A Critical Review of Li/Air Batteries

Transcription

Journal of The Electrochemical Society, 159 (2) R1-R30 (2012)
0013-4651/2012/159(2)/R1/30/$28.00 © The Electrochemical Society
R1
A Critical Review of Li/Air Batteries
Jake Christensen,a,∗,z Paul Albertus,a,∗ Roel S. Sanchez-Carrera,b,∗ Timm Lohmann,a
Boris Kozinsky,b Ralf Liedtke,c Jasim Ahmed,a and Aleksandar Kojica
a Robert
b Robert
c Robert
Bosch LLC, Research and Technology Center, Palo Alto, California 94304, USA
Bosch LLC, Research and Technology Center, Cambridge, Massachusetts 02142, USA
Bosch GmbH, Gerlingen-Schillerhöhe, Baden-Wuerttemberg 70839, Germany
Lithium/air batteries, based on their high theoretical specific energy, are an extremely attractive technology for electrical energy
storage that could make long-range electric vehicles widely affordable. However, the impact of this technology has so far fallen short
of its potential due to several daunting challenges. In nonaqueous Li/air cells, reversible chemistry with a high current efficiency
over several cycles has not yet been established, and the deposition of an electrically resistive discharge product appears to limit
the capacity. Aqueous cells require water-stable lithium-protection membranes that tend to be thick, heavy, and highly resistive.
Both types of cell suffer from poor oxygen redox kinetics at the positive electrode and deleterious volume and morphology changes
at the negative electrode. Closed Li/air systems that include oxygen storage are much larger and heavier than open systems, but
so far oxygen- and OH− -selective membranes are not effective in preventing contamination of cells. In this review we discuss the
most critical challenges to developing robust, high-energy Li/air batteries and suggest future research directions to understand and
overcome these challenges. We predict that Li/air batteries will primarily remain a research topic for the next several years. However,
if the fundamental challenges can be met, the Li/air battery has the potential to significantly surpass the energy storage capability of
today’s Li-ion batteries.
© 2011 The Electrochemical Society. [DOI: 10.1149/2.086202jes] All rights reserved.
Manuscript submitted April 15, 2011; revised manuscript received October 24, 2011. Published December 29, 2011. This article
was reviewed by Kuzhikalail Abraham (kmabraham@comcast.net) and Yang Shao-Horn (shaohorn@mit.edu).
The Li/air cell has received significant interest in the past several
years as researchers look at couples that may achieve a specific energy
significantly higher than current lithium-ion cells with two intercalation electrodes (e.g., C6 /LiMO2 , where “M” refers to a transition
metal such as Ni, Mn, or Co). The main application driving interest
is transportation, where specific energy and energy densityd are most
important, although applications in portable electronics and grid energy storage are also of interest. Of particular interest in the context of
transportation is the fact that, with the specific energy and energy density of today’s automotive Li-ion cells, one’s driving range is limited
to about 70 miles for a 200 kg pack, as shown in Figure 1. We assume
a specific energy of 150 Wh/kg at the cell level and 105 Wh/kg at the
pack level (70% of a pack’s weight is the cells). This means that in order to enable electric vehicles with a range similar to today’s vehicles
powered by liquid fuels, a battery system with a specific energy and
energy density much higher than today’s state-of-the-art is required.
Indeed, while some observers predict that Li-ion cells may eventually
reach 400 Wh/kg through the use of high-capacity cathode materials
(275 mAh/g) and alloy anode materials (2000 mAh/g), significantly
higher values can only be obtained with even higher capacity cathodes,
Li metal, and improved packaging of active materials. A Li/air battery
has the potential to truly surpass the battery technology used today,
as well as that under development for deployment in the medium
term (i.e., that which may achieve 400 Wh/kg). A cell-level specific
energy value for a Li/air cell remains uncertain, but our cell energy
calculations show that 1000 Wh/kg or more should be attainable if
∗ Electrochemical Society Active Member.
z
E-mail: jake.christensen@us.bosch.com
d By “specific energy” we mean energy per unit mass, and by “energy density” we
mean energy per unit volume. Some authors instead use the terms “gravimetric energy
density” and “volumetric energy density,” respectively.
several fundamental challenges can be overcome (Figure 3 and the
associated discussion address the issue of practical specific energy in
detail). This specific energy could enable an electric driving range of
more than 380 miles on a single charge at the beginning of a battery’s
life, a value approaching that of a gasoline-powered vehicle. Additionally, the cost of a system that achieves today’s driving range may
be reduced significantly with a much higher specific-energy battery.
This could bring electric vehicles to the mass market, since consumer
demand is expected to be driven strongly by vehicle cost reduction,
provided adequate range is available.
There are many possible reactions involving Li and air.— From
the outset it is important to realize that several chemical products may
result from the reaction of Li with O2 , depending on the chemical environment and mode of operation. The main distinction is whether the
medium in which Li is combined with O2 is aqueous or nonaqueous,
and this distinction will be noted throughout this review. Some researchers have also explored hybrid aqueous/nonaqueous Li/air cells,
a concept in which a Li-conducting ceramic is used to separate one
compartment with a nonaqueous electrolyte containing Li metal and
another compartment with an aqueous electrolyte.1
It is important to note that throughout this review we refer to
aprotic solvents as “nonaqueous,” which is not particularly precise but
is consistent with much of the Li/air literature. We shall not explicitly
consider protic nonaqueous Li/air chemistry, as published work is
lacking in this area, although it is expected that it shares many general
characteristics with aqueous Li/air chemistry. Likewise, mixtures of
water and other protic solvents are outside the scope of this review.
We also note that while many researchers refer to Li/air batteries, in
fact most of the laboratory work has focused on Li/oxygen batteries,
as components in air such as H2 O and CO2 can interfere with the
desired electrochemical behavior.
Downloaded 16 Jan 2012 to 209.221.240.193. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp
R2
may form as a film on the surface of Li metal.8 An example of the
reaction of Li with O2 in a mildly acidic environment is the formation
of LiCl:9
Driving range (miles)
500
1,000 Wh/kg,
future Li/air
400
300
2 Li + 1/2 O2 + 2 NH4 Cl ↔ 2 LiCl + 2 NH3 + H2 O.
400 Wh/kg,
future Li-ion
200
100
200 Wh/kg, current
state of the art Li-ion
0
0
100
200
300
400
500
Battery pack weight (kg)
Figure 1. Driving range and battery weight for different cell-level specific
energy values. It is assumed the battery cells weigh 70% of the battery pack,
the Li/air cell has an 83% energy efficiency, the Li-on cells have a 93% energy
efficiency, and 300 Wh/mile are required from the battery. The range is given
at the beginning of a battery’s life and assumes 100% of the capacity can be
used; in practice not all the energy can be used, and the available energy falls
with increasing battery age. The US Department of Energy has a goal for an
EV battery of 200 kg.176
In nonaqueous Li/air batteries there are two principal electrode
reactions of interest:
2 Li + 1/2 O2 ↔ Li2 O,
[1]
and
2 Li + O2 ↔ Li2 O2 .
[2]
In the absence of practical considerations the full reduction of O2
to Li2 O is desired because of its higher specific energy and energy
density, but it appears that Li2 O2 is a product that forms more readily
than Li2 O.2–4 In addition, when Li2 O2 is formed full cleavage of the
O-O bond may not be necessary, which is important from a kinetic
point of view.5, 6 These reactions will be discussed in more detail in
later sections.
Reactions involving Li and O2 in an aqueous medium depend on
the pH. In a basic aqueous environment O2 reduction includes H2 O
as a reactant and results in the formation of LiOH:
2 Li + 1/2 O2 + H2 O ↔ 2 LiOH.
[3]
The product of this reaction is aqueous LiOH, which has a solubility limit of about 5.25 M at standard temperature and pressure.7 If
LiOH exceeds its solubility limit it will precipitate out of the solution
as a monohydrate, LiOH · H2 O, rather than LiOH.7 This is a critical
point for calculating the specific energy and energy density of aqueous
Li/air cells. We are presently unaware of any solvent system that leads
to the precipitation of LiOH rather than LiOH · H2 O, although LiOH
[4]
Another example of a reaction in an acidic solution is the formation
of Li2 SO4 from Li, O2 , and H2 SO4 .9 Although acidic solutions have
the advantage that carbonates are not formed as in basic solutions (e.g.,
the formation of K2 CO3 in KOH solutions), in the present review we
exclude from detailed consideration aqueous reactions involving Li
and O2 in acidic media because the examples known to the authors
have a lower specific energy than in basic media (e.g., reaction 4 has
a lower specific energy than reaction 3). Neutral solutions have also
been discussed, and we exclude them from consideration here for
the same reason.10 There may also be other chemical considerations
for a given reaction, such as the fact that in reaction 4 NH3 has a
high vapor pressure, limiting the reversibility of an open cell due to
evaporation. We also note that if the only criterion for a “Li/air” cell is
a reaction that includes Li and O2 , there are additional reactions that
fall into this broad classification. However, in this review we focus
our attention on the reactions that have thus far received the most
attention, 1–3. We also exclude from consideration the somewhat
related reactions between Li and H2 O (e.g., the “seawater battery”
developed by PolyPlus Inc.), because O2 is not a reactant and H2 gas
is evolved, significantly limiting the possibility of creating a secondary
system.
What, then, are the specific energy and energy density values for
the Li/air cells included in our analysis? We approach this question
by first looking at the active materials alone and then including the
components of a typical cell sandwich.
Energy estimates for the active materials alone.— Calculations
of the specific energy and energy density based on the weight of
the active materials alone provide a benchmark for values that can
be obtained by practical cells, although a practical cell should not
be expected to achieve more than about half of the energy per mass
or volume of the active materials alone. “Active materials” refers to
Li, O2 , and H2 O in the charged state, and Li2 O, Li2 O2 , LiOH, and
LiOH · H2 O in the discharged state. As mentioned above, to the best
of our knowledge LiOH · H2 O and not LiOH will precipitate from an
aqueous solution, so its inclusion here is for the sake of comparison
only. In Table I we summarize the physical properties of the Li/air
active materials in the discharged state we consider in this review, as
well as a current and “next generation” Li-ion intercalation material
for comparison.11 Figure 2 shows specific energy and energy density
numbers based on active materials alone (i.e., excluding the mass or
volume of all cell components besides the active materials defined
above); in the charged state the weight of O2 is excluded. For the
LiMO2 material we assume a specific capacity of 275 mAh/g and
a density of 4.25 g/cm3 , values appropriate for an advanced oxide
material.12 Energy calculations for an open system like Li/air are
different from those for other battery systems that are closed to the
external environment because the mass of the battery increases during
Table I. Physical properties of select Li/air and Li-ion materials positive-electrode active materials in the discharged state, as well as Li metal.13
Active
material
Specific
capacity
(mAh/g)
Li2 O
Li2 O2
LiOH · H2 O
LiOH
LiMO2 , M = Mn, Ni, Co
LiFePO4
Li metal
1794
1168
639
1119
275
170
3861
Density
(g/cm3 )
2.01
2.31
1.51
1.46
4.25
3.6
0.534
Capacity
density
(mAh/cm3 )
3606
2698
965
1634
1169
612
2062
Uθ vs. Li
metal
(V)
Theoretical specific
energy (vs. Li metal)
(kWh/kg)
Theoretical energy
density (vs. Li metal)
(kWh/L)
2.91
2.96
3.45
3.45
3.75
3.42
0.0
5.22
3.46
2.20
3.86
1.03
0.58
10.49
7.99
3.33
5.60
4.36
2.09
14
R3
14
(a)
12
10
Charged
Charged w/ H2O weight
Discharged
8
6
4
2
0
Active-only energy density (kWh/L)
Active-only specific energy (kWh/kg)
(b)
12
10
Figure 2. (a) specific energy and (b) energy
density values based on active materials alone
for selected Li/air active materials, and an insertion reaction for comparison.
8
6
4
2
0
Li2O
Li2O2 LiOH·H2O LiOH Li/LiMO2
Li2O
the discharge process (assuming oxygen is not carried on board).
For that reason, the specific energy and energy density numbers are
shown in Figure 2 for both the fully charged and the fully discharged
states. As Figure 2a shows, a cell producing Li2 O or Li2 O2 has about
the same specific energy in the charged state, but in the discharged
state the Li2 O2 cell has a lower energy per mass due to only one O2
molecule being consumed per 2 Li atoms, rather than 1/2 O2 in the
case of Li2 O. The specific energy calculations for the LiOH · H2 O and
LiOH systems in the charged state depend on the assumption about
where the H2 O in the discharge product originates. In Figure 2a we
show one case in which the H2 O is provided by an external source so
that only the weight of Li metal is included in the charged state, and
another case in which the H2 O in the product is stored in the charged
cell along with the Li (e.g., in a water reservoir). The main assumption
for the LiOH and LiOH · H2 O values is that of a single equilibrium
potential. In practice the cell potential will vary with the activity of
the reactants and products during the cycling process, but we use
only the standard cell potential here. Compared with a Li/LiMO2 cell,
all four Li/air discharge products have a significantly higher specific
energy.
While specific energy is important, energy density can be just as
important in automotive and other applications. Figure 2b shows the
energy density based on the active materials alone. The higher energy
density of the discharged cells than charged cells is partly a result of
the low density of Li metal (0.534 g/cm3 ). The figure shows that the
energy density of a discharged Li/LiMO2 cell is higher than that of an
aqueous Li/air cell, and within about a factor of two of a nonaqueous
Li/air cell. Thus, the advantages of Li/air cells from a specific energy
point of view are more dramatic than from an energy density point of
view because of the relatively low density of Li/air active materials
compared to metal oxide intercalation materials.
Energy estimates for practical cells.— Energy estimates based
on the weight and volume of active materials alone should be followed with energy estimates for practical cells. Such estimates for
Li/air cells are uncertain due to the absence of well developed cell
designs. However, in this section we make assumptions about possible cell designs in order to arrive at initial estimates for practical
cells that can later be refined. We focus on an optimistic practical cell
design that will require additional materials development rather than
only looking at cell components that are available today. For example,
we assume that an ionically conductive lithium metal protection layer
with a thickness of 50 μm and density of 3.0 g/cm3 will be developed
that can be manufactured and used in practical cells.
Our assumptions are summarized in Table II. We assume the Li/air
charged cells have a significant volume fraction of gas in the positive
electrode (70%) while in the discharged state a small amount of gas
remains (5%). A gas phase provides volume into which the solid
active materials can be deposited and provides good transport of O2
into and out of the cell. We assume the cell contains 20% excess
Li relative to the capacity obtained by filling 65% of the positive
electrode volume with discharge product. While some authors have
suggested using a large excess of Li (100 to 300%), we consider
this impractical, as it implies the tolerance of a significant degree of
parasitic reactions, likely involving electrolyte decomposition, over
the lifetime of the battery. The products generated by such significant
parasitic reactions would likely impair cell performance well before
all of the excess Li was consumed. Assumptions about the source of
H2 O for the aqueous Li/air cells are very important. If humidity from
Table II. Cell and tank properties for practical cell energy
calculations. Thickness values are at full charge while volume
fraction values are at the end of discharge. ε values indicate
volume fractions, LPSL = Lithium protection separator layer.
GDL = gas diffusion layer. CC = current collector. The practical
cell energies shown here are nominal, that is they do not include
the practical energy efficiency.
Property
Value
Units
Lpositive
LLPSL
LGDL, positive
LCC, negative
LCC, positive
Amount of excess Li for all cells
εactive material, pos at the end
of discharge
εelectrolyte, pos, Li/air cells
εelectrolyte, pos, Li/LiMO2 cell
εgas phase, pos at the end of discharge
εinerts, pos (carbon)
εGDL, pos
Mass packing factor based
on charged cell w/o tank
Volume packing factor based
on charged cell w/o tank
ρLiOH (aq) elyte. (saturated)
ρLiPF6 in PC elyte.
ρinert (carbon)
ρLPSL
ρGDL, positive
ρCC, negative
ρCC, positive
Oxygen volume in tank
Battery pack energy from which
oxygen pressure is calculated
Tensile strength of tank
(Stainless steel)
Tank safety factor (additional wall
thickness beyond tensile strength)
Additional mass and volume above
that of the tank for tank components
200
50
50
5
7.5
20
0.65
μm
μm
μm
μm
μm
%
0.20
0.25
0.05
0.10
70
80
%
%
70
%
1.105
1.2
2.2
3.0
1.8
8.9
2.7
75
140
g/cm3
g/cm3
g/cm3
g/cm3
g/cm3
g/cm3
g/cm3
L
kWh
460
MPa
50
%
20
%
R4
2.5
(a)
Charged
Charged w/ tank
Discharged
Discharged w/ tank
2.0
1.5
1.0
0.5
0.0
Practical energy density (kWh/L)
Practical specific energy (kWh/kg)
2.5
(b)
2.0
Figure 3. Practical (a) specific energy and
(b) energy density values for selected Li/air
active materials, and an insertion reaction
for comparison. The energy of soluble LiOH
in the electrolyte phase is excluded for the
LiOH · H2 O and LiOH cases as it would lead
to an increase of less than 5% for this cell
design.
1.5
1.0
0.5
0.0
Li2O
Li2O
the external air can be used to supply some of the H2 O required by
reaction 3 and the LiOH · H2 O precipitate, that significantly reduces
the weight and volume basis in the charged state. However, adding
a system to capture water may add complexity and cost, and we
therefore use a design in our calculations that would either involve
filling a water reservoir (like filling current cars with fuel) or having a
water-impermeable membrane that completely prevents water ingress
and egress. Therefore, in our practical cell numbers for the charged
state we include the weight and volume of water present in LiOH
and LiOH · H2 O. These assumptions can be revised as practical cell
designs are more fully developed.
Energy results for “practical” cell designs are shown in
Figure 3, with results for systems with and without an oxygen tank
shown. We exclude the energy content of soluble LiOH in the electrolyte for the LiOH and LiOH · H2 O energy calculations because, for
this cell design, including it will increase the energy values by less
than 5%, and it is unclear whether it is better to cycle with a saturated
electrolyte solution or have a lower concentration in the fully charged
stage. The theoretical amount of energy stored when cycling between
a 0 M and a saturated solution (5.25 M at 25◦ C) of aqueous LiOH is
about 430 Wh/kg and 475 Wh/L. Although an oxygen tank is, strictly
speaking, not part of a Li/air cell, including its mass and volume in the
calculation underscores the potentially large disparity in the energy
density of closed vs. open systems. We assume the use of a stainless
steel oxygen tank in the shape of a 1.25 m-long cylinder with two
hemispherical ends.
