<s>
A	O
metal	B-Architecture
–	I-Architecture
air	I-Architecture
electrochemical	I-Architecture
cell	I-Architecture
is	O
an	O
electrochemical	O
cell	O
that	O
uses	O
an	O
anode	O
made	O
from	O
pure	O
metal	O
and	O
an	O
external	O
cathode	O
of	O
ambient	O
air	O
,	O
typically	O
with	O
an	O
aqueous	O
or	O
aprotic	O
electrolyte	O
.	O
</s>
<s>
During	O
discharging	O
of	O
a	O
metal	B-Architecture
–	I-Architecture
air	I-Architecture
electrochemical	I-Architecture
cell	I-Architecture
,	O
a	O
reduction	O
reaction	O
occurs	O
in	O
the	O
ambient	O
air	O
cathode	O
while	O
the	O
metal	O
anode	O
is	O
oxidized	O
.	O
</s>
<s>
The	O
specific	O
capacity	O
and	O
energy	O
density	O
of	O
metal	B-Architecture
–	I-Architecture
air	I-Architecture
electrochemical	I-Architecture
cells	I-Architecture
is	O
higher	O
than	O
that	O
of	O
lithium-ion	O
batteries	O
,	O
making	O
them	O
a	O
prime	O
candidate	O
for	O
use	O
in	O
electric	O
vehicles	O
.	O
</s>
<s>
While	O
there	O
are	O
some	O
commercial	O
applications	O
,	O
complications	O
associated	O
with	O
the	O
metal	O
anodes	O
,	O
catalysts	O
,	O
and	O
electrolytes	O
have	O
hindered	O
development	O
and	O
implementation	O
of	O
metal	B-Architecture
–	I-Architecture
air	I-Architecture
batteries	I-Architecture
.	O
</s>
<s>
The	O
remarkably	O
high	O
energy	O
density	O
of	O
lithium	B-Language
metal	I-Language
(	O
up	O
to	O
3458	O
Wh/kg	O
)	O
inspired	O
the	O
design	O
of	O
lithium	B-Language
–	O
air	O
batteries	O
.	O
</s>
<s>
A	O
lithium	B-Language
–	O
air	O
battery	O
consists	O
of	O
a	O
solid	O
lithium	B-Language
electrode	O
,	O
an	O
electrolyte	O
surrounding	O
this	O
electrode	O
,	O
and	O
an	O
ambient	O
air	O
electrode	O
containing	O
oxygen	O
.	O
</s>
<s>
Current	O
lithium	B-Language
–	O
air	O
batteries	O
can	O
be	O
divided	O
into	O
four	O
subcategories	O
based	O
on	O
the	O
electrolyte	O
used	O
and	O
the	O
subsequent	O
electrochemical	O
cell	O
architecture	O
.	O
</s>
<s>
Nonetheless	O
,	O
efficiency	O
of	O
lithium	B-Language
–	O
air	O
batteries	O
is	O
still	O
limited	O
by	O
incomplete	O
discharge	O
at	O
the	O
cathode	O
,	O
charging	O
overpotential	O
exceeding	O
discharge	O
overpotential	O
,	O
and	O
component	O
stability	O
.	O
</s>
<s>
During	O
discharge	O
of	O
lithium	B-Language
–	O
air	O
batteries	O
,	O
the	O
superoxide	O
ion	O
(	O
O	O
)	O
formed	O
will	O
react	O
with	O
the	O
electrolyte	O
or	O
other	O
cell	O
components	O
and	O
will	O
prevent	O
the	O
battery	O
from	O
being	O
rechargeable	B-Architecture
.	O
</s>
<s>
Sodium	O
–	O
air	O
batteries	O
were	O
proposed	O
with	O
the	O
hopes	O
of	O
overcoming	O
the	O
battery	O
instability	O
associated	O
with	O
superoxide	O
in	O
lithium	B-Language
–	O
air	O
batteries	O
.	O
</s>
<s>
Sodium	O
,	O
with	O
an	O
energy	O
density	O
of	O
1605	O
Wh/kg	O
,	O
does	O
not	O
boast	O
as	O
high	O
an	O
energy	O
density	O
as	O
lithium	B-Language
.	O
</s>
<s>
Since	O
NaO	O
will	O
decompose	O
reversibly	O
to	O
an	O
extent	O
back	O
to	O
the	O
elemental	O
components	O
,	O
this	O
means	O
sodium	O
–	O
air	O
batteries	O
have	O
some	O
intrinsic	O
capacity	O
to	O
be	O
rechargeable	B-Architecture
.	O
</s>
<s>
Potassium	O
–	O
air	O
batteries	O
were	O
also	O
proposed	O
with	O
the	O
hopes	O
of	O
overcoming	O
the	O
battery	O
instability	O
associated	O
with	O
superoxide	O
in	O
lithium	B-Language
–	O
air	O
batteries	O
.	O
</s>
<s>
The	O
homogeneous	B-General_Concept
deposition	O
of	O
Mg	O
metal	O
makes	O
Mg	O
–	O
air	O
systems	O
interesting	O
.	O
</s>
<s>
However	O
,	O
the	O
cell	O
's	O
reversibility	O
is	O
limited	O
,	O
and	O
the	O
especially	O
visible	O
during	O
recharging	B-Architecture
.	O
</s>
<s>
Aluminium	O
battery	O
cells	O
are	O
not	O
rechargeable	B-Architecture
,	O
so	O
new	O
aluminium	O
anodes	O
must	O
be	O
installed	O
to	O
continue	O
getting	O
power	O
from	O
the	O
battery	O
,	O
which	O
makes	O
them	O
expensive	O
to	O
use	O
and	O
limited	O
to	O
mostly	O
military	O
applications	O
.	O
</s>
<s>
Aluminium	O
–	O
air	O
batteries	O
are	O
better	O
for	O
the	O
environment	O
compared	O
to	O
traditional	O
lithium-ion	O
batteries	O
.	O
</s>
<s>
Aluminium	O
is	O
the	O
most	O
abundant	O
metal	O
in	O
the	O
Earth	O
's	O
crust	O
,	O
so	O
mines	O
would	O
not	O
have	O
to	O
be	O
as	O
invasive	O
to	O
find	O
a	O
similar	O
amount	O
of	O
aluminium	O
compared	O
to	O
lithium	B-Language
.	O
</s>
<s>
Another	O
factor	O
is	O
that	O
aluminium	O
recycling	O
plants	O
already	O
exist	O
,	O
while	O
lithium	B-Language
recycling	O
plants	O
are	O
just	O
starting	O
to	O
emerge	O
and	O
become	O
profitable	O
.	O
</s>
<s>
Iron	O
–	O
air	O
rechargeable	B-Architecture
batteries	I-Architecture
are	O
an	O
attractive	O
technology	O
with	O
the	O
potential	O
of	O
grid-scale	O
energy	O
storage	O
.	O
</s>
<s>
In	O
conjunction	O
with	O
a	O
fuel	O
cell	O
,	O
this	O
enables	O
the	O
system	O
to	O
behave	O
as	O
a	O
rechargeable	B-Architecture
battery	I-Architecture
,	O
creating	O
H2O/H2	O
via	O
the	O
production	O
and	O
consumption	O
of	O
electricity	O
.	O
</s>
<s>
The	O
combination	O
of	O
both	O
of	O
these	O
cycles	O
is	O
what	O
makes	O
the	O
system	O
operate	O
as	O
an	O
iron	O
–	O
air	O
rechargeable	B-Architecture
battery	I-Architecture
.	O
</s>
