applications such as man-portable power for a host
of military equipment. In addition to these there are so
called boutique applications such as electric vehicles that
leverage the unique forms/configurations of the cell.
Although the principal application is for primary cells, the
zinc-air battery has also been successfully developed as an
electrically or mechanically rechargeable system.
Electrically rechargeable systems have been successful
with the development of bifunctional electrodes using
perovskites. Mechanically rechargeable systems typically
come in the form of replacement cartridges which also
refreshes the electrolyte. The anode can be metallic or in a
mix with the electrolyte.
The overall net reaction can be expressed as follows:
Zn + ½ O2  ZnO
Magnesium-Air:
There has been limited success in
commercializing magnesium-air batteries. The corrosion of the
magnesium is somewhat influenced by the direct reaction with the
electrolyte thus forming magnesium hydroxide and a large amount
of hydrogen. The electrolyte is water with either caustic or sea
salt. The overall net reaction is as follows:
(oxygen). An example of a cathode preparation would
entail two layers of a carbon, binder, and catalyst mixture
being deposited on both sides of a metal current collector
(Figure 1). Carbon mats are impregnated with slurry of
carbon, binder, and catalyst. Two of these impregnated
mats are laminated around a current collector. All air
cathodes would have a microporous PTFE layer added to
the side exposed to the environment. This produces a
double-sided air cathode.
The overall net reaction can be expressed as follows:
PT
Fi FE
lm
CLa
ye
r
density and operating voltage, and relatively easy to fabricate and
operate safely. Although the discharge of aluminum anodes is
irreversible it can be recharged mechanically by replacing the
anode and the electrolyte.
Ni
M
es
h
Aluminum-Air: Aluminum-air batteries exhibit both high energy
CLa
ye
r
Mg + ½O2 + H2O → Mg (OH)2
Figure 1. A diagram of a layered air cathode
4 Al + 6H2O + 3O2 → 4 Al (OH)3
The electrolyte for aluminum-air batteries can be either salt
water or an alkaline. The conductivity of the salt water is
lower than the alkaline with a corresponding energy
reduction as follows:

Saline electrolyte: 0.05 AH/ml

Alkaline electrolyte: 0.4 AH/ml
Although a saline electrolyte is safe, other challenges exist
to achieve optimal performance. In order to minimize the
voltage drop a narrower gap between the electrodes is
required. As a consequence the byproducts of the anodic
dissolution of aluminum reaction accumulate as a
gelatinous product between the anode and cathode. This
can cause anode passivation and the need for additional
water since it binds the electrolyte. Some solutions include
the use of flocculating agents to precipitate the aluminum
hydroxide or the addition of 18% citrate in the NaCl
solution. In either case these changes would impact energy
density.
The use of alkaline electrolyte provides for excellent
coulombic efficiency (e.g., from 60 to 98%) and increased
voltage during discharge (e.g., 500mV). Depending on the
electrolyte molarity the aluminum reaction product can be
managed two ways. The first is to precipitate out the
aluminum hydroxide (e.g., < 6M KOH) or operate as a
solids-free design with a flowing electrolyte system. In
both cases the electrolyte requires the addition of a low
concentration (e.g., <1%) of tin (as a stannate) to control
corrosion of the aluminum anode. The optimal operating
temperature for the flowing electrolyte system is 60° to
80°C.
Cells & Batteries
Metal-air cells come in a variety of physical types, with the
main type for the man portable and reserve power units
being a prismatic or rectangular solid shape. The metal
anode is either a slab or slurry. For separating the
electrodes a physical space filled with electrolyte can be
used or a sheet of separator. An example of a prismatic
lithium-air cell is shown in Figure 2. The window in the
center of the cell is the exposed air cathode that allows air
to enter the cell. This cell was developed for a low rate, 12
volt battery system measuring 5 in. wide by 5 in. high by 5
in. long. Cells like this can be stacked together with
electronics to for a man portable unit like the one shown in
Figure 3 which depicts a 16 volts, 2.9 kWh aluminum air
system for man-portable applicationvi.
Figure 2. A generalized drawing of a prismatic lithium-air
cell
The stoichiometric rate for the air reaction is 0.01 liters per
minute per ampere or 10 cc/min.
Air Electrode: In simplistic terms, the gas diffusion
electrode or air cathode is the site for three phase contact of
a solid (carbons and catalyst), liquid (electrolyte) and gas
Figure 3. A photograph of a 12/24 volt man portable
aluminum-air battery (SOFAL)
Applications
The main commercial application of metal-air cells seen
today is the use of Zn-air cells to power hearing aids. The
majority of metal-air work is in research and development.
Recent interest is in applying metal-air technology to
automotive, grid storage and military applications. The
automotive sector is focusing on Li-air cells as secondary
systems, and also Al-air as primary batteries to be used as
range extenders.
The grid storage area is focusing on many metal-air
systems as both primary and secondary batteries to help
provide peak power. Reserve power units, to be used
during a power outage, are focusing on Al-air and Zn-air.
The reserve power unit systems are primary and secondary.
The military is an early adapter of metal-air systems. They
currently use Zn-air primary batteries due to the high
specific energy. The military is interested in metal-air
systems for man portable power to provide energy to
sensors, communications, electronics, weapons, and
computers. There is also military interest for providing
power to the grid as a backup power sources, or for peak
power demands as reserve power units.
Future Directions
With recent advancements in the battery industry involving
materials, processing, instrumentation, and modeling, the
metal-air systems are posed for a renaissance.
Advancements in carbons, catalysts, and electrodes, will
allow for increased voltages and energy. Newer processes
will allow for lightweight cell and battery designs.
Instrumentation and modeling will help increase
knowledge into the fundamental properties of the metal-air
chemistry and bring forth improvements. Due to the high
theoretical specific energy, the metal-air batteries hold
great promise to fill the energy requirements for military
portable and reserve power.
Conclusions
This is an exciting time for metal-air batteries with their
extremely high theoretical specific energy, and the
worldwide interest in energy storage. A large amount of
effort was dedicated to getting the Zn-air system into the
commercial and military market. Several other metal-air
systems hold the promise of greater performance than the
Zn-air battery and should be investigated further. Metal-air
batteries because of their high energy to weight ratio offer a
great advantage to portable power and reserve power units.
i
A.P. Karpinski et al, Progress on the Development of a
Lightweight, Rechargeable Zinc-Air Battery, 6th Workshop
for Battery Exploratory Development, Williamsburg, VA,
June 1999
ii
Alupower technical booklet
iii
O. Wagner, Cadmium-Air Cell Studies, Research and
Development Technical Report ECOM-3451, July 1971
iv
G. Cohn et al, Silicon-air batteries, Electrochemistry
Communications 11 (2009) 1916-1918
v
K.F. Blurton, H.G. Oswin, Refuelable Batteries,
Energetics Science Inc., NY, Symposium of Non-Fossil
Chemical Fuels, Boston, 1972
vi
A.P. Karpinski et al, The SOFAL Aluminum-air Battery
for Man-Portable Applications, 33rd IECEC, Colorado
Springs, August 1998