First considering systems without an oxygen tank, Figure 3a shows
that a “practical” discharged Li2 O2 Li/air cell may achieve a specific
energy more than twice that of a Li/LiMO2 cell, while a LiOH · H2 O
cell may have a “practical” specific energy only slightly higher than
a Li/LiMO2 cell. In terms of energy density, Figure 3b shows that a
Li/LiMO2 cell has a modestly lower energy density than a Li2 O or
Li2 O2 cell, and a modestly higher value than a LiOH · H2 O cell. The
major change that would allow the aqueous Li/air system to have a
significantly higher specific energy and energy density would be the
formation of pure LiOH rather than LiOH · H2 O, which we also show
for the sake of comparison in Figure 3. For comparison with our “practical” numbers here, PolyPlus, a company focused on the development
of protected lithium metal electrodes, has claimed a practical specific
energy of almost 1.0 kWh/kg for their basic-electrolyte aqueous Li/air
cells.14 The numbers given in Figure 3a (0.70 kWh/kg for charged,
0.66 kWh/kg for discharged), are about 30% below the number given
by PolyPlus. A number of factors may contribute to this difference,
including our use of a 80% packing weight factor (they may have a
lower-weight packaging technique) and how much of the weight of
the water stored in the LiOH · H2 O is included in their weight basis.
In particular, if they have a design that takes some water from the
external environment, that could significantly lower their weight basis for the charged cell. Another factor is our use of a relatively thin
200 μm positive electrode thickness; with a positive electrode 1 mm
in thickness and all else the same, the specific energy for our practical
cell design is also about 1.0 kWh/kg. Note that such a thick electrode
is more realistic for aqueous than nonaqueaous cells because of the
much higher conductivity of aqueous electrolyte solutions. Again, we
stress that the “practical” cell energy numbers presented here will
certainly be revised as more detailed designs are developed, and are
meant to represent optimistic estimates.
Figure 3 also shows the results of calculations including an oxygen
tank. We include these numbers because it is important to see how
Li/air cells compare with a Li/LiMO2 cell if the problems associated
with making an open system cannot be solved. For these calculations
the mass of the oxygen is included when the cell is charged, as it is
stored in the tank. We assume the oxygen in the tank has a specified
volume (75 L) and the tank is sized for a battery system that stores
140 kWh. Additional specifications are given in Table II. Figure 3
shows that the use of an oxygen tank results in a significant reduction in the specific energy and energy density. In terms of specific
energy, the Li2 O and LiOH cells with a tank still have a higher value
than a Li/LiMO2 cell, but the Li2 O2 cell and LiOH · H2 O cells have
a slightly lower value than a Li/LiMO2 cell. In terms of energy density, if an oxygen tank is used the values for all the Li/air cells will
be lower than for a Li/LiMO2 cell. These calculations demonstrate
the importance of creating a Li/air battery system that is able to use
oxygen from the atmosphere rather than store it onboard, although the
tank results may be improved if a lighter weight tank material (e.g.,
carbon fiber) or a higher pressure (and thus a smaller tank volume)
could be used. Table III shows the pressure of a fully charged oxygen
tank for each Li/air active material, as well as the isothermal energy
of compression required to go from 1 bar to the final pressure. The
non-unity compressibility of oxygen was accounted for using the van
der Waals equation. Interestingly, the LiOH and LiOH · H2 O active
materials require the smallest amount of oxygen because they react
4 electrons per mole of O2 (as does Li2 O) and have a higher equilibrium potential than Li2 O. Li2 O2 requires significantly more oxygen,
and therefore a higher pressure and heavier tank, than the other active materials because only 2 electrons per mole of O2 react. The
isothermal work of compression is relatively small in each of these
cases (<3% of the practical discharge energy) but isothermal compression is probably more practical if done electrochemically; however,
Table III. Details on the oxygen tank pressures and compression
energies that would enable a closed Li/oxygen battery system.
Active material
Li2 O
Li2 O2
LiOH · H2 O
LiOH
Fully charged
O2 pressure (bar)
Isothermal compression
work (kWh/kWh practical
discharge energy)
134
275
114
114
0.0108
0.0244
0.0088
0.0088
2
Electrode capacity (mAh/cm )
having 100 to 300 bar oxygen pressure in the stack presents challenges of its own, including safety. In addition, while electrochemical
compression has been explored for hydrogen it is not well established
for oxygen.15 A major advantage of using electrochemical compression and having the oxygen electrode operate at the same pressure
as the tank, besides avoiding the need for a mechanical compressor,
is that the compression work can be returned on discharge, resulting
in a theoretical round-trip compression energy efficiency of 100%. If
adiabatic mechanical compression is used (which is typical for mechanical gas compression) the theoretical compression work is higher,
although again if the oxygen electrode is directly exposed to the oxygen tank pressure at least some of the compression energy can be
returned on discharge. Mechanical compression seems more plausible if the compressor could be located and powered from outside the
vehicle.
The calculations presented in Figure 3 are for secondary cells. The
specific energy and energy density of primary cells may be higher,
depending on the cell designs that an application allows. Because
many primary cells are intended for low-rate applications, a thicker
piece of Li metal may be used, increasing the mass and volume of
active materials compared to packaging and thereby increasing the
specific energy and energy density. In addition, primary cells typically
do not require any (or as much) excess Li metal because a non-unity
current efficiency does not affect later cycling.
Along with our calculations of the cell energy, it is important to
present the capacity per area for the cells described above. In Figure 4
we show the electrode capacity (in mAh/cm2 ) for each active material
using the specifications given in Table II. The figure shows that each
material has an electrode capacity above 10 mAh/cm2 , and in the case
of Li2 O it approaches 50 mAh/cm2 . Typical values for a Li-ion cell are
in the range of 3–7 mAh/cm2 . Of course, a thicker electrode with the
same active material and active material volume fraction has a higher
area-specific capacity, but there is a limit to how thick an electrode
can be made due to mechanical issues. In later sections we discuss the
importance of presenting not only the amount of capacity per, for example, gram of carbon, but also the capacity stored per electrode area.
In conclusion, the specific energy of nonaqueous Li/air batteries
that form Li2 O or Li2 O2 is extremely appealing, as in a “practical”
cell it may be about 2 to 4 times higher than a comparable cell with
a Li metal negative electrode and an advanced Li-ion intercalation
positive electrode. The specific energy of the aqueous Li/air cell is
somewhat higher than a comparable Li/LiMO2 cell, but its energy
density is lower. The relatively low density of the Li/air active materials means that their specific energy is more appealing than their
energy density when comparing against an advanced Li-ion intercalation material. Storing oxygen onboard the vehicle in a tank will
significantly reduce the specific energy and energy density of a Li/air
cell to the point that a Li/air cell is no longer compelling compared
with a Li/LiMO2 cell, unless a low-weight and high-pressure tank
is used. The magnitude of reductions in the practical numbers in
Figure 3 when a tank is used will result from a constrained optimization for the tank pressure that results in an acceptable tank volume
and gives the lowest cost (taking into account compression losses).
Overview of the critical challenges for the Li/air system.— Now
that we have established the promising specific energy of both nonaqueous and aqueous Li/air systems, we turn our attention to the
significant challenges that stand between the Li/air concept and commercialization. In the next section, we address the critical issues pertaining to nonaqueous Li/air cells, including those that are relevant to
both nonaqueous and aqueous systems. In the subsequent section, we
discuss issues that are relevant to aqueous systems only. Finally, we
summarize the main challenges and provide an outlook for the future
of this technology.
We consider issues that limit the practical achievable specific energy, specific power, and cycle life of Li/air systems, as well as the
additional complexity involved in the open nature of the cell. Because
we estimate that the commercialization of this technology will be possible only with solution of several very difficult challenges we defer
questions of cost until the design of a viable system becomes clear.
However, a significant increase in the specific energy will result in a
commensurate reduction in cost if the cost per mass remains constant;
this relationship remains to be established. Similarly, we suspend discussions of system safety. While nonaqueous systems, if achievable,
contain flammable liquid electrolyte and a Li anode with the potential
to generate dendritic shorts, there is evidence to suggest that basicelectrolyte aqueous systems with protected Li metal have insufficient
reactivity to create the conditions for explosion due to the formation
of a film that limits the reaction rate.16, 17
Not every hurdle that lies in the path of commercialization can
be discussed in this review, as it is impossible to predict all of the
challenges considering that the ultimate design of a Li/air system may
be significantly different from present concepts. Here we shall focus
on a set of significant challenges that have high priority within the next
five years. Depending upon the results of research being carried out
over that time period it may be that certain widely studied concepts are
shown to be impossible or unattractive, while other related systems
(e.g., aqueous Li cells18 ) showing more promise may be discovered.
The issues we consider are listed here and correspond to the subsection
headings.
Issues that are broadly applicable to Li/air systems, or only
nonaqueous systems:
40
r
r
r
r
r
r
30
Issues that apply only to aqueous systems:
50
R5
Establishing truly reversible electrochemical reactions.
Obtaining high capacity in the positive electrode.
Accommodating significant volume changes.
Stabilizing the Li metal negative electrode.
Achieving adequate power capability and efficiency.
Supplying contaminant-free O2 to the system.
r
Managing precipitation and dissolution of the discharge
product.
r Catalyzing discharge and charge when O-O bonds are broken.
20
10
Nonaqueous Li/Air Systems
0
Li2O
Li2O2 LiOH·H2O LiOH
Li/LiMO2
Figure 4. Positive-electrode area-specific capacity for a thickness of 200 μm
and an active material volume fraction of 0.65. For comparison, most nonaqueous Li/air cells built thus far have an electrode capacity of less than 5 mAh/cm2 ,
even though they may achieve a large amount of capacity per weight of carbon
(≥1000 mAh/g carbon).
While the first paper on a Li/air cell came out in the 1970s,19
and the first paper on a nonaqueous Li/air cell in the late 1990s,2
interest has grown significantly in the past ten years, with much of
the attention focused on nonaqueous cell designs.3–5, 20–26 Several review papers have already been published;3, 27–29 we avoid repeating
that task here and instead focus on the critical issues that must be
understood and solved for the Li/air system to move toward commer-
R6
Figure 5. Example of a Li/air gas diffusion cathode based on Ni-foam. The
coating consists of 50 wt% Super P carbon black, 40 wt% PVDF binder and
10 wt% MnO2 additive. (a) SEM-image of a coated Ni-foam. (b) Optical image
of uncoated Ni-foam. (c) Optical image of a coated Ni-foam-cathode.
cial viability. However, we first briefly discuss the components and
performance of a nonaqueous Li/air cell. One advantage of nonaqueous Li/air cells over aqueous Li/air cells is that a solid-electrolyte
interphase (SEI) forms on Li metal in many nonaqueous electrolytes,
allowing some experiments to be conducted without using a solidelectrolyte separator. Thus, a typical experimental cell is composed
of a piece of Li metal, a nonaqueous solvent, a separator (e.g., glass
fiber or a Celgard separator), and an air electrode. Air electrodes for
Li/air batteries can be prepared following several techniques,30–33 and
the most common used thus far involve carbon black (e.g., Super P,
Ketjen Black, Active coal), a polymer binder (e.g. PVDF, PTFE, cellulose), and an organic solvent (e.g. NMP, Acetone) or water (in case of
PTFE-suspensions and cellulose binders) being homogenized to form
a viscous slurry. The slurry is typically coated onto a metal grid, metal
foam or conducting fleece by a film-casting process30 or by ultrasonic
treatment. The supporting and conducting grid or sheet influences
the positive electrode performance if a 3-phase boundary between
oxygen, reaction product(s), and electrolyte is formed. Common positive electrode supports are Al grids,4, 34 Ni meshes35 or porous Ni
foams.36 The resulting air cathode has to fulfill several requirements.
It should have a high surface area at reasonable pore volumes, good
electronic (>1 S/cm) and ionic (>10−2 S/cm) conductivities, and a
design that supports fast gas transport to the reaction centers during
discharge.
Figure 5 shows an SEM image of a coated Ni foam (a) together
with optical images of uncoated (b) and coated samples (c). The
coating consists of 50 wt% Super P carbon black, 40 wt% PVDF
binder and 10 wt% MnO2 particles. From the SEM image one can
visualize the functionality of the gas diffusion air electrode. The large
pores ensure gas transport to the coated branches (covered with active
slurry). The electrochemical reaction presumably takes place within
the mesopores of the coating. It is still an open question how the
reaction distributes itself over the pore volume and it likely depends
on the specific electrode formulation and operating conditions. In
order to provide good reactivity and efficiency, the wetting behavior,
oxygen solubility, ionic conductivity, and stability of the electrolyte
on the cathode surface are of great importance.
Figure 6 shows a sample discharge and charge cycle for a nonaqueous Li/air cell. There is typically a significant offset between the
discharge and charge curves, often more than 1 V, indicating a high
cell impedance and/or a different reaction mechanism on discharge
and charge. As discussed in subsequent sections, several physical processes contribute to the high resistance. The abscissa label in Figure 6
shows that a common way to express the charge stored in a nonaqueous
Li/air cell is in mAh/g of cathode material, including carbon, binder,
and catalyst. Some authors define the capacity in mAh/g of carbon, the
Figure 6. Sample discharge and charge curve for a nonaqueous Li/air battery
with a hydrophobized gas diffusion cathode cycled at a current density of
0.2 mA/cm2 . The electrolyte contains 1 M LiPF6 in TEGDME.). The cathode
(50 wt% Super P carbon ± 40 wt% PVdF ± 10 wt% MnO2 ) loading in
each case is 2 mg/cm2 , and the electrode thickness is 50 μm. The capacity
is normalized to the total mass of the cathode excluding the current collector.
For comparison the top x-axis shows the capacity normalized to the electrode
surface.
idea being that the reaction happens on the carbon surface. We stress
that it is also valuable to include the capacity in mAh/cm2 , as this
metric in combination with the electrode thickness can be compared
to Li-ion and other electrodes.
Figure 7 shows a typical result for the fall in capacity with cycling
for a nonaqueous Li/air cell containing a carbonate-based electrolyte.
Some authors have shown more stable capacity, but no publication
has demonstrated 100 cycles with at least 90% of the initial capacity
retained. Different types of carbon material typically demonstrate
different amounts of capacity and have different rates of capacity
fade, as Figure 7 shows. For example, Super P carbon delivers the
best performance although its surface area is only 60 m2 /g. Active
coals like Supra 30 have more than 1700 m2 /g but deliver much lower
capacity. This demonstrates that a carbon with a higher surface area
Figure 7. Variation of carbon materials for Ni-foam based cathodes in a
propylene carbonate electrolyte with 1 M LiPF6 . Super P carbon black (60
m2 /g) shows the best performance in this comparison regarding maximum
capacity. The other materials are carbon black (Ensaco 350 G, 400 m2 /g),
carbon fibers (CF), and two active coals, Supra 30 (1760 m2 /g) and PAK1000C
(1000 m2 /g). The cathode (50 wt% carbon ± 40 wt% PVdF ± 10 wt% MnO2 )
loading in each case is 2 mg/cm2 , and the electrode thickness is 50 μm.
Figure 8. Influence of three different cathode designs on the electrochemical
performance of a Li-O2 cell. The error bars indicate the capacity range observed
for many different samples of the respective type. The electrolyte is 1 M
LiPF6 in propylene carbonate. The Al-grid supports a solid coating and allows
gases to diffuse through the layer. The all-carbon-based electrode design is a
gas diffusion layer (GDL) consisting of a dense carbon fiber network with a
hydrophobic coating (a wet chemical treatment using a sliane-based precursor)
and a carbon black coating of about 50 μm. The cathode (50 wt% Super P
carbon black ± 40 wt% PVdF ± 10 wt% MnO2 ) loading in each case is
2 mg/cm2 . The slurry was applied to the carbon black layer of the GDL using
a coating knife. The Al-grid was coated by dipping it into the slurry similar to
Ni-foam. Note that the discharge capacity increases reproducibly between the
first and second cycle for two of the cathode compositions. This may be due
to some rearrangement or modification of the electrode surface.
does not necessarily have a higher capacity. Typical electrolytes most
likely cannot penetrate the nanopores present in high-surface-area
carbons; hence, reactive sites are restricted to the outer surface of
these carbons and the available surface area is reduced. In addition,
the particular air electrode design (e.g., all carbon treated with a
silane-based hydrophobic layer vs. carbon/Ni-foam vs. carbon/Algrid (expanded metal) electrodes, see Figure 8) has a great influence
on the capacity achieved.
The identity of the solvent also has a significant impact on the
capacity obtained and the discharge and charge potentials. In our own
studies we have tested n-methyl pyrrolidone (NMP) and tetraethylene glycol dimethyl ether (TEGDME)e in comparison with propylene carbonate (PC). Figure 9 shows data of the first cycle for PC,
TEGDME, and NMP measured at a current density of 0.2 mA/cm2 .
The air electrode used in these experiments consists of silane-treated
carbon black, PVDF binder and a MnO2 catalyst. The differences in
the capacity obtained and the potentials indicate possible differences
in the reaction pathways, discharge product morphology and growth
mechanism, electrode wetting, and oxygen solubility and transport. In
this comparison NMP shows the highest discharge potential of about
2.7 V and, at approximately 35 hours, the longest discharge time.
In summary, while a basic Li/air cell can be assembled using
materials commonly available and often used in Li-ion cells, many
experimental results and physical processes remain difficult to understand. In the coming sections we identify several of the challenges
that have been identified, and seek to delineate where clarity has been
established and where additional evidence is required before a clear
explanation can be provided.
Truly reversible electrochemical reactions need to be
demonstrated.— Although efforts to commercialize primary Li/air
batteries are already underway, the ultimate objective in automotive
electrification is to produce a high-specific-energy storage battery that
can be cycled thousands of times. At this stage, limited cyclability of
both nonaqueous and aqueous Li/air cells has been demonstrated;
e We note that recent interest in TEGDME follows early work on poly(ethylene glycol)
dimethyl ethers in Li/oxygen cells.37
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Figure 9. Potential vs. time of Li/air cells in three different nonaqueous electrolytes. All samples have been cycled at a current density of 0.2 mA/cm2 . The
air electrode consists of hydrophobized carbon, PVDF, and a MnO2 catalyst.
It is important to mention that the plateau around 3.8 V for NMP is due to
oxidative decomposition, and is not related to the desired charge reaction (i.e.
reversible Li2 O2 decomposition). The same plateau is present during charging
of NMP cells without a prior discharge. The cathode (50 wt% Super P carbon
± 40 wt% PVdF ± 10 wt% MnO2 ) loading in each case is 2 mg/cm2 and the
thickness is 50 μm.
however, in the case of nonaqueous systems, it has not been shown
that they can be cycled with a current efficiency sufficiently high
for a practical reversible cell (>99.95% is required to reach about
450 cycles with 80% of the capacity remaining). The distinction between cycling and cycling reversibly is crucial; a cycle is only reversible if the chemical makeup of the system is the same at the start
and end of the cycle. For the nonaqueous battery in particular, detailed
investigation of the chemistry of discharge and charge products is a
critical issue that has thus far received insufficient attention. There is
now conclusive evidence that the electrochemical reactions that occur in nonaqueous cells containing carbonate solvents during charge
are not, in part or whole, the reverse of those that occur during discharge. Hence, the system can be cycled several times, but only at the
expense of some component, likely the electrolyte solvent, which is
consumed irreversibly on each cycle. Without a continuous supply of
fresh reactant and a way to remove irreversibly generated discharge
products, hundreds of cycles are unattainable. In addition, any number of non-electrochemical reactions may also be occurring. In this
section we provide a critical description of recent experimental and
theoretical work that has examined the fundamental processes occurring in the nonaqueous Li/air electrochemical cell, as well as specify
the types of studies that will be helpful in overcoming the critical
issues.
Experimental evidence strongly supports irreversible reactions in
carbonate-based solvents.—The first paper on a nonaqueous Li/air
cell (published in 1996) described the use of an electrolyte with a
polymer solvent (polyacrylonitrile) as well as the plasticizers ethylene
carbonate (EC) and propylene carbonate (PC) and a cobalt pthalocyanine catalyst.2 It identified Li2 O2 as the probable discharge product
based on qualitative analysis and Raman Spectroscopy. Although they
used some but not all of the same materials as Abraham and Jiang,
later authors appear to have taken the formation of Li2 O2 in carbonate solvents as already established.2, 4, 20, 33, 34 However, several recent
papers have specifically focused on the reaction chemistry and established conclusively that in carbonate solvents (without the presence
of any polymers) Li2 O2 is not a principal discharge product, and on
charge CO2 rather than O2 is evolved.38–42
In the most definitive article published so far, McCloskey et al.
carried out differential electrochemical mass spectroscopy (DEMS)
experiments on Li/oxygen cells fed with isotopically labeled O2 as
R8
Figure 10. Raman spectra of discharged carbon cathodes from a pure DMEbased cell, a 1:1 (v:v) EC/DME-based cell, and a 1:2 (v:v) PC/DME-based
cell. Neat P50 carbon paper is included for comparison. Discharge conditions: 0.09 mA/cm2 under O2 to 2 V. Caption and figure reproduced from
reference 38.
well as spectroscopic analysis of discharged and charged electrodes.38
In a solvent composed of either 1:1 EC:DMC or 1:2 PC:DME, significantly more than 2 e− /O2 molecule were consumed on discharge
(indicating reactions besides formation of Li2 O2 ), XRD of discharged
air electrodes showed the absence of Li2 O2 , and Raman Spectroscopy
showed both the absence of Li2 O2 and the presence of Li2 CO3 and
possibly Li alkyl carbonates (see Figure 10). Upon charging a cell
that had undergone a first discharge, the principal gas evolved was
CO2 rather than isotopically labeled oxygen. The evidence in this
paper clearly establishes the absence of reversible cycling when, at
the very least, the specific carbonate solvents used by the authors
are present. We consider the use of isotopically labeled oxygen in
quantitative, on-line DEMS experiments to be a key method for establishing the reversibility of the Li/air system; in a reversible system the
same isotopically labeled O2 consumed on discharge should emerge
on charge. If CO2 is evolved, isotopic labeling of carbon in the solvent
may help determine whether the solvent or, if carbon based, the electrode decomposes. We note that when evaluating new chemistries for
reversibility, it is important to quantify the number of electrons and
O2 molecules consumed on discharge and charge as well as the purity
of the discharge product; reversible systems have charge/discharge
capacity ratios of very nearly 1 and e− /O2 ratios of very nearly 2
(assuming Li2 O2 is the discharge product). Other ratios and/or the
presence of discharge products other than Li2 O2 indicate the presence
of electrochemical or chemical side reactions. Quantifying electron
and O2 consumption and generation as functions of current density and
potential window can help identify the nature of these side reactions
if they exist.
The conclusions of McCloskey et al. are supported by several
other publications. For example, Freunberger et al. also carried out
DEMS analysis on a carbonate-containing Li/air cell, as well as spectroscopic analysis that included FTIR, NMR, and Surface-Enhanced
Raman Spectroscopy (SERS).39 They found that in an electrolyte of
LiPF6 in PC on discharge the products include Li2 CO3 and several
alkyl carbonates, and on charge CO2 rather than O2 evolves. Their
spectroscopic and gas-analysis findings are consistent with those of
McCloskey et al. and clearly point to irreversible reactions during the
cycling of a Li/oxygen cell with a PC solvent. In addition, similar results have been reported by Mizuno et al. who carried out FTIR and gas
analysis;40 Zhang et al. who carried out gas analysis during charging,
FTIR, and XRD;41, 43 and Veith et al. who carried out XPS, FTIR, and
Raman on Li/oxygen cells with an electrolyte composed of LiPF6 in
a mixture of ethylene carbonate and dimethyl carbonate.42 While sev-
eral other articles state that Li2 O2 forms in a Li/oxygen cell containing
a carbonate solvent (e.g., the original paper by Abraham and Jiang
and a paper by Thapa et al. who built carbon-free electrodes44 ), none
of them employed a set of analytical tools (including gas analysis and
spectroscopy) that could consistently account for the exclusive electrochemical formation of Li2 O2 . We conclude from the articles that did
employ rigorous spectroelectrochemical techniques and demonstrated
the absence of significant Li2 O2 formation and reversible cycling that
carbonate solvents do not have sufficient stability for long-term Li/air
cell cycling.
While the spectroscopic techniques mentioned thus far have been
helpful for identifying the discharge and charge chemistry, Zhang and
co-workers demonstrated that the characteristic lithium NMR chemical shift for Li2 O2 is difficult to distinguish from that of a reaction
by-product of their carbonate-electrolyte Li/air cells.43 Similar results
were also obtained in our recent theoretical work.45 Therefore, a direct
assignment for the chemical shift that indicates the presence of Li2 O2
could not be established by means of lithium NMR spectroscopy. The
sometimes conflicting results for the carbonate-based Li/air chemistry
underscore the importance of carefully selecting appropriate analytical tools to identify discharge and charge products.
No solvent has yet demonstrated highly reversible cycling; further
exploration and quantification are necessary.—Some noncarbonate
solvents have been explored for use in a Li/oxygen cell, including
ethers, ionic liquids, and acetonitrile. In this section we assess critically the results for each of these solvents. Considering, first, the class
of ethers, McCloskey et al. also explored the use of dimethoxyethane
(DME) in the same work that addressed carbonate solvents.38 During discharge in DME, 2.05 ± 0.05 e− /O2 were consumed and
the principal product identified by spectroscopy was Li2 O2 (see
Figure 10), while on charge the same isotopically labeled oxygen
consumed by the cell on discharge was released. However, during the
charging process, in which the cell was charged by a capacity equivalent to the discharge capacity, 3.2 e− /O2 released were consumed,
indicating a current efficiency for O2 release of only 60%. Interestingly, even with such a low current efficiency, no Li2 O2 was found at
the end of charge, which the authors suggest may be due to a chemical
or electrochemical reaction of Li2 O2 during the charging process that
does not have O2 as a product. The authors conclude that while DME
does provide for the formation of Li2 O2 on discharge, it is unstable
during the charging process and is therefore an unsuitable solvent for
reversible cycling. However, much of the charging process was carried out at high potential (∼4.5 V vs. Li), at which DME is likely to
decompose electrochemically,46, 47 and additional experiments (e.g.,
charging at lower currents; using a cell design that ensures intimate
electrical contact between Li2 O2 and conductive carbon) should be
performed before eliminating DME as a candidate solvent for nonaqueous Li/oxygen cells. Peng et al.48 showed that electrodes packed
with Li2 O2 could be charged in PC solvent, whose oxidation potential
is ∼5.1 to 6.0 V,4, 46, 47, 49, 50 without evidence of solvent decomposition.
Freunberger et al. also report on the chemistry of Li/oxygen cells
that make use of ethereal solvents, and found that for linear ethers like
tetraglyme, as well as cyclic ethers like 1,3-dioxolane, some Li2 O2
forms on discharge, although significant amounts of electrolyte decomposition products are also found.51 They did not report results
for DME as a solvent, possibly explaining the difference between the
fraction of Li2 O2 produced during the first discharge they found compared with McCloskey et al. Like McCloskey et al.,38 they conclude
that ethers are unsuitable solvents for reversible Li/oxygen cells, although they are more stable than carbonates as some Li2 O2 forms
on the first discharge. Other authors have also discussed the fact that
Li2 O2 does form in ethereal solvents, and there is at least limited
reversibility.26, 52 In our view, while additional work on ethereal solvents is warranted, there are already indications that they too undergo
decomposition reactions.
A different class of solvents that has recently received attention is
ionic liquids. For example, Mizuno and Iba discuss the use of an ionic
liquid, N-methyl-N-propylpiperidinium bis-trifluoromethansulfonylamide (PP13TFSA), as a solvent that demonstrates better reversibility
than PC, and results in O2 rather than CO2 evolution during charge.53
Other authors have also explored the use of ionic liquids for Li/air
batteries, although to date there has been no paper that demonstrates
quantitatively a sufficiently high current efficiency for a practical
cell.31, 54–56
Finally, several authors have explored the use of acetonitrile as a
solvent for Li/air batteries.26, 48 While they have found that oxygen
reduction and evolution appear to be reversible, they have not adequately quantified the current efficiency of Li2 O2 formation, and point
out that the higher vapor pressure and inferior safety characteristics
of acetonitrile make it a poor choice for a practical Li/air cell.
Laoire et al.26 have suggested that suitability of a solvent for a Li/air
cell be evaluated in terms of Pearson’s “Hard Soft Acid Base” (HSAB)
Theory.57 By comparing two cations (e.g., Bu4 N+ , a soft Lewis acid,
and Li+ , a hard Lewis acid), they showed that O2 − is more stable
in a soft acid environment, but is reduced to O2 2− and ultimately
O2− in a hard acid environment. Moreover, solvation decreases the
hardness of the Li+ acidity in proportion to the donor number of
the solvent. This in turn leads to a longer lived superoxide and
reversible LiO2 formation. Such theories can help guide the solvent
exploration process.
In conclusion, while several noncarbonate classes of solvents have
been explored for use in Li/oxygen batteries, to date there has been no
report that clearly establishes highly reversible cycling, and finding
a salt and solvent combination that allow truly reversible cycling
remains perhaps the first and most important challenge for Li/air
batteries at this time.
Hypothesized mechanistic routes for the formation of the desired
reaction product include reactive intermediates.—There is now sufficient evidence that the solvent plays a critical role in determining the
nature of the Li/air reaction products and the cell’s electrochemical
cyclabilty. The search for a stable electrolyte could be greatly assisted
by acquiring detailed knowledge of the prevalent reaction mechanisms
of the Li/air cell, including the identification of rate determining steps
and intermediate reactive species. For example, the experimental work
of Abraham and co-workers25, 26 resulted in a proposed reaction mechanism that includes the reduction of oxygen to lithium superoxide:
O2 + Li+ + e− → LiO2
[5]
2LiO2 → Li2 O2 + O2
[6]
LiO2 + Li+ + e− → Li2 O2
[7]
Li2 O2 + 2Li+ + 2e− → 2Li2 O
[8]
followed by
or
and possibly
Equation 5 involves the irreversible reduction of O2 via a oneelectron process to form LiO2 . This product disproportionates to Li2 O2
and O2 (equation 6) or is further reduced to form Li2 O2 (equation 7).
Li2 O is also probably formed as the ultimate reduction product of O2 ,
as indicated in equation 8.25, 26 Recent in-situ spectroscopic data have
provided direct experimental evidence of the formation of LiO2 as an
intermediate species for the formation of Li2 O2 .51 LiO2 formation involves reduction of O2 to superoxide, O−
2 , which has been implicated
both experimentally39 and theoretically58 as the species responsible for
the decomposition of the carbonate-based electrolytes. As discussed
extensively by Sawyer and Valetine, the superoxide ion is a powerful
reducing agent, which is thought to react very rapidly with a variety of
organic substrates.59 Others have also ascribed significant reactivity to
the electrochemically formed LiO2 species, which rapidly reduces the
carbonate solvent and forms solvent-decompositions products.38, 43
Although the rate of formation of LiO2 and its reactivity toward
carbonate solvents remain unmeasured, preliminary XRD results by
Zhang and co-workers indicate that the attack of the carbonate-based
solvent molecule by the superoxide ion proceeds at a faster rate than
+
60
To summarize, the
the coordination of O−
2 and Li to form LiO2 .
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evidence for whether O2 − or LiO2 is more influential in unwanted
parasitic reactions is not yet definitive.
Independent of whether the solvent decomposition reaction proceeds via O−
2 , LiO2 , or another Li/air reactive species, it is now
clear that a solvent that is resistant to attack by reduced O2 species
must be discovered in order to achieve a practical Li/air cell with
a long cycle life. Recent advances in surface-sensitive spectroscopy
and microscopic analyzes38, 51 coupled with first-principles modeling
simulations58 could help unveil the fundamental elementary processes
of the Li/air electrochemical cell and explain the reasons for the poor
reversibility of the reactions in typical Li-ion battery solvents.
When screening electrolytes that do not form a stable SEI on Li metal
an appropriate counter electrode should be used.—The carbonate solvents of widespread use in Li-ion batteries can form solid-electrolyte
interface layers on Li metal electrodes that are sufficiently stable to
allow tens or even hundreds of cycles. When noncarbonate solvents
are being screened for their stability toward positive-electrode reactions, care must be taken that side reaction products generated at the
counter electrode do not diffuse across the cell and interfere. Ideally, a
counter electrode and reference electrode that have a potential at which
a solvent is stable should be used. For example, Li4 Ti5 O12 , with an
equilibrium potential of 1.5 V vs. Li metal may be a good candidate,
although it does not have cyclable Li as synthesized (active materials
are typically synthesized in the discharged state). To circumvent this
problem Li2 O or Li2 O2 could be packed into the positive electrode.
Another possible counter and reference electrode is LiFePO4 . In any
case, we stress the importance of a careful experimental design even
for research cells in order to ensure the electrochemical stability of
the reactions occurring at all electrodes and the avoidance of soluble
products that may obscure the interpretation of the electrochemical
signals. A protected lithium electrode could likewise serve as a counter
electrode for screening nonaqueous solvents, and it may be necessary
for the function of certain solvents.
Outlook: Nonreactive nonaqueous solvents remain elusive and require a carefully designed screening procedure.—It is now clear that
carbonate solvents are poor candidates for reversible Li/air systems
because of their susceptibility to react with reaction intermediates.
The search for noncarbonate solvents should involve screening in
cells with appropriate negative electrodes (e.g., protected Li or highpotential intercalation electrodes). The amount and identity of O2
consumed and generated during discharge and charge should be measured and compared to the discharge and charge capacity to quantify
reversibility. The identity of the discharge product and its absence
after recharge should also be confirmed. Thus far linear and cyclic
ethers, acetonitrile, and some ionic liquids have been explored; while
Li2 O2 is formed on the first discharge in some cases, highly reversible
cycling has not been quantified and reported for any of these noncarbonate solvents. In our view, finding a stable solvent that allows
truly reversible cycling remains perhaps the most important current
challenge for Li/air research. Using a variety of analytical techniques,
including gas analysis (preferably with isotopically labeled oxygen
and possibly carbon), and spectroscopy such as Raman and XRD,
is required to ensure that cycling is truly reversible. Identifying and
understanding the reaction pathways that influence reversibility via a
combination of spectroscopy, microscopy, and first-principles simulations is a worthy effort that the scientific community should undertake.
A high positive-electrode capacity needs to be achieved for a
Li/air cell to achieve a high specific energy and energy density.—
Conventional positive-electrode materials like LiMn2 O4 , LiFePO4 ,61
or LiNi1/3 Co1/3 Mn1/3 O2 have maximum capacities of 100 to
200 mAh/g. A practical sulfur-electrode (elemental sulfur dispensed
in a carbon or polymer matrix) reaches 400 to 900 mAh/g62, 63 referred
to the mass of carbon, binder and sulfur. The experimentally observed
capacity range for Li/air gas diffusion electrodes is about 600 to
5000 mAh/g (the typical range of loadings is 3 to 6 mg/cm2 )
depending on the electrolyte, carbon, binder, carrier material, and
oxygen partial pressure.21, 22, 31 The capacity is referred to the total
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mass of the carbon, binder, and additives in the cathode. This large
capacity range shows that many parameters may influence the specific
energy of a Li/air cell. Therefore the cathode has to be understood
in terms of maximum capacity and relevant mechanisms that limit
the capacity. Three major capacity-limiting issues, passivation, pore
blockage, and O2 transport limitations, are discussed in the literature
and are assessed critically in the following sections.
However, before moving to those capacity-limiting issues we stress
that in order for a Li/air cell to achieve a high energy the cell, at the end
of discharge, needs to have a high volume fraction of active material
(whether it be Li2 O, Li2 O2 , or LiOH · H2 O). Reporting the capacity in
mAh/g carbon, mAh/g cathode, or even mAh/cm2 somewhat obscures
this extremely important point. Unlike standard Li-ion cells where the
active material is built directly into the electrodes with a volume
fraction of around 50 to 75%, in a Li/air cell the active material
is essentially being synthesized during the discharge process. While
many authors report the capacity in mAh/g carbon, we emphasize that
carbon is not the true active material in the Li/air cell, as Table I shows.
To enable a reader to calculate the volume fraction of active material
in an electrode or cell at the end of discharge it is important to include
the information necessary to make that calculation. For example, if the
capacity is reported in mAh/g carbon, an author should also provide
the carbon loading (in mg-carbon/cm2 ) and the electrode thickness to
allow the calculation of mAh/cm3 , and from that the volume fraction
of active material.
Passivation by insulating discharge products appears to limit the
capacity.—Recent flat-electrode experiments and Li/air cell modeling indicate that passivation of the electrode surface by electronically insulating discharge products severely limits the capacity of
Li/air cells even at low rates of discharge, although the results may
depend on the electrolyte used and the discharge rate.23 In particular, Figure 11 shows the rapid passivation that occurs during discharge on a flat glassy-carbon surface. The maximum thickness obtained for the discharge product is less than 100 nm. The cell in
this case used a carbonate solvent; hence, the discharge reaction
picture is complicated by the fact that an array of discharge products are generated. Researchers carrying out rotating-disk electrode
experiments have also commented on passivation.25 This limitation
may be general because the desired reaction product in the nonaqueous Li/air cell, Li2 O2 , has the electronic properties of an insulator (to be more specific, bulk Li2 O2 is insulating,5, 64 although it
Initial open-circuit potential
3.0
2
High
kinetic
resistance
Cell potential (V)
2.8
0.75 µA/cm (~12.2 hr)
2
1.50 uA/cm (~ 3.4 hr)
2
3.76 µA/cm (~1.0 hr)
2
Fit to 0.75 µA/cm data
2.6
Exponential
resistance
rise
is possible that its surface states and grain boundaries may be more
conductive).
We stress that besides Li2 O2 , other reaction products such as Li2 O,
Li2 CO3 and Li-organic compounds may appear depending on the electrolyte system. The mechanism of electrical passivation in noncarbonate electrolyte systems is at an early stage of investigation.52
The impedance rise associated with passivation leads to an increasing overpotential that may end the discharge before the available
pore volume is even moderately filled. It will therefore be necessary
to understand the detailed growth mechanism of the desired reaction
product, which is not yet well understood. Defects or grain boundaries
in the Li2 O2 crystal could result in enhanced electronic conduction to
the Li2 O2 /electrolyte interface, or diffusion of Li and O through the
film could enable growth from the electrode/Li2 O2 interface. Theoretical computations of electronic conductivity through the film have been
limited to the consideration of fully dense, monocrystalline films.23, 65
More detailed characterization of Li2 O2 films grown on porous electrodes bathed in noncarbonate electrolytes, using a combination of
imaging (e.g., SEM, TEM, AFM) and electrochemical techniques
(e.g., use of ferrocene couples, GITT), is required to elucidate the
Li2 O2 growth mechanism. Depending upon the results, models that
incorporate grain boundaries and/or defect chemistry could be developed to improve our understanding of Li2 O2 formation and growth.
Pore blocking may also restrict the practical capacity.—Besides direct
passivation of the electrochemically active surface, blockage of micropores and some mesopores by Li oxides or other products formed at
the beginning of discharge can limit the accessibility of some electrode
surface for electrochemical reaction. Figure 12 shows schematically
how Li2 O2 growth on the surface of carbon in an electrode with restricted pores could result in either passivation or pore blocking. Both
phenomena result in unused pore volume and hence limit the discharge
capacity.
In electrodes where passivation does not occur, pore blocking may
be the mechanism that most severely limits capacity. To overcome
pore blocking the selection of a suitable carbon material with a sufficiently large pore diameters to allow the entire electrochemically
active surface to react is important.
In order to generate innovative designs that prevent air electrode
passivation and pore blocking one needs to understand the chemistry of the reaction process (discussed in Section 2.1) and how the
carbon/air electrode microstructure affects the discharge performance.
Hence, it is necessary to characterize the porous network of the real gas
diffusion electrode and relate it to its electrochemical performance. In
the literature N2 -adsorption measurements (BET66 ) are applied for the
selection of suitable carbon materials. Information about pore size distribution, pore volume, and the available surface area can be related to
state-of-charge dependent impedance measurement data (EIS).67 This
has been used to track the electrical passivation and relate the behavior
to structural properties of the carbon material determined by BET. For
mesocellular carbon foam a capacity increase by 40% compared to
other carbon black materials has been achieved recently.68 The authors
2.4
2.2
2.0
0
2
4
6
10x10
8
-3
2
Capacity (mAh/cm )
0
10
20
30
40
50
60
Thickness of discharge products (nm)
70
Figure 11. Demonstration of the passivation of a flat-electrode surface during
a discharge on glassy carbon. Reproduced from reference 23.
Figure 12. Schematic illustration of the pore filling during discharge. The
growing Li2 O2 layer leads to cathode passivation by electrical isolation (top
right) and pore blocking (bottom right).
attribute the enhanced performance to their large pore volumes and
ultra-large mesoporous structures, which allow more lithium oxide
deposition during discharge.
It should be possible to explore in a quantitative way the degree
of pore blocking that develops. For example, an electrode may be
harvested, its porosity checked, and a Gurley test conducted to determine the resistance to flow through the porous structure for a defined
pressure drop. In addition, a careful design of experiments that separates the influence of total surface area, overall porosity, pore size,
and pore size distribution would be helpful to truly quantify the degree of pore blocking that occurs. Such experiments should also take
into account differences in electrode wetting among different pore
designs. Coupled with flat-electrode experiments that are useful for
assessing passivation, such a study may help separate the effects of
pore blocking and passivation, which may have a similar overall effect
on electrochemical performance.
Oxygen transport limitations may arise in flooded electrodes; gas
channels will improve oxygen transport; 3-phase percolation is
important.—Oxygen transport limitations are expected to become important even at low current densities (i.e., <1 mA/cm2 , although the
precise value depends on the electrode thickness and other parameters) in fully flooded Li/air cells because the solubility of oxygen in
typical nonaqueous electrolytes is less than 5 mM, and thus at least
a factor of 20 lower than the typical concentration of the salt in a
nonaqueous electrolyte. The diffusion coefficient of oxygen is probably somewhat higher than that of typical electrolyte salts, although
rigorous measurements are not available.21, 69
While optimizing the composition of the electrolyte involves a
very large permutation of salts, solvents, and additives, the ability of
an electrolyte to dissolve oxygen can be investigated systematically.
Read et al. measured the ionic conductivity σ, the viscosity η and
the Bunsen coefficient α of various electrolytes and compared these
values to the capacities obtained from cycling experiments of lithium
half-cells.20, 21 Furthermore they tested the influence of the oxygen
partial pressure on the cell capacity. With respect to each parameter,
the cell capacity increases and then saturates as shown in Figure 13,
R11
although the relationship is not monotonic. At low discharge rates (i.e.,
the lines labeled f with a current density of 0.05 mA/cm2 ), the capacity
reaches its maximum at relatively low values of the inverse viscosity
1/η and the Bunsen coefficient α, while at high rates (i.e., the lines
labeled a with a current density of 0.5 mA/cm2 ) the capacity increases
steadily over the measured range (Figure 13a and 13b). The viscosity
and Bunsen coefficient influence oxygen transport in the electrolyte,
which becomes more critical at higher rates. Similarly, the capacity plateau shifts to higher oxygen partial pressures if the discharge
rate increases (Figure 13c). The plateau’s position is related to the
rate-dependent saturation concentration of oxygen in the respective
electrolyte.
While the oxygen partial pressure, and the electrolyte’s oxygen
solubility and diffusion coefficient, have a strong influence on the
high-rate capacity, the fact that the low-rate capacity saturates at
less than 2000 mAh/g carbon implies that O2 and Li+ transport in
the electrolyte do not ultimately limit the discharge capacity. However, if passivation or other limitations can be avoided, selecting electrolytes with high oxygen solubility, low viscosity, and high conductivity, and using a high oxygen partial pressure may afford higher
rate capability. The work of Read et al. suggests that ether-based
electrolytes and solvent blends made of propylene carbonate and
tris(2,2,2-trifluoroethyl) phosphate possess favorable properties related to oxygen transport.70, 71
To further enhance oxygen transport, continuous gas-diffusion
paths could be created within the porous framework of the air
electrode.72, 73 Adjusting the O2 partial pressure slightly above ambient pressure may be sufficient to force the electrolyte into wetting
pores and create a desirable 3-phase percolation throughout the air
electrode. This can also be achieved by employing suitable combinations of binders, carbons, and current collector grids, as well as
by fabricating the composite electrode with a particular pore-size
distribution.74 We performed measurements under systematic variation of the amount of electrolyte in the cell, which suggest that the
cell performance goes through a maximum with the amount of electrolyte. While too much electrolyte impedes the supply of oxygen
Figure 13. Specific cell capacity (per g carbon) depending on four different electrolyte parameters at various discharge rates: (a) Electrolyte viscosity η; (b)
Bunsen coefficient α which represents the oxygen solubility; (c) oxygen partial pressure; (d) ionic conductivity σ. The letters a-f at the individual curves indicate
the discharge rate: a) 0.5, b) 0.4, c) 0.3, d) 0.2, e) 0.1, f) 0.05 mA/cm2 . Figures reproduced from reference 21.
R12
3.0
With O2 transport limits,
without passivation
~20,500 mAh/g Super P carbon
Cell potential (V)
2.8
2.6
2.4
2.2
with passivation
2.0
Without O2 transport limits,
with passivation
(a)
1.8
3.0
Cell potential (V)
2.8
without passivation
~7,350 mAh/g Super P carbon
2.6
2.4
2.2
2.0
With O2 transport limits, Without O2 transport limits,
with passivation
with passivation
(b)
1.8
0
200
400
600
800
Specific capacity (mAh/g Super P carbon)
Figure 14. Simulation-only results demonstrating the relative impacts of eliminating oxygen transport limitations and eliminating the electronic resistance of
the discharge products. (a) shows results at a current density of 0.08 mA/cm2 ;
the two simulations with passivation superpose. (b) shows results at a current
density of 0.47 mA/cm2 . Caption and figure reproduced from reference 23.
gas, too little electrolyte results in poor effective ionic conductivity
and diffusivity. If, as described in the introduction, a Li/air system
is designed with a pressurized oxygen tank and the stack is exposed
directly to those high pressures, the quantity of oxygen in the electrolyte of a flooded cell would increase significantly, which would
have the effect of minimizing oxygen transport limitations. In other
words, running the stack at a high oxygen pressure could have the
practical effect of increasing the capacity at a given current density,
or moving to the right in Figure 13c.
Albertus et al. carried out modeling work to assess the principal
limits on the capacity of a Li/air cell. While the experimental system
on which their model was based contains carbonates, they nevertheless concluded that electrical passivation by the discharge product is
a key limitation in nonaqueous systems, as both Li2 O and Li2 O2 are
electrically resistive and have a minimal solubility in typical nonaqueous electrolytes. Figure 14 shows the effect of electrical passivation
by the discharge product and oxygen transport limitations for a nonaqueous Li/air cell. The figure shows that at low current densities
(<0.5 mA/cm2 ) removing the passivation limitation leads to a more
significant increase in capacity than removing oxygen transport limitations. At current densities at and above about 0.5 mA/cm2 oxygen
transport limitations are important for the particular cell design and
assumptions used by the authors, but again the removal of passivation results in a more significant increase in capacity (i.e., removing
oxygen transport limitations while keeping passivation increases the
capacity by less than a factor of two; removing passivation while
keeping oxygen transport limitations increases the capacity by about
a factor of 30). This result demonstrates that while efforts to improve
gas flow channels can make notable improvements to the capacity that
can be achieved by nonaqueous Li/air cells, if the discharge product
is electrically insulating and remains at the reaction site that will be
the principal capacity-limiting mechanism.
Enhancing Li2 O2 solubility could enable non-passivating Li2 O2
precipitation.—Passivation of the electrode surface occurs because
bulk Li2 O2 is electrically insulating and is insoluble in the electrolyte. Enhancing the solubility of Li2 O2 could potentially enable
nucleation and precipitation of solid Li2 O2 away from reactive sites
in the air electrode. PolyPlus has proposed the use of noncarbonate solvents with limited Li2 O2 solubility such as ethylene glycol and dimethylformamide, which could be enabled through the
use of a protected lithium anode.75 Researchers have also proposed
the use of boron-based anion receptor additives to significantly enhance the solubility of Li2 O2 ,76, 77 some of which may assist passivation of the negative electrode at low potentials.77 However, these
studies have mainly been carried out in carbonate solvents, and one
must be careful to distinguish between Li2 O2 solubility in and reactivity with the solvent.
Novel designs may address passivation and improve capacity.—To
overcome capacity limitations related to passivation, it may be necessary to find radically new solutions that address the specific properties
of the Li/air system. Very recent work indicates that tailored electrode
structures,78 surface treatments,35, 79 introduction of defects into the
discharge product,5 and elevating the operating temperature may lead
to higher discharge capacities without passivation.
Mitchell et al. recently described an electrode consisting of a hollow carbon-nanofiber “carpet” grown via chemical vapor deposition
on porous anodized alumina coated with thin Ta and Fe layers.78
High discharge capacities (7200 mAh/g carbon at 63 mA/g carbon)
were obtained, and the Li2 O2 discharge product, identified by XRD,
grew as nodules on the fibers and developed into toroids (up to
1 μm) over the course of discharge before eventually forming a
monolithic mass (see Figure 15). It should be noted, however, that
the carbon loading was rather low (∼0.1 mg/cm2 ), and that the effective low-rate capacity (0.7 mAh/cm2 ) is roughly a factor of 2–10
lower than in Li-ion cells. Additional work with higher carbon loadings would be very illuminating. Similar air electrode designs might
be realized using carbon nanotubes grown on catalyst particles (e.g.,
nickel) which are placed on the electrode surface. Alternatively one
could grow metal nanowires by chemical vapor deposition80 or electrochemically. We also note that a systematic study of the morphology
of Li2 O2 that forms during discharge, including as a function of substrate type, current density, and electrolyte, is a high-impact area for
research.
Surface treatment (e.g., hydrophobization35 ) of the electrode surface may also be an appropriate method for mitigating passivation. For example, C. Tran et al. recently demonstrated that surface treatment of the carbon material leads to an enhanced capacity and a discharge curve that implies diminished passivation (see
Figure 16).79 In our own experiments on electrodes with a hydrophobized carbon surface, we found that the oxygen reduction products
form small crystallites on the hydrophobized surface instead of a
dense film. Open pores between the crystallites may ensure charge
and mass transfer to the reaction site. Nevertheless, conclusive evidence of this coating-passivation correlation is still lacking in the
literature.
Besides sophisticated air electrode geometries it may be possible
to increase the conductivity of the reaction product by doping. Hummelshøj et al. predict by DFT calculation of the Li2 O2 density of states
that the introduction of Li-defects (vacancies) increases the density
of states around the Fermi energy.5 The calculated density of states
for pure Li2 O2 and Li2 O2 with Li vacancies is shown in Figure 17.
If one can find a way to create Li-vacancies in-situ during discharge
the maximum capacity could be increased and the subsequent charge
is believed to be facilitated. Similarly, introduction of dopants during discharge could facilitate continuous Li2 O2 growth. A practical
solution for achieving such defects is unavailable.
Another possibility to make the discharge product more conductive is to operate the Li/air cell at elevated temperatures (e.g., 60◦ C)
because the conductivity of the insulating or semiconducting dis-
R13
Figure 15. Evolution of Li2 Ox discharge product morphology on carbon nanofibers (electrolyte = 0.1 M LiClO4 in DME). Insets show the corresponding
discharge voltage profile. (a–b) Galvanostatic discharge to a capacity of 350 mAh/g carbon at 68 mA/g carbon. Li2 O2 particles appear to first form on the carbon
nanofiber sidewalls as small spheres with ≤100 nm diameters. (c–d) Intermediate galvanostatic discharge to 1880 mAh/g carbon at 64 mA/g carbon. Particles
appear to develop a toroidal shape as the average particle size increases to 400 nm. (e–f) Full discharge to 7200 mAh/g carbon at 63 mA/g carbon. The discrete
particles merge to form a monolithic Li2 O2 mass with low porosity. Caption and figure reproduced from reference 78.
charge product increases with temperature. First measurements in our
laboratories indicated an increase of the discharge capacity by 50%
compared to room temperature. For certain electrolytes a significantly
(∼500 mV) lower charge potential was also observed. While O2 solubility and transport of both O2 and Li+ would also be improved,
operating Li/air cells at elevated temperature could make the search
for a stable electrolyte even more challenging.
Outlook: Overcoming product resistivity and transport limitations is
the key to achieving high-energy nonaqueous Li/air cells.—Understanding the morphology and conductivity of the discharge product
in Li/air cells will help pave the way to high-capacity nonaqueous
Li/air batteries. Experimental measurements of film conductance, using both flat and roughened substrates, as well as first-principles and
Figure 16. Discharge curve for uncoated and coated carbon in a gas diffusion
electrode. The different slopes, indicated by the black lines, suggest that the
coating mitigates electrical passivation, leading to a larger discharge capacity.
Reproduced from reference 79.
continuum scale modeling of film conductance and growth are new
and open areas of research that could help improve this understanding. In addition, there is currently no systematic understanding of the
growth process of Li2 O2 and the morphology of particles. This is a
key area for exploration.
The use of solubilising electrolyte solvents or additives may reduce
or eliminate the product resistivity problem, while the development of
novel electrode architectures could enable high pore volume utilization by avoiding or in spite of passivation. Appropriate pore structures
are required to avoid pore blockage, and a combination of wetting
Figure 17. Calculated density of states for (a) pure Li2 O2 and (b) Li2 O2 with
a concentration of 1/16 Li vacancies. The black curve shows the DFT singleparticle spectrum and the red curve shows the GW quasiparticle spectrum. In
(a), the top of the valence bands have been aligned and in (b) the Fermi levels
have been aligned. Reproduced from reference 5.
R14
Li2O 2 cell before dis charge
CC
Li/LiMO 2 cell before dis charge
CC
Li metal
(w/ 20%
exces s )
CC
LP S L P os itive electrode G DL
(70 vol. % gas )
Li2O 2 cell after dis charge
CC
CC
Li metal
(w/ 20%
exces s )
LP S L
P os itive electrode
(65 vol % MO 2)
Li /LiMO 2 cell after dis charge
CC
CC
20% LP S L P os itive electrode G DL
(65 vol. % Li2O 2)
exces s
Li metal
CC
20% LP S L P os itive electrode
exces s
(65 vol. % LiMO 2)
Li metal
Figure 18. Schematic showing the significant amount of volume change that occurs when Li metal cells are discharged. The cell on the left has a high-capacity
Li/air active material (Li2 O2 ) while the cell on the right has a lower-capacity intercalation active material (LiMO2 ). LPSL = Lithium Protection Separator Layer.
CC = current collector. GDL = gas diffusion layer. The layer thicknesses are drawn in proprtion.
Significant volume changes in Li/air cells need to be
accommodated.— In a Li/air cell both the Li metal electrode and
the positive electrode have materials that undergo significant volume
changes. In earlier sections we discussed the formation of new phases
in the positive electrode, and in the section on aqueous Li/air cells we
will discuss the formation of a solid LiOH · H2 O phase. However, in
this section we focus on quantifying the magnitude of volume changes
for a cell with the parameters given in Table II, and then critically reviewing solutions that help manage those significant volume changes.
Volume changes in Li/air cells are particularly challenging because
the cell sandwich accumulates mass (oxygen) during discharge and
releases mass during charge. Note that it is important to distinguish
between electrodes that undergo significant volume change and a cell
that undergoes significant volume change. Balanced volume changes
at each electrode are possible if the density of the products and reactants are matched, but this is not true for the active materials alone
in the case of Li/air cells. We consider the general topic of reversibly
accommodating major volume changes in solid systems to be a highimpact area for research.
The high capacity of the cathode materials and the formation of new
phases make volume changes significant.—A cell with a Li metal
negative electrode undergoes significant changes in the thickness of
the cell sandwich during a cycle, provided a significant amount of
the Li metal is actually cycled. Volume changes are a feature of any
metal electrode in which cycling involves plating/striping, but are
particularly significant in the case of Li metal because the density of
Li metal is so low (0.534 g/cm3 at 25◦ C). Remarkably, the density of
the Li (in mol Li/cm3 , which is directly proportional to the capacity
density in mAh/cm3 ) stored in Li2 O and Li2 O2 is significantly higher
than in pure Li metal, as shown in Table I. Indeed, the Li/air cell is
an interesting case of a cell that increases in mass and decreases in
volume during discharge. In a Li metal cell with a positive electrode
that intercalates Li and has minor volume changes (<15% in many
positive-electrode intercalation materials), the Li metal is the only
region of the cell undergoing significant volume changes. However, in
a Li/air cell the discharge process involves the creation of a new phase
rather than the incorporation of Li atoms into a pre-existing phase.
The creation of a new phase in Li/air cells implies that electrolyte
displacement will occur unless the new phase displaces a gas phase.
Figure 18 shows a diagram of the volume changes in a Li2 O2 and
a Li/LiMO2 cell for the cell specifications given in Table II; for the
Li2 O2 cell it is assumed that the positive electrode begins with a
volume fraction of gas of 0.70 that is pushed out of the cell sandwich
during the discharge process by the Li2 O2 that forms.
A more quantitative depiction of volume change is given in
Figure 19. The results in this figure depend strongly on assumptions about the initial components in the positive electrode, particularly on the quantity of electrolyte vs. gas phase in the charged
cell. As discussed above, for our “practical” nonaqueous Li/air cells
we assume the active materials that are produced displace a gas
phase rather than electrolyte, while in the aqueous cases we include an initial reservoir of H2 O. The results in Figure 19 show
that the volume change is more significant for Li2 O than Li2 O2
1.0
Final / initial volume
and nonwetting pores may enable high transport rates for both lithium
ions and oxygen to the reaction sites during discharge. We reiterate
that it is important to report directly the volume fraction of product
in the discharged electrode (or cell), or provide sufficient information
for the reader to do so. Reporting only the capacity in mAh/g carbon
is not enough to determine whether an experimental cell can actually
achieve a high capacity and energy. However, reporting the mAh/cm2
and the electrode thickness is sufficient.
0.8
0.6
0.4
Cell sandwich alone
Including H2O reservoir
0.2
0.0
Li2O
Figure 19. Plot showing the volume change of four Li/air cells and a Li/LiMO2
cell. LiOH has not been observed to precipitate and is included only for comparison. Volume change is calculated as discharged volume / charged volume. The
LiOH · H2 O case shows the volume change when an H2 O reservoir is excluded
and included (it is assumed the H2 O is stored outside the cell sandwich).
because of the higher capacity density of Li2 O. The volume changes
for the LiOH · H2 O, LiOH,and Li/LiMO2 cases are similar. The volume change for LiOH · H2 O and LiOH are smaller than the Li2 O and
Li2 O2 cases because their capacity density is lower than that of Li2 O or
Li2 O2 (see Table I) and therefore less Li is required. In the LiOH · H2 O
and LiOH cases, when taking into account the volume of water in a
reservoir in a charged cell the overall volume change of the cell is
larger. Note that we exclude the additional volume from packaging in
Figures 18 and 19.
Pressure should be applied to cells changing volume to maintain good
contact.—Given the significant volume changes that occur in Li-metal
cells it is important to maintain good contact between the layers of the
cell sandwich in order to prevent significant contact resistances from
arising. In particular, maintaining a good solid-solid contact between
Li metal and the lithium protection separator layer is important, as
there will be a great deal of lithium transport across that interface.
Application of external pressure on the cell, depending on its design,
may alleviate this variability.62 Another way of dealing with this issue
is to use a block copolymer that maintains good adhesion through the
ability of the conducting block to flow.81
Minimizing changes in contact resistance may prove more difficult for wound cells. Establishing relationships between mechanical
design and cell performance and durability will become an important
area of research after fundamental chemical challenges are overcome.
It is important to quantify the evolution of contact resistances with
cycling and the influence of pressure on this evolution.
Volume change can be accommodated through the use of a flexible seal.—How can cell packaging respond to significant volume
changes? One promising idea for dealing with volume change is to
use a flexible seal that allows the cell sandwich to move without providing significant stress on the cell packaging. Such a sealing method
has been outlined by the PolyPlus battery company in a patent.82 The
patent outlines a method to maintain good ionic, mechanical, and
electrical connectivity between a Li metal electrode and both the Li
metal backplane and a Li protection layer. However, this approach has
so far been applied only to planar double-sandwich cells. Wound and
stacked cells may require more intricate packaging.
Volume change can be accommodated through the use of an electrolyte
reservoir.—If a Li/air cell is discharged to the point that the active
material being formed (e.g., Li2 O2 ) displaces all of the gas volume, it
will begin to move electrolyte within or out of the cell sandwich. If
the electrolyte remains within the cell sandwich and is pushed toward
the Li metal, it may lead to poor contact between the Li metal and
lithium protection separator layer or between the lithium protection
separator layer and the positive electrode. Alternatively, it may be
drawn off in a direction perpendicular to the direction of current flow.
Pollard and Newman treated an electrolyte reservoir in the context of
the Lithium-Aluminum, Iron Sulfide cell.83
Outlook: The operating principle and durability of cells that repeatedly undergo significant volume changes have to be evaluated.—Accommodating the significant volume changes in both the Li metal
electrode and within the positive electrode in a highly reversible manner is one of the principal challenges for the successful commercialization of Li/air cells. Most Li/air cells demonstrated thus far have
cycled only a small amount of Li, although primary cells have discharged significant thicknesses.9, 14, 84 While there are promising ideas
for maintaining good seals and contact between cell layers, this is an
area in which major progress will need to be made.
Li metal electrodes are chemically and morphologically unstable in most electrolytes and must be protected.— Reversible charge
storage in the Li metal electrode is accomplished by plating (depositing) and stripping (dissolving) Li on/from the surface of the electrode.
This deposition-dissolution reaction can be simply modeled by an
appropriate kinetic expression, such as the Butler-Volmer equation.
However, two factors complicate the process. First, the nonaqueous
liquid electrolyte commonly used in Li metal cells is unstable at the
potential of Li, and a passivating film (the solid electrolyte interphase,
R15
or SEI) must form at the surface in order to stabilize the interface. The
SEI imparts additional ohmic and mass-transfer resistance to dissolution and deposition of the metal. Second, the plating and stripping of
Li inherently involves change in volume of the active material. This
volume change often involves roughening of the Li surface, evolution of the metal morphology (i.e., formation of grain boundaries),
and macroscopic shape change. In addition, the contact resistance between the Li and the electrolyte or separator may change depending
on the direction of current, electrode state of charge, age of the cell,
and pressure applied to the cell sandwich. Although SEI formation
allows for research on nonaqueous Li/air cells without ex-situ application of protection layers, SEIs are not robust against volume and
surface changes of the Li anode.85 Hence, formation of an SEI should
be inhibited by using a solid electrolyte separator that is stable against
Li. This solid electrolyte should also provide mechanical resistance to
Li morphology development and roughening.
We note that despite decades of interest and development, thus far
there has been no successful and widespread introduction of secondary
Li-metal cells into the market. Research on methods to enable the use
of Li metal in secondary cells remains a high priority.
It is challenging to form stable SEIs on Li metal.—Li is an alkaline
metal and hence quite electropositive, chemically reactive, and rather
susceptible to oxidation. This property, which makes it very attractive
for use as a negative electrode, also makes it prone to react with other
components in the cell. Although passivation layers on Li were previously examined in aqueous16, 17, 86, 87 and nonaqueous88 electrolytes,
Peled coined the now popular term “Solid Electrolyte Interphase”
(SEI) for the layer that forms on the surface of Li metal via decomposition of the electrolyte,89 which is unstable at the Li potential.
Comprehensive reviews of the SEI, which forms on lithium, graphite,
and other Li-insertion anodes, include those of Aurbach,90 Balbuena
and Wang,91 Winter et al.,85 and Ogumi and Inaba.92
The SEI on Li differs significantly from that on graphite in that it
forms immediately upon contact between the electrode and electrolyte.
Hence, it is much more challenging to control the chemistry and
morphology of the SEI during formation on Li. Rather, the electrolyte
salt, solvents, and additives must be selected carefully to achieve a
highly stable SEI.
Evolution of the SEI on Li is also very different from that on
graphite due to the dramatic volume change of the electrode. Depending on the chemistry, temperature, mechanical design of the cell,
and cycling conditions, the Li-metal SEI may evolve more or less
dramatically along with morphology changes in the underlying metal
(see Figure 20). If films are not elastic enough to accommodate the
volume change of the active material, fracture and reformation or
thickening of the film may occur as the cell is cycled.93 Spotnitz modeled SEI growth on graphite using an empirical relationship that takes
into account the rate-dependence of film fracture.94 However, detailed
modeling of the SEI during evolution of the Li metal morphology is a
challenging endeavor that has apparently not yet been undertaken.
Dendrites and morphology changes limit cyclability.—The most common failure mode for cells with Li metal anodes is that of dendrite
growth and increase in electrode surface area. Needle-like dendrites
can grow through the separator during charging of the cell, resulting
in an internal short. “Soft shorts” that burn out rapidly result in a temporary self discharge of the cell, while “strong shorts” consisting of a
higher, more stable contact area can lead to complete discharge of the
cell, cell failure, and possibly thermal runaway. While dendrites grow
through the separator during charge, shorts can sometimes develop
during discharge depending on the external pressure placed on the
cell and/or internal volume changes that occur in both the negative
and positive electrodes.
Because Li metal is highly electronically conductive, surfaces tend
to roughen as the metal is plated and stripped. Peaks in the surface
grow as dendrites during charge; while the surface is smoothed during
discharge, some roughness typically remains at the end of discharge,
and, depending on the depth of discharge, the overall roughness can be
amplified from one cycle to the next. Because the metal is essentially
R16
Figure 20. Illustration of differences in SEI formation and evolution on the surfaces of (a) graphite and (b) metal (Li or Li-alloys). Reproduced from reference 85.
at the same electrochemical potential throughout, potential and, to a
lesser extent, concentration gradients in the electrolyte phase drive the
change in morphology. Previous Li dendrite growth modeling work
has shown that the moving front of a dendrite tends to accelerate during
cell charge due to the higher current density localized at the dendrite
tip relative to its base.95 Application of thermodynamic models has
shown that dendrite initiation (i.e., initial roughening of an almost
perfectly smooth surface) can be suppressed by applying mechanical
stress and selecting solid electrolytes with shear moduli on the order
of 10 GPa at room temperature.96, 97 The same models indicate that
surface tension at metal-fluid interfaces is insufficient to suppress
dendrite initiation.96, 97
Related to dendrite initiation and growth is development of the
Li morphology, which tends to increase the electrode surface area
with cycling and consumes solvent to generate fresh passivation
layers. Formation of high-surface-area mossy Li tends to occur during low-rate deposition from a liquid electrolyte, especially if the salt
concentration is high.98 The high surface area combined with high
reactivity of Li and flammability of the organic solvent makes for a
very reactive and dangerous cell.
Sion Power has reported a diminishing grain size in its Li/S cells
without application of external pressure to the cell. This results in
a gradual increase in the volume of the anode as the cell is cycled.
Moderate pressures (∼10 bar) tend to mitigate this type of morphology
development (see Figure 21). Sion Power also proposed a strategy to
minimize the surface morphology changes in the Li anode, which
consists of ensuring complete stripping and re-plating of all lithium
on the rigid current collector in an anode-capacity-limited cell.62
Modeling needs in the area of morphology change include models
of grain nucleation and growth during Li deposition, which include
the influence of pressure; cell-level aging models that account for
solvent consumption at the anode; and cell-level electrochemome-
chanical models that account for gradual swelling of the anode with
cycling.
Alloying Li with other elements such as Mg may help elevate
the surface energy and reduce morphology changes,99, 100 and more
studies are needed in this direction.
Shape change can occur due to nonuniform current densities.—Provided that Li dendrites and other forms of microscopic morphology
development can be suppressed, macroscopic changes in the shape of
the anode may occur due to nonuniform current density distribution
throughout the cell. When there is high resistance to electronic current
flow from the tab to the outermost edges of the current collector (e.g.,
if the current collector thickness is too small relative to its area), the
current density may be very nonuniform, with a higher current density near the tabs. Hence, Li will deposit and dissolve preferentially
near the tabs, as depicted conceptually in Figure 22. Application of
pressure may alleviate this problem.
Li-conducting solid electrolytes can provide chemical and mechanical
protection of Li metal.—Because of the enormous challenge involved
in stabilizing the Li surface chemically and mechanically through
the use of electrolyte additives, such that passivation remains in effect over hundreds to thousands of cycles, the preferred treatment for
rechargeable Li-based cells is the use of a solid-electrolyte membrane
that is mechanically robust and chemically stable against both electrodes. Such a barrier removes several simultaneous constraints that
the liquid electrolyte otherwise must satisfy, but the requirements for
its properties are nonetheless multifaceted and challenging to obtain in
a single material. The barrier must be chemically stable with respect to
some or all of the following: the liquid electrolyte in the positive electrode, electronic conductors and catalysts in the positive electrode,
the metallic Li negative electrode, reactive species such as oxygen
molecules and reaction intermediates, and (in aqueous cells) water.
Solid electrolytes must also have sufficient Li+ conductivity over the
operating temperature range of the cell, negligible electronic conductivity, and high elastic modulus to prevent Li dendrite initiation.
In order to provide cheap, robust, lightweight protection, a method
must be developed to produce relatively thin (< 50 μm), pinhole-free
solid-electrolyte layers at a reasonable cost.
Al tab
Al tab
Cathode
Cathode
1000 cycles
Separator
Separator
Li metal
Cu tab
Figure 21. SEM images of Li foil anodes, cycled 50 times, (a) with and
(b) without application of 10 kg/m2 nonisotropic pressure. Reproduced from
reference 62.
Li metal
Cu tab
Figure 22. Depiction of macroscopic Li redistribution in a cycled Li/air cell
at the end of charge.
R17
Figure 23. Ionic conductivities of several classes
of solid electrolytes (LLTO = (La,Li)TiO3 ; LAGP
= Li1+x Alx Ge2−x (PO4 )3 ; PVdF-HFP = poly
(vinylidene fluoride)-hexafluoropropylene; PEO
= polyethylene oxide). Reproduced from reference
103. Figure numbers refer to those in the original
paper.
Mechanical stability is another important aspect of Li-metal electrode protection that influences the age and safety of the cell. The
two primary classes of Li solid electrolytes, inorganic ceramics and
solid organic polymers, have different mechanical properties. Ceramics have high elastic moduli and are thus more suitable for rigid flat
cell designs. Polymers are softer and more flexible, but at the same
time less chemically and thermally stable, and also susceptible to
absorption and transport of liquids. Flexibility is an advantage due
to the high anode volume change, and in wound designs in which
changes in the separator curvature could cause fracturing of ceramic
membranes. At the same time, the lower shear moduli of polymers
are likely insufficient to prevent Li metal dendrite initiation,97 leading
to safety concerns due to possible electrical shorts. Elasticity may be
important in order to reduce fracture and tearing after multiple cycles
and to achieve wound cell designs.
A number of candidates that satisfy some of the requisite properties
for Li protection have been proposed.10, 82, 101–128 For extensive reviews
see references101–103 and references therein.
A variety of inorganic compounds (sulfides, oxides, phosphates)
in crystalline, polycrystalline or amorphous morphologies, as well
as solid dense polymer-based materials, have been investigated with
conductivities at room temperature ranging from 10−8 to 10−2 S/cm
(see Figure 23).10, 82, 101–128 Most inorganic crystalline and glass materials have lower conductivities than liquid electrolytes, by at least 1-2
orders of magnitude. It is worth mentioning that because Li motion
in solid state systems is a thermally activated Arrhenius-type process,
conductivity increases with temperature, sometimes by two orders of
magnitude or more over the range 0 to 200◦ C. While operating a Li/air
cell at elevated temperature (>80◦ C) may increase the rate capability
and capacity, this presents numerous engineering challenges at the
system level.
Li3 N has high conductivity (∼10−3 S/cm at room temperature),
but is unstable at high potentials (>0.445 V vs. Li).101, 104 Li3 P
has an order of magnitude lower conductivity, but is stable up to
2.2 V.101, 105 The Li analog to sodium β-alumina, Li2 O·11Al2 O3 , has a
high room-temperature single-crystal conductivity of 3×10−3 S/cm,
but is extremely hygroscopic and challenging to prepare dry.101, 106
The so-called Li Super-Ionic CONductors (LiSICONs) are γII Li3 PO4 type oxysalts101, 102 (e.g., Li14 Zn(GeO4 )4 107, 108 ) that contain
interstitial Li ions, but show a drop in conductivity over time at low
temperature because of Li trapping by the immobile sublattice via defect complex formation.101, 109 Room temperature conductivities are
generally less than 10−4 S/cm.101 Bates and coworkers found that ra-
dio frequency magnetron sputtering of lithium silicates, phosphates,
or phosphosilicates resulted in N2 incorporation to form LiPON, an
amorphous analog to LiSICON.110 Thin-film batteries with Li anodes
and Lithium Phosphate OxyNitride (LiPON) separators have demonstrated thousands of cycles.111, 112 However, mechanical stability sufficient for long cycle life has not been established in a thick-electrode
cell design, and the low conductivity of LiPON (2×10−6 S/cm at
25◦ C110 ) precludes the development of cells with thick LiPON membranes.
Thio-LiSICON, the S analog to LiSICON (e.g., Li4 GeS4 113 ), can
achieve high room-temperature conductivity and low activation energy (e.g., 2.2×10−3 S/cm and 20 kJ/mol for Li3.25 Ge0.25 P0.75 S4 102 ).
Glass ceramics with structures related to thio-LiSICON exhibit even
better performance (3.2×10−3 S/cm, Ea = 12 kJ/mol for 70Li2 S30P2 S5 102, 114 ). For these systems glass ceramics have roughly an order of magnitude higher conductivity at room temperature than their
amorphous counterparts (glasses), although there are several exceptions to this trend.103 Despite high conductivities (10−3 S/cm at room
temperature), ceramic films are not easy to fabricate and often have
poor chemical durability.115
Early work on the Li analog to NaSICON, LiM2 (PO4)3 , led to
discoveries of some materials with high conductivity, but poor chemical stability against Li (M = Ti116, 117 ), and others with good stability, but poor conductivity (M = Ge118, 119 ). It was found that Al
substitution via solid state reactions resulted in NaSICON structures
(Li1+x Alx Ge2−x (PO4 )3 , or LAGP) with 4 orders of magnitude higher
conductivity.120
Workers at Ohara found that by heat treating glasses they
formed glass ceramics including a NaSICON crystalline phase
(Li1+x Alx Ti2−x (PO4 )3 , or LTAP), with a high conductivity up to
1.3×10−3 S/cm, depending on heat-treatment temperature.115, 121 Not
only is the material stable against Li metal, it is also stable in aqueous solutions, making it a suitable candidate for aqueous Li/air cells,
and was recently shown to be stable in acidic solutions.122 However,
this material tends to be difficult to manufacture with large area and
low thickness. The general composition of water-stable glass ceramics produced by Ohara and used by PolyPlus in its aqueous Li/air
and Li/water cells is Li1+x (M,Al,Ga)x (Ge1-y Tiy )2−x (PO4 )3 , where x<
= 0.8, 0< = y< = 1, and M is a member of the Lanthanide series;
and/or Li1+x+y Qx Ti2−x Siy P3−y O12 , where 0 < x < = 0.4 and 0 < y <
= 0.6, Q = Al or Ga.10, 82, 121, 123
The garnet family of ceramics have lower conductivity than
NaSICON-type phosphates and perovskite-type oxides (8×10−4 S/cm
R18
at 300 K124 ), but they have a lower grain boundary resistance and are
stable against Li metal.102 The garnet chemical space is still being
widely explored.
Grain boundaries often limit the conductivity of solid electrolytes;
for instance, perovskites (ABO3 ) such as Li0.5−3x La0.5+x TiO3 125–128
have bulk conductivities as high as 1×10−3 S/cm, but with grain
boundaries the composite conductivity is only 2 × 10−5 S/cm.125, 126
The grain boundary contribution to total resistance is commonly sensitive to the ceramic composition and depends on foreign phases that
precipitate at the boundaries. Thus making progress in ionic conductivity of a ceramic polycrystalline material requires understanding
optimization of each contribution separately. To get a clear picture on
the limitations of bulk transport, one needs to synthesize and perform
transport measurements on single crystal samples, which is often very
challenging. First-principles computations of hopping barriers and
molecular dynamics offer an alternative route which is proving useful
to understand atomic-level mechanisms.
Once the bottlenecks in a given material are identified, it is critical
to engineer the composition and fine-tune the synthesis process to
optimize the microstructure and density. Even more difficult is to
achieve these optima for a film that is only 50 μm thick. Thin-film
manufacturing processes, which vary from sputtering to chemical
vapor deposition, tend to be expensive or result in inhomogeneous
coatings.
Multi-layer encapsulation of Li is needed to meet stability and
impedance requirements.—It is unlikely that a single material will be
discovered that satisfies all the requirements for protecting Li metal
in Li/air cells. Most Li/air cell designs to date involve the use of interlayers between Li and the solid-electrolyte membrane that are stable
at the Li potential and reduce considerably the contact resistance.
PolyPlus has patented a method for producing a thin layer (∼0.2 to 1
μm) of Li3 N between Li and the water stable NaSICON-type separator (LTAP) supplied by Ohara.123 This interlayer reduces the contact
resistance that results from decomposition of the LTAP surface exposed to Li. Electricite de France (EDF) has deposited a thin layer
(0.5 to 2 μm) of LiPON onto LTAPf to prevent degradation of the separator; however, imperfections in the coating allowed reaction of Li
with the LTAP during cycling, ultimately resulting in the formation of
cracks in the separator, and Li morphology development and detachment from the separator were observed.129 Nonaqueous electrolytes
have also been used to eliminate contact issues between Li and the
separator.1, 27, 130
In its Li/S cells, Sion Power uses multi-layer membrane assemblies
including ceramic and polymer components to prevent reaction of the
electrolyte with Li,62 which may contribute to morphology development and capacity fade. Polymer components may provide sufficient
flexibility to achieve wound cell designs, while ceramic components
may provide improved stiffness to prevent dendrite initiation.
Outlook: A low-resistance multi-layer solid-electrolyte encapsulation
of Li metal is required for stable operation and good performance.—
We identify three critical issues in Li metal protection: good ionic
conductance, chemical stability, and mechanical stability. There is
often a tradeoff between the three properties, depending on the cell
design and positive-electrode electrolyte chemistry. In-situ protection
via an SEI is unlikely to prevent morphology development during
cycling. Solid-electrolyte membranes that are flexible, strong, thin,
pinhole free, easy to manufacture, and have high conductivities are still
missing. More experimental and computational work to understand
and design such materials is needed in the direction of mechanical
strength and ionic conductivity. Interlayers between Li metal and
the solid-electrolyte membrane are required to improve the stability
and decrease the contact resistance of the interface. Application of
pressure may be required to prevent Li metal morphology changes
during cycling.
f While the authors of reference 129 refer to the material as “Lisicon,” their reference
to an article from PolyPlus and the fact that no other Li-protection material besides
Ohara’s LTAP is known to be stable in water suggest that this is a misnomer. We shall
assume that LTAP was intended.
Cell impedance must be significantly reduced to achieve adequate
power density and efficiency.— In addition to providing high specific
energy and cyclability, the specific power (or power density) and
energy efficiency of the Li/air cell must be acceptable for the intended
application. We shall assume that this potentially high-energy system
is mainly used in high-energy applications, such as electric vehicles,
portable electronics, stationary storage, and potentially hybridized
applications, in which the Li/air battery provides the base load while
a second battery or a capacitor provides load leveling.
A long-range EV with advanced high-energy batteries can be expected to travel roughly 387 miles on a single charge. Assuming
300 Wh of energy are required to propel the vehicle one mile, and
that only 83% of the battery’s energy is utilized, this range can be
satisfied by a 140-kWh battery. If the maximum pulse discharge (i.e.,
acceleration) and charge (i.e., regeneration) power is 140 kW, then
the maximum pulsing rate of the battery should be 1C, while the average discharge rate, assuming a maximum sustained freeway speed
of 77 mph, could be as high as C/5. Average charging rates could
be even higher, but we assume that overnight or partial charging can
be tolerated for drivers demanding long range. A hybrid storage system, consisting of a high-energy, low-rate battery and a low-energy,
high-rate battery or capacitor could limit the high-energy battery rate
requirement to the average discharge C rate. It is clear that the required
power capability for a long-range battery strongly depends upon the
vehicle range, charging strategy, and level of hybridization. However,
we can conclude that EV storage systems incapable of achieving ∼C/5
continuous discharge are uninteresting, and that systems that achieve
> 2C pulsing are not required. Hence, the target battery-level specific
power is 140 to 1400 W/kg for a 700-Wh/kg battery, depending on the
degree of hybridization. Our cell energy calculations indicate that, to
achieve this high specific energy, an area-specific capacity of approximately 40 mAh/cm2 (similar to the capacities for Li2 O and Li2 O2
cells showin in Figure 4, and corresponding to 194 μm of cycled Li)
is required. Hence, the target current density for Li/air cells should be
in the range 8 to 80 mA/cm2 .
The power density of a cell can be evaluated at the cell-sandwich
level as the product of the voltage (the open-circuit potential, U, minus
the cell overpotential, ηcell ) and the current density, i (in A/m2 ), divided
by the cell-sandwich thickness, Lcs :
P̃ =
i (U − ηcell )
L cs
The specific power is obtained from the power density and the
average material density, ρ:
P̂ =
P̃
ρ
For a typical Li/air cell, the average material density is
approximately 1 g/mL (0.53 g/mL for Li, ∼3 g/mL for a
solid-electrolyte separator,120 8.9 g/mL for the Cu current collector, 2.7 g/mL for the Al current collector, ∼1 g/mL for
polymers, water, and nonaqueous electrolytes); by comparison,
Li-ion cells (without Li metal) have an average density of
∼2 g/mL.
The cell overpotential is a function of the current density, cell
design, state of charge, and materials used. As a function of current density, it is approximately linear at low current density and at
high current density increases exponentially due to increasing masstransfer limitations. Hence, the optimum power is a balance between
overpotential and current density. Requirements of acceptable energy
efficiency make this a constrained optimization (see Figure 24). Even
if the cost of energy is not of concern, a lower system efficiency implies a higher rate of heat generation during charge and discharge, and
therefore a higher cost, weight, and volume of the thermal management system.
Several phenomena limit the rate capability of Li/air cells.—While
the major rate-limiting processes include poor oxygen reduction and
evolution reaction (ORR and OER) kinetics and large ohmic drops
1.0
efficiency
0.8
0.6
0.4
0.2
~
P
η cell
0.0
current density
Figure 24. Representative qualitative dependence of overpotential (ηcell ),
power density ( P̃), and discharge voltage efficiency upon current density. The
vertical dashed line indicates the current density below which the efficiency is
at least 80%.
through protection layers on top of Li metal, there are a number of
processes that contribute to the poor rate capability and efficiency of
Li/air cells. Impedance contributions in a battery can be classified as
kinetic, ohmic, or mass-transport limitations. The kinetic impedance
for elementary electron-transfer reactions tends to obey the ButlerVolmer expression, with approximately linear behavior at low surface
overpotentials and logarithmic (or Tafel) behavior at high overpotentials. However, ORR and OER in the Li/air system consist of multi-step
mechanisms that exhibit more complex behavior.
At the negative electrode, the kinetics of Li deposition and dissolution are typically described by a Butler-Volmer expression, with
an exchange current density that can depend on the electrolyte. Depending on the electrolyte, electrode preparation, and measurement
technique, reported values of the Li exchange current density are generally on the order of 1 to 10 mA/cm2 .131–133 Assuming charge transfer
is described by Butler-Volmer kinetics with a symmetry factor of 0.5,
these current exchange densities imply that 100 to 250 mV of overpotential will drive a current density of 100 mA/cm2 , which is much
higher than typical Li/air current densities.
In aqueous Li/air cells, the kinetics of LiOH · H2 O precipitation and
dissolution could limit the achievable specific power, as is discussed
in a later section.
Ohmic limitations include the high-frequency impedance of the
electrolyte, the ionic resistance of separators or membranes that are
single-ion conductors, and contact resistances. Nonaqueous electrolytes exhibit a conductivity of about 0.01 S/cm (1M LiPF6 in
PC/EC/DMC) at 25◦ C,134 while aqueous systems exhibit a conductivity of 0.35 S/cm (saturated LiOH in H2 O) at 25◦ C.7 Li-conducting
ceramic separators have conductivities of 10−6 to 10−2 S/cm at
25◦ C.110, 115, 120, 121 Some Li-conducting layers can be made thin
(< 10 um)110, 111 so as to minimize the overpotential associated with
the separator; however, such thin separators have not been successfully
employed in Li/air cells. Conduction of electrons through the carbon
matrix in the positive electrode, Li metal, current collectors, and tabs
can also contribute to the cell impedance, but these contributions are
minor for a well-designed cell.
Contact resistances also tend to be ohmic in nature (i.e., with
overpotential proportional to current density). These resistances can
appear at a number of different interfaces within the cell, including
weld points, the Li/separator interface, interparticle contacts in the
carbon matrix of the positive electrode, and between the insulating
discharge product and carbon matrix. Because of the large volume
R19
changes that occur during charge and discharge, some contact resistances, especially at the Li/separator interface, may evolve during
charge/discharge and as a function of cycle number. Likewise, gradual
physical separation of the discharge product from the carbon matrix
may result in significant changes in contact resistance as the cell is
cycled.
Mass transfer limitations are those that involve concentration gradients resulting from finite diffusivities in multicomponent solutions.
For instance, when oxygen is consumed at positive electrode reaction
sites during discharge, it must be replenished by oxygen diffusing
through the liquid electrolyte. Because the solubility of oxygen in
nonaqueous electrolytes is limited,20, 21 even the relatively high oxygen diffusivity is insufficient to maintain the reaction site oxygen
concentration near its nominal bulk value in flooded cells discharged
at high rates. Hence, the ORR kinetics may suffer from a diminished
local O2 concentration. Limitations on the gas-phase oxygen transport
in diffusion media and Li-ion transport in the electrolyte can also impact the cell impedance, but these contributions are typically smaller
than that of oxygen transport limitations in the electrolyte in flooded
cells.
Passivation of the positive electrode by an electronically insulating
discharge product is a complex phenomenon that may involve kinetic,
ohmic, and mass-transfer processes. At this stage, the phenomenon is
not well understood, although it has been identified as a main contributor to the limited capacity and power capability of nonaqueous
Li/air cells. Empirically, the impedance of the discharge product appears to increase exponentially with film thickness, at least in some
solvent-salt combinations.23
In nonaqueous Li/air cells, current densities as high as 1 mA/cm2
have been demonstrated.135 However, the capacity begins to fall off
at current densities above about 0.1 mA/cm2 , depending on the cell
design and other parameters.23, 135 Moreover, most measurements in
the literature have been carried out in carbonate-based solvent systems, which show no evidence of reversible chemistry. There are
limited data available for noncarbonate systems. Passivation of the
electrode surface has the strongest influence on rate capability in
carbonate systems.23 There is some evidence that it also plays a
role in noncarbonate systems.136 If passivation can be overcome
then the introduction of oxygen gas channels should allow sufficient quantities of oxygen into the cell to support any reasonable
current density for a Li/air cell, as hydrogen-oxygen fuel cells can
operate at over 1 A/cm2 while much lower current densities are expected for Li/air cells given their significantly higher area-specific
impedance.
Aqueous systems, without O2 transport limitations or passivation,
have achieved current densities of 15 mA/cm2 in primary applications with protected lithium electrodes137 and 2 mA/cm2 (at 30◦ C)
in secondary applications.129 However, these high current densities
have not been demonstrated over large capacity windows. In primary applications, where cells are designed to discharge over weeks
or months, typical average discharge currents and specific power
are 0.5 mA/cm2 and 2 W/kg, respectively.14, 84 By way of comparison, more than 100 mA/cm2 has been demonstrated for rechargeable
Zn/air systems using a bifunctional air electrode.138 Likewise, current densities well over 100 mA/cm2 have been achieved in primary
Li/air cells without protection layers.87 This underscores the limitation associated with the water-stable Li protection layer, which
has a Li conductivity of up to 3.5 × 10−4 S/cm at 25◦ C, but
due to its fragility is difficult to manufacture at thicknesses below
∼150 μm.139 At 50 mA/cm2 , this represents a voltage drop of 2.15 V
through the separator.
Oxygen reduction and evolution are kinetically slow; the nature of
electrocatalysis is unclear given that a solid product is generated.—
In order to improve air electrode performance and selectively assist the formation of the desired reaction products, recent research
has concentrated on the use of various metal and metal oxide particles in the carbon matrix.22, 24, 33, 34, 44, 141 Despite these efforts, the
molecular-level mechanistic processes that could explain the experimental observations (e.g., reduction of overpotentials for the charge
R20
Conductivity (S/cm)
Mass breakdown: Total = 1.5963 kg/m2
Binder, etc.
Separator too Separator too
heavy
heavy
-2
10
Electrolyte
Separator
150 m
100 m
-3
10
50 m
Anode cc
Cathode cc
Anode
10-4
-5
10
0
0.2
0.4 0.6
C rate
0.8
1
Cathode
Figure 25. (Left) The ionic conductivity of the separator required to attain a cell-sandwich specific energy of 1250 Wh/kg at the given C rate and separator
thickness. The conductivity of Ohara’s LTAP is indicated as a dashed line at 3.5e-4 S/cm. For a 150-μm separator, there is a C rate above which no cell design,
even if the separator had infinite conductivity, will achieve 1250 Wh/kg because the separator is too heavy. (Right) The mass breakdown by component in the
discharged state is shown for this limit. Assumptions are described in the text. cc = Current Collector.
and discharge reactions and/or extended capacity upon discharge of
the Li/air cells) are not yet clearly understood. In particular, if a
solid, insoluble product forms and remains at the reaction site the
catalyst may be quickly covered, such that the catalytic activity is reduced. Nevertheless, several reports on catalysis have been published.
Débart et al.34 have shown that a cell containing α-MnO2 as the catalyst
significantly increases the discharge capacity of the Li/air cell. The authors suggested that the improvement in the capacity of the cell is due
to the presence of a tunneling structure that has the ability to accommodate in close proximity the Li+ and O2− ions, which in turn leads to
a subsequent incorporation of Li+ and O2−
2 into a compact film. Such
proximity and incorporation is not possible in other manganese oxide materials. More recently, Lu et al.33, 142 incorporated bifunctional
catalysts into Li/air cells. By implementing a nano-structured PtAu/C
bifunctional catalyst into their cells, the authors were able to lower the
charge voltage and raise the discharge voltage, in order to obtain one
of the highest round-trip efficiencies of rechargeable Li/air batteries
reported to date. While there have been reports on catalysis, a recent
paper by McCloskey et al gives compelling evidence that catalysis
observed in carbonate systems only aids electrolyte decomposition,
while in DME no catalysis is observed for Au, Pt, or MnO2 .177
Thus far, the ORR and OER kinetics have not been well characterized for this system, although Xu and Shelton143 have recently
applied DFT methods to investigate the Li-based oxygen reduction
reaction (Li-ORR) on two different metallic surfaces, Au (111) and Pt
(111), and found that Au(111) is the most active surface for Li-ORR.
Their results also indicated that on both metallic surfaces, lithiation
significantly weakens the O–O bond and most likely will lead to the
formation of monoxides (Lix O), which will tend to aggregate to form
cluster-like oxides. To the best of our knowledge, this is so far the only
study that has used theoretical approaches to understand the mechanistic details of the electrochemical reactivity of the Li/air cell on
metallic surfaces, suggesting the need of further reports on this topic.
Reducing the thickness of lithium-protection layers can significantly
improve rate capability.—Solid Li-ion conductors generally have Liion conductivities at least an order of magnitude lower than liquid Li
electrolytes. One of the most attractive Li-protection layers, due to
its relatively high conductivity (>10−4 S/cm) and stability against a
variety of solvents, including water, is the class of glass ceramics from
Ohara.115, 121, 139, 140 Unfortunately, this material is brittle and difficult
to manufacture at thicknesses below 150 μm.
We used a 0D Matlab model to provide a rough estimate of the
separator materials design required to attain cell-level performance
targets. An optimization routine in Matlab (fmincon) was used to
compute the minimum ionic conductivity in the separator required
to obtain a cell-sandwich specific energy of 1250 Wh/kg at speci-
fied values of C rate and separator thickness.g The positive electrode
thickness and carbon loading were free to change in order to obtain
this optimum. Losses in the cell, which increased with C rate, were
included in the calculation of delivered specific energy (i.e., the discharge energy was 1250 Wh/kg, while the nominal specific energy
was higher). To underscore the critical influence of the separator, only
ohmic losses were considered in the optimization. In a real cell, kinetic losses, and potentially mass-transfer limitations, would further
lower the delivered specific energy of the cell. The resulting discharge
efficiency for all optimized cell-sandwich designs was between 75
and 85%.
Figure 25 shows the conductivity of the separator required to
attain a cell-sandwich specific energy of 1250 Wh/kg at various C
rates in a Li/air cell with a solid-state separator. We assume that the
density and conductivity of the electrolyte in the positive electrode
are 1 g/mL and 0.01 S/cm, respectively, that the density of the
separator is 3 g/mL, and that the current collectors are Cu and Ni. An
anode/cathode capacity ratio of 1.2 was fixed for the optimization.
The results imply that decreasing the thickness of a lowconductivity separator may be a more effective route to attaining
high practical specific energy at moderate C rates than increasing the
conductivity of a thick separator. For a 150-μm (100-μm) separator,
it is impossible to achieve the target practical specific energy at a
discharge rate above 0.4 C (0.7 C) with any cell design (i.e., the maximum falls below 1250 Wh/kg). Put simply, a thick separator, even
if highly conductive, requires correspondingly thick electrodes in order to achieve high energy density, but thick electrodes imply a poor
power density. On the other hand cells with thin separators and thin
electrodes could achieve both high energy density and power density.
Outlook: Power density improvements require eliminating passivation in nonaqueous systems and reducing the thickness of lithiumprotection layers.—In nonaqueous systems, passivation phenomena
appear to be the most restrictive with regard to achieving high current density. If passivation can be eliminated, poor solubility of O2 in
nonaqueous solvents may require new cell designs with continuous
gas, electrolyte, and electronically-conductive solid networks in order
to achieve even higher current density. Catalysts that promote facile
ORR and OER in nonaqueous systems may be required to improve
the voltage efficiency of nonaqueous Li/air cells.
In aqueous systems, the high thickness and low room-temperature
ionic conductivity of available water-stable Li-protection layers are
what limit the current density far below that of other metal-air sysg For a cell-sandwich specific energy of 1250 Wh/kg, an 80% cell packaging factor
implies a cell level specifc energy of 1000 Wh/kg. A further 43% increase in mass at
the pack level implies a system energy of 700 Wh/kg.
tems. Preliminary calculations indicate that reducing the protection
layer thickness would have the greatest impact with regard to the
practically achievable specific energy. However, the mechanical
stability of thinner layers must be adequate for the hundreds to
thousands of large-volume-change cycles expected for automotive
applications. Similar requirements may also apply to solid separators
for nonaqueous systems.
Supplying oxygen to the cells requires new membranes or onboard storage.— As a required reactant for discharge, oxygen must
be drawn in from the environment or stored on board the vehicle. The
most attractive scenario is an open system in which the cell “breathes”
oxygen from the ambient air. In this case, specific energies higher than
1000 Wh/kg could be achieved for a Li/air cell, as shown in Figure 3.
Conversely, storing oxygen inside a closed system (directly in the
cells or in a tank) leads to practical specific energies between 500 and
1000 Wh/kg. While a truly “air-based” system would afford higher
specific energy, the system most widely investigated in the literature so
far is the lithium-oxygen cell. In this section we discuss the challenges
related to ambient air operation of Li/air batteries and present some
concepts from recent literature, including membrane solutions as well
as closed systems using pure-oxygen tanks. Note that for an aqueous
Li/air cell it may be desirable for H2 O to enter the cell from ambient
air.
Open systems are susceptible to contaminants and electrolyte evaporation, which result in severe performance degradation.—The design
of an open lithium/air system presents many challenges associated
with preventing contaminants from entering the cell and keeping the
electrolyte in the cell. Any molecules that can conceivably be found
in the atmosphere may enter the system and affect the cell chemistry,
through catalyst poisoning, corrosion, or side reactions. Several undesired side reactions may occur if carbon dioxide, nitrogen, or water
enter the cell. The most important side reactions include:
CO2 :
4Li + O2 + 2CO2 → 2Li2 CO3
[9]
Li2 O + CO2 → Li2 CO3
[10]
Li2 O2 + CO2 → 1/2 O2 + Li2 CO3
[11]
LiOH + CO2 → Li2 CO3 + H2 O
[12]
2Li + 2H2 O → 2LiOH + H2
[13]
LiOH + H2 O → LiOH • H2 O
[14]
6Li + N2 → 2Li3 N
[15]
Li3 N + 3H2 O → 3 LiOH + NH3
[16]
H2 O:
N2 :
If the Li metal anode is protected from the electrolyte in the positive electrode (e.g., with a solid-electrolyte separator), some of the
reactions involving contaminants (9, 13, 15, and 16) can be prevented.
In this case, introduction of CO2 can still result in the formation of
electronically insulating bulk lithium carbonate and, in nonaqueous
electrolytes, H2 O may result in solvent degradation or the formation of
corrosive acids.144–146 Li2 CO3 formation may be partially irreversible,
resulting in capacity fade.
While the reactions with CO2 and N2 only degrade the cell performance, introduction of water is more critical if the negative electrode
is not protected. In this case, additional gas evolution (e.g., H2 and
R21
Table IV. Gibbs free energies of formation for important lithium
compounds, and the reactants from which they are formed that
was used to calculate the Gibbs formation energy. Data taken
from.11
Compound and reactants
Gr (kJ/mol)
Li2 CO3 (Li, CO2 , O2 )
Li2 O2 (Li, O2 )
Li2 O (Li, O2 )
LiOH (Li, O2 , H2 O)
LiOH·H2 O (Li, O2 , H2 O)
Li3 N (Li, N2 )
−809
−571
−562
−320
−329
−129
NH3 ) may lead to a hazardous overpressure and flammable gases may
react with oxygen in a chain reaction.
From a comparison of the Gibbs free energies of formation of the
relevant compounds (see Table IV) one can see that lithium carbonate
is more stable than the lithium oxides while lithium nitride has a
relatively small formation enthalpy. This implies that nitrogen entering
an open Li/air cell may be less critical than CO2 , although the reaction
kinetics must also be considered.
Ideally the cell would include a gas selective membrane that allows
only oxygen to enter and exit the positive electrode. Optimization of
the system implies a balance between effective separation of entering gases and low weight and cost achieved by minimizing system
complexity.
A second critical issue in open Li/air systems is electrolyte evaporation. Most recent studies of Li/air batteries use propylene carbonate as the electrolyte solvent because of its high boiling point
(BP = 240◦ C) and good polarity, although it is chemically unstable,
while some apparently more stable electrolytes26, 48 have lower boiling
points (e.g., acetonitrile, BP = 82◦ C). In order to use the electrolyte
with the best properties it is preferable to avoid the evaporation issue
entirely. A gas selective membrane may serve this function as well.
Besides the fundamental aspect of gas separation and electrolyte enclosure a realistic solution must be able to handle the
expected gas flows at realistic current densities. A 90-kWh electric vehicle Li/air system discharged at 1 C has an oxygen consumption of 3.85 L/s at 25◦ C and 1 atm. Such practical constraints
should be considered together with the gas selectivity of potential
membranes.
Membranes may be an effective means of keeping contaminants out
of the cell and keeping the electrolyte in the cell.—In an open system,
contaminants must be separated from the air that enters the positive
electrode. We preclude membraneless gas separation processes such
as adsorption, absorption, and distillation, the components of which
would likely be too complex, costly, large, and inefficient for automotive applications. Most researchers are instead exploring membranes
implemented at the cell level that prevent ingress of contaminants
present in the air. A further advantage of membranes over external
air filters is that they often prevent evaporation of electrolyte from
the cell. In primary aqueous Li/air cells, desiccants inside the cell
have been used to absorb water vapor from the atmosphere and retain
water,9 but this concept does not appear suitable for rechargeable systems. If membranes are used to purify the air their weight and volume
will need to be included in energy calculations.
Membranes with O2 -selective silicone oils.—Zhang et al. demonstrated an O2 -selective membrane for operation in ambient air of
20–30% relative humidity, as shown in Figure 26.147 The cell design contained a nonaqueous electrolyte and the membrane allowed
permeation of O2 while blocking moisture. Such membranes can be
prepared by loading O2 -selective silicone oils into porous supports
such as porous metal sheets and Teflon (PTFE) films. The O2 gas was
continuously supplied through a membrane barrier layer at the interface of the cathode and ambient air. The authors reported that silicone
oil of high viscosity showed reasonable performance. The immobilized silicone oil membrane (see Figure 26) in the porous PTFE film
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Figure 26. Schematic of two kinds of O2 membrane structures. (a) Liquid
immobilized in homogeneous porous substrate, and (b) liquid immobilized in
silicalite membrane coated porous metal substrate (asymmetrical). Reproduced
from reference 147.
enabled Li/air batteries with carbon black air electrodes to operate
in ambient air (at 20% RH) for 16.3 days with a specific capacity
of 789 mAh/g carbon and a specific energy of 2182 Wh/kg carbon.
Its performance was much better than a reference battery assembled
with a commercial, porous PTFE diffusion membrane as the moisture barrier layer on the cathode, which only had a discharge time of
5.5 days, corresponding to a specific capacity of 267 mAh/g carbon
and a specific energy of 704 Wh/kg carbon.
The silicone based membrane prepared by Zhang et al. obtained
an oxygen permeance of 1.6 · 10−6 mol m−2 s−1 Pa−1 at an O2 /H2 O
selectivity between 1.5 and 3.6. In a different work Reynolds et al.
used a polyperfluorocarbon liquid in a porous Celgard 2500 polymer
substrate but they obtained a four orders of magnitude lower oxygen
permeance of 1.7 · 10−10 mol m−2 s−1 Pa−1 at an O2 /H2 O selectivity
of 3.9.148
Polymer protection membranes.—In another study, Zhang et al. used a
heat-sealable polymer membrane (Melinex 301H) as both an oxygendiffusion membrane and a moisture barrier in the operation of nonaqueous Li/air cells under ambient conditions with an oxygen partial
pressure of 0.21 atm and relative humidity of ∼20%.149 A schematic
is shown in Figure 27. The membrane also minimized the evaporation
of the electrolyte solvent. These batteries operated in ambient conditions for more than one month with a specific energy of 362 Wh/kg,
based on the total weight of the battery including packaging. The
thickness of the polymer membrane was 25 μm, representing ∼1%
of the cell volume (see Figure 27). Current densities between 0.05
and 0.1 mA/cm2 were demonstrated, but at higher current densities
oxygen permeation through the membrane was insufficient.
A promising step toward more suitable membranes has been very
recently reported by MaxPower Inc.150 The authors describe the development of oxygen-selective membranes based on polysiloxane and
methacrylate–polysiloxane copolymers which allow current densities of up to 2 mA/cm2 and protect the cell against humidity and
moisture. Thinner membranes with high O2 selectivity must be developed in order to meet the high power requirements of practical
applications.
Anion exchange membranes for aqueous systems.—For aqueous Li/air
cells, EDF has developed an anion exchange membrane (AEM) between the ORR electrode and discharge product reservoir that allows
transport of OH− , but mitigates influx of O2 or contaminants such as
CO2 .129 Their AEM is composed of interpenetrating polymer networks
(IPN): hydroxyl-conducting polycationic crosslinked polyepichlorhydrine (PECH) provides ionic conduction and poly(hydroxyl ethyl
methacrylate) provides mechanical strength and reduces swelling.
Current densities up to 6 mA/cm2 (∼C/12) were obtained at 60◦ C.
However, some air can permeate the polymer, resulting in slow
degradation. Improvement of the membrane’s ability to screen CO2
could enable the use of more alkaline electrolytes, which are particularly susceptible to Li2 CO3 formation but have a lower OER
overpotential.29
Use of tanks and compressors results in a closed system, protected
from contaminants.—As an alternative to membrane-enabled open
systems, Li/air batteries can be designed as fully closed or partially
closed systems that use oxygen from tanks rather than from the atmosphere. This idea was already discussed in the introduction in the
context of energy calculations, and it was concluded that the added
mass and volume of a tank significantly dampen the appeal of a Li/air
system. The major advantage of using an oxygen tank is that all of the
complexities associated with air purification can be avoided. While
the ultimate commercial viability of a Li/oxygen, rather than a Li/air,
system is perhaps limited, the use of oxygen tanks will be critical during research and development phases to ensure well-defined chemistry
free of possible contaminants from air.
Outlook: Until effective membranes are developed, a bulky closed system may be required.—Thus far no membrane has been completely
effective in preventing the introduction of contaminants into the cell
when ambient air is used as an oxygen source. O2 -selective membranes are preferred for nonaqueous cells, whereas anion-exchange
membranes with good OH− selectivity are an attractive option for
aqueous cells. In addition to achieving good selectivity and preventing solvent evaporation, candidate membranes must enable high rates
of O2 or OH− transport.
Figure 27. Schematic of a sealed test cell used by Zhang et al. for ambient operation of Li/air. Reproduced from reference 149.
Onboard storage of O2 provides a more reliable but bulkier solution
to the contaminant problem. In this case, the O2 tank and compressors
add weight and complexity to the system, and the power required to
run the compressors impacts the overall system efficiency. However,
at least theoretically the efficiency loss may be small, as shown in
Table III, and in open systems efficiency can also be diminished by
resistive separation membranes. The added weight and volume are
a significant barrier to the development of closed systems, as the
achievable specific energy falls close to the threshold that can be
obtained by a Li/LiMO2 cell (∼500 Wh/kg), while the energy density
could be much worse (see Figure 3).
Aqueous Li/Air Systems
One of the major advantages of the aqueous Li/air system is that the
discharge reaction product is soluble in water, eliminating an apparent shortcoming of the nonaqueous Li/air batteries, namely formation
of electrically resistive products that may passivate the air electrode.
Moreover, selection of an electrolyte system that achieves reversible
chemistry does not appear to be a bottleneck for the development of
aqueous Li/air cells. However, effective storage of the discharge product, stability of the separator against both the Li anode and the aqueous
air electrode, and catalysis of both the ORR and OER reactions remain
considerable challenges for rechargeable aqueous Li/air cells.
The operating principle of the aqueous Li/air cell is shown
schematically in Figure 28. Under the proposed scheme, the metallic
lithium anode is protected by a Li-ion conducting ceramic film, which
prevents the vigorous reaction of metallic lithium with water. Another
important characteristic of these cells is the need for catalysts in the
positive electrode that reduce the activation barriers for both ORR
and OER.
The idea of using a protected lithium electrode (PLE) was introduced in 2004 by the PolyPlus Battery Company,10, 82, 151 who demonstrated long-term stability, high-discharge capacity, and some cyclability of lithium metal in aqueous solutions when the anode was protected
by a water-stable glass-ceramic electrolyte (LTAP) from Ohara.115, 121
Imanishi et al. have also looked at the concept of using protected
Li anodes to fabricate rechargeable Li/air batteries using an aqueous electrolyte.152, 153 More recently, Zhou and co-workers introduced
the concept of a Li/air system including an organic/aqueous hybrid
electrolyte. In this case, the catalytic reduction of O2 occurs in an
aqueous electrolyte, while the metallic lithium remains in a nonaqueous (organic) electrolyte; the two electrolyte systems are separated
by a LiSICON separator.1, 27 Several other groups have adopted approaches similar to PolyPlus’ PLE, many of which rely on NASICONtype lithium conductors.129, 130, 140
Figure 28. Schematic of an aqueous Li/air cell with electrically isolated cathode and anode in the positive electrode. Reproduced from reference 27.
R23
The reaction mechanisms for aqueous Li/air batteries are different
from those of the nonaqueous Li/air batteries and can be summarized
as follows:
Positive electrode :
O2 + 2H2 O + 4e− ↔ 4OH−
Negative electrode :
Li ↔ Li+ + e−
[17]
[18]
The overall cell reaction is shown in equation 3.
During the discharge process, O2 is electrocatalytically reduced to
produce hydroxyl ions (OH− ) at the positive electrode (Equation 17),
while Li+ ions are generated (Equation 18) at the negative electrode.
LiOH is soluble in water, but precipitates to form LiOH·H2 O above
the solubility limit (5.25 M at 25◦ C).7 During the charge process, O2
gas is generated at the positive electrode and Li is plated at the negative
electrode. Catalytic materials are needed to accelerate the kinetics of
both ORR and OER. The electrocatalytic reduction of O2 in aqueous
solution often requires the use of expensive catalysts such as Pt,152, 153
particularly in acidic electrolytes.29
Like the nonaqueous Li/air cell, the rechargeable aqueous system is
also at a development stage, and new concepts continue to be explored.
Here we discuss two challenges specific to the aqueous Li/air system:
controlling the location and morphology of the precipitated discharge
product, and durably catalyzing both the ORR and OER.
Precipitation and dissolution of the discharge product must be
managed to enable high energy and acceptable power.— The discharge process in the aqueous Li/air cell differs from that of the nonaqueous cell in that the discharge product, LiOH, has a relatively high
solubility. Rather than forming a film directly on the surface of the
positive electrode, the dissolved product can reach a concentration of
approximately 5.25 M (at room temperature) before it precipitates as
LiOH · H2 O.154 A phase diagram is given in Figure 29. When cycled
Figure 29. Phase diagram for the LiOH ± H2 O system, reproduced from
reference 154. The solid lines are fits to the data, which comes from various
sources given in reference 154. The eutectic is at a temperature of 255.15 K
and a composition of 5.27 mol/kg, while the peritectic is at a temperature of
382.05 K and a composition of 7.70 mol/kg. The phase diagram shows that at
temperatures below the peritectic the monohydrate LiOH · H2 O is the favored
form of LiOH, while above that temperature anhydrous LiOH is favored. The
use of supporting salts (such as LiCl or NaOH) could change the phase diagram.
R24
to the solubility limit, the theoretical specific energy of the system is
430 Wh/kg; hence significant formation of LiOH · H2 O is key to attaining an attractive specific energy. Because precipitation and dissolution of LiOH · H2 O does not involve electron transfer, the solid
product need not be in contact with the positive electrode. This poses a
challenge, as impedance rise and restriction of accessible porosity can
result from an uneven product distribution, but also provides many
opportunities for novel cell and system designs as well as battery
management system algorithms155 that control the discharge process.
In order to achieve high cell specific energy, it is desirable to fill
a significant fraction of the available reservoir or positive-electrode
pore volume with the solid discharge product. Figure 3 shows that
when 65% of the volume of a 200 μm positive electrode is filled
with LiOH · H2 O a specific energy of about 700 Wh/kg results at the
cell level for a practical design. If only smaller volume fractions are
achieved the cell-level specific energy will be even lower. The objective of maximizing the discharge capacity of the cell by going to
a high volume fraction of LiOH · H2 O must be balanced against the
undesired impedance rise resulting from restriction of the pores. Ideally, the pores should fill uniformly with discharge product in order to
maintain a homogeneous current distribution through the cell separator. Preferential precipitation of the product in certain locations due
to gravitational fields, concentration and thermal gradients, current
heterogeneity, or electrolyte flow would tend to amplify nonuniformity of the current distribution and result in more rapid impedance
rise, and at worst could reduce the accessibility of large regions of
the reservoir or electrode. Current nonuniformity would also result
in uneven Li plating/stripping during charge/discharge. Coverage of
the anode protection layer or positive electrode with a dense layer of
monohydrate can likewise result in impedance rise and a premature
end of discharge.
As with nonaqueous systems, aqueous Li/air cells involve substantial volume change, and the precipitation of large amounts of
solid LiOH · H2 O within the pores of a reservoir or electrode could
impart deleterious mechanical stress on the system. So far high cell
reversibility involving large volumes of LiOH · H2 O (more than a few
percent of the porosity) has not been demonstrated.
Some existing prototype cells demonstrate high discharge capacity
but have limited reversible capacity.—While much of the recent Li/air
literature has focused on nonaqueous systems, several companies have
published the results of their aqueous Li/air research and development.
Although each company describes a unique cell design, a common
feature is the concept of a discharge product reservoir that is distinct
from the positive electrode. PolyPlus has demonstrated the discharge
of primary aqueous Li/air cells in which discharge product is stored
in a porous reservoir of the cathode (see Figure 30).84 One of their
patents describes a porous zirconia felt reservoir that expands with the
discharge product as it precipitates and is filled with a catholyte that
contains hygroscopic salts used to draw in water from the atmosphere.9
While this decreases the mass of the cell (in the charged state) and
prevents evaporation of catholyte, the use of such salts may not be
appropriate for rechargeable cells.
Toyota has demonstrated reversibility of aqueous Li/air system including precipitation/dissolution of LiOH.130 A symmetric aqueous
cell with Pt electrodes and Ohara separator was used to show the reversible exchange of 200 mAh capacity in 5M saturated solution. The
200 mAh of charge passed corresponded to precipitation/dissolution
of solid LiOH in/from one of the two electrodes. Raman spectra of the
solids confirmed LiOH as the product, and formation of O2 bubbles
was observed during dissolution of the LiOH. In a second experiment,
two nonsaturated solutions were used, and the electrodes were cycled
between 4 and 5 M without precipitation. In a second cell type, the
air electrode consisted of LaSrCoO3 , carbon, and PTFE on carbon
paper, and the anode consisted of Li metal in a LiTFSA/PC electrolyte, with an Ohara separator between the electrodes. In this case,
34 mAh were passed, corresponding to 30 mg of LiOH (composition
confirmed by XRD). During charge (from a second cell constructed in
the discharged state, with solid LiOH in the cathode), the solid LiOH
was consumed.
EDF has proposed solutions to several critical LiOH storage issues
that enable up to 100 reversible charge/discharge cycles (100 cycles
at 0.1 mAh/cm2 , 40 cycles at 2 mAh/cm2 ).129 Improvements include
application of an anionic polymer layer on the air electrode side of their
LTAP separator, which prevents nucleation and preferential deposition
of a resistive LiOH layer at the LTAP/reservoir interface. Without this
layer an exponential rise in overpotential with time results. Humidified
air is supplied to the air electrode, where oxygen is reduced and
reacted with water to form hydroxyl ions (Equation 17) which in turn
are conducted through the IPN to the reservoir. The IPN mitigates
influx of CO2 , which reacts with LiOH to form Li2 CO3 , resulting in
cell degradation. An oxygen evolution electrode is embedded in the
reservoir to enable recharge of the cell (see Figure 31).
By using a transparent cell housing EDF observed the formation of
a solid discharge product at the bottom of the reservoir (see Figure 31).
Thus far, only a small amount of solid LiOH · H2 O, relative to the
total pore volume of the reservoir, has been cycled. In cells that were
initially charged to 73 mAh/cm2 (for an available cell specific energy
of 500 Wh/kg), 40 cycles were achieved at a depth of discharge (DOD)
Figure 30. Cross section of PolyPlus’ aqueous Li/air cell. Reproduced from
reference 84.
Figure 31. Schematic of EDF’s rechargeable aqueous Li/air cell. Reproduced
from reference 129.
of 2 mAh/cm2 (13 Wh/kg) and 100 cycles were achieved at a DOD of
0.1 mAh/cm2 .129
While the current EDF cell has a vertical orientation, such that
LiOH · H2 O settles to the bottom of the cell and evolved gases exit the
top of the cell, this has the disadvantage that a non-uniform current
distribution may result, with more current flowing in the top region
of the cell that does not contain LiOH · H2 O. This would lead to a
non-uniform thickness of Li metal as a function of height in the cell.
An alternative orientation would be to place the cell in a horizontal
configuration such that the LiOH · H2 O precipitate would settle either
on the AEM or the LTAP. This should result in a more uniform current
distribution but may make it more difficult for oxygen produced at the
OER electrode to exit the cell. The incorporation of the OER electrode
in the cell sandwich may also be more difficult.
High reversible capacities must be demonstrated.—While Toyota and
EDF have shown good reversibility of the Li/air system involving a
small amount of solid discharge product, PolyPlus has demonstrated
large discharge capacities in a primary cell. Further research on the
reversibility of LiOH · H2 O reservoirs may pave the way to a cyclable high-energy Li/air cell. There is as yet limited understanding
of where the solid product precipitates and how this affects current
distribution in the cell. The mechanical stability of candidate porous
reservoirs, such as expandable zirconia felt, over many cycles should
be investigated. The influence of current density, reservoir geometry
(porosity and thickness), and cell orientation with respect to gravitational field on the product distribution requires a more comprehensive
analysis.
Discharge product morphology impacts recharge rate capability.—
During charge the solid particles of LiOH · H2 O must dissolve and the
LiOH must diffuse to the OER electrode. The rate of particle dissolution will depend on their size and shape, as well as the porosity of
the precipitate layer that forms. Providing forced convection in some
way could significantly improve the dissolution rate. One important
question is whether, if the LiOH · H2 O particles that form are very
small, they will undergo Ostwald ripening during a rest step. If this
were to occur the larger particles would take longer to dissolve. There
could also be changes to the porosity of the layer that would impact
the dissolution rate. We consider the issue of the physical form of
LiOH · H2 O deposits to be a critical area for research for aqueous
Li/air cells.
Novel system concepts may involve external LiOH · H2 O reservoirs.—
While reservoirs internal to the cell sandwich have been the focus
of recent aqueous Li/air cell developments, earlier work on metal-air
systems included electrolyte circulation,29, 87 which may allow for external reservoirs. Storing the product outside the path of ionic current
flow would ensure that the pores of the separator and positive electrode remain open for ionic transport. Sufficiently high flow rates,
relative to the applied current density, should minimize LiOH concentration gradients orthogonal to the applied electric field and maintain
a uniform current density.
External storage of the product could be enabled by a flow-through
cell connected to a storage reservoir, in which the aqueous LiOH
solution is continually circulated between the two. Several general
concepts could be explored for separating the monohydrate product
from the circulating stream, including gravitational (e.g., the use of a
settling tank as proposed for Al/air cells156 ), mechanical, evaporative,
and thermal separation, as well as filtration. The energy requirements
for the separation process and for maintaining flow should be included
in overall energy efficiency calculations.
Controls strategies involving thermal management and current profiles could be explored as a means to improve the uniformity of product
distribution in the reservoir.
Outlook: LiOH · H2 O formation appears to be reversible; highcapacity and long-term cycling need to be demonstrated.—There has
recently been rapid progress in demonstrating reversibility and identifying challenges to long-term cyclability of aqueous Li/air systems.
Several companies have proposed promising engineering approaches
to improve the energy density (adding desiccants to the discharge
R25
product reservoir) and durability (polymer protection of the reservoir
to separate it from the anode protection layer and the air electrode) of
aqueous Li/air cells.
Success of companies like Sion and PolyPlus in protecting Li metal
could be leveraged to increase cyclability of the negative electrode,
which appears to limit cell life. Other cycle-life limitations involving
storage of LiOH · H2 O may become apparent as the anode reversibility
is improved.
The solutions proposed here should be attempted in rechargeable
Li/air cells in which a much higher fraction of the theoretical capacity
is cycled. Volume changes associated with high-capacity charge and
discharge may require significantly different cell designs from those
proposed today.
Charge and discharge need to be catalyzed to reduce kinetic
losses.— In the aqueous Li/air system, where OH− is the cathodic
reaction product, the O-O bond must be broken during oxygen reduction and reformed during oxygen evolution. These bond-breaking and
bond-forming reactions may result in a considerable overpotential increase relative to the ORR and OER in nonaqueous systems, in which
Li2 O2 may be formed and one of the O-O bonds possibly remains
intact.
In order to lower the overpotential of the ORR, there are few alternatives to Pt with comparable performance, particularly in acidic
environments.29 In alkaline electrolytes, most materials are unstable
at the anodic potential required for the OER, and even Pt dissolves.157
Hence, to avoid expensive catalysts, it may be necessary to use an
alkaline electrolyte and electrically isolate the ORR and OER electrodes.
Alternatively, a bifunctional electrode consisting of two different
catalysts for ORR and OER may be feasible, provided the active
components for the reduction and evolution of the O2 molecule are
stable over a wide range of potentials, from 0.6–0.7 V (RHE) during discharge (ORR), to over 2.1 V (RHE) during charge (OER).158
Such bifunctional electrodes are already employed in metal-air
systems.138, 159, 160
Work on PEM and other fuel cells has significantly reduced the Pt
loading required for ORR.—Investigation of ORR mechanisms in fuel
cell cathodes is much more advanced than what has been recently proposed for metal-air batteries. ORR has been extensively studied in the
field of fuel cells because of its fundamental complexity, sensitivity
to the electrode surface properties, and slow reaction kinetics. It has
also been demonstrated that the ORR is a significant component of the
cell overpotential and limits the efficiency of electrochemical energy
devices that use air as the oxidant.161 Despite the complexity of this reaction, two mechanisms have been proposed for oxygen reduction: the
so-called direct 4-electron pathway, in which peroxide is not formed,
except as a possible adsorbate; and the peroxide pathway, involving
a 2-electron reduction of O2 to form peroxide, which subsequently
reacts electrochemically to form H2 O or chemically to form H2 O
and O2 .162 The 2-electron pathway is more common in alkaline solutions and on mercury, gold, carbon, and most transition metal oxides,
whereas the 4-electron pathway proceeds via dissociative adsorption
on noble metallic catalysts and some transition metal catalysts and
macrocycles.162, 163 To obtain the maximum redox efficiency and to
avoid the corrosion of the electrode material by the peroxide intermediate of the 2-electron reaction mechanism, it is preferable to achieve
a 4-electron ORR. Therefore, finding suitable inexpensive catalysts
that promote direct 4-electron reduction of the O2 molecule is still an
active area of research.
Bifunctional electrodes are used in Zn/air systems.—Metal-air systems involve a significant leap in complexity compared to fuel-cell
systems because OER must be catalyzed in addition to ORR. Since
the ORR reaction mechanisms have been discussed in the previous
section, here we describe the reaction pathways involved in oxygen
evolution. According to Jörissen,162 the OER mechanisms are rather
complex and susceptible to change depending on the electrode potential. On metallic surfaces, the rate-determining step for the OER is the
R26
electron transfer from H2 O in acidic media or OH− in basic media
to form adsorbed OH radicals at the electrode surface. OER has also
been investigated on metallic-oxide surfaces;164 in these systems the
intermediate formation of H2 O2 from metal-desorbed OH species has
been suggested to be the rate-determining step. Upon formation of the
physisorbed H2 O2 species, O2 evolves in the final step of the proposed
OER mechanism.164
Without a third electrode in the cell, the reduction and evolution
of oxygen in rechargeable (secondary) metal-air batteries requires
bifunctional air electrodes, which contain separate ORR and OER
catalysts connected electronically. Dating back several decades,159, 160
bifunctional electrodes have emerged as a practical solution to the
lack of highly active and stable bifunctional catalysts for both ORR
and OER. For example, it is well-known that the most effective ORR
catalysts are those based on platinum (Pt),165 but Pt has only moderate
activity for the OER. On the other hand, ruthenium (Ru) and iridium
(Ir) oxides are among the best OER catalysts,166 but they are not
as active as Pt for ORR. Although, alloys of these compounds have
shown a better bifunctional-catalytic performance,167 the development
of bifunctional catalysts for practical applications still represents a
challenge as the best catalytic materials still consist of precious metals,
which are in turn scarce and expensive.
Since the composition and design principles used in bifunctional
electrocatalysts for metal-air batteries have been recently reviewed by
Neburchilov et al. and Jörissen et al.162, 168, 169 here we make only a
few remarks relevant to the Zn/air cell.
While several metal-air chemistries have been proposed, Zn/air
batteries are the most mature technology and have contributed significantly toward the development of metal-air batteries.158, 169 The
overall discharge reaction can be summarized as follows:170
Positive electrode :
Negative electrode :
O2 + 2H2 O + 4e− → 4OH−
Zn → Zn2+ + 2e−
[19]
Zn2+ + 4OH− → Zn(OH)2−
4
[20]
−
Zn(OH)2−
4 → ZnO + H2 O + 2OH
[21]
Net reaction :
2Zn + O2 ↔ 2ZnO
[22]
In contrast to the Li/air cell, hydroxyl ions generated at the air
electrode migrate to the Zn anode compartment to complete the cell
reaction, with a cell equilibrium potential of 1.65 V.170
In spite of the challenges involved in catalysis for secondary Zn/air
batteries, some success (150 cycles) with cells that contain bifunctional air electrodes has been reported by the ReVolt Company.138 In
their work, a bifunctional electrode is constructed with a catalyst that
promotes ORR (MnSO4 , Fe, Pt, and Pd among other materials) and
a bifunctional catalyst that shows high reaction rates for both ORR
and OER (for example, La2 O3 , Ag2 O, Ag, spinels and perovskites).
According to Neburchilov et al., although several companies have
patented bifunctional electrodes (ReVolt among others), all of them
have insufficient ORR catalytic lifetimes at the high OER potential
(∼2.1 eV).169 It is therefore reasonable to expect some delay in the
commercialization of the Zn/air batteries while inexpensive, abundant,
active, and stable combinations of catalytic materials are sought.
3-electrode cells remove restrictions on ORR and OER catalysts, but
complicate the system design.—Perhaps the most promising solution
to the catalysis of ORR and OER in aqueous Li/air cells is the use
of two separate and electrically isolated electrodes for charge and
discharge. This sidesteps the catalyst stability issue, as the catalysts
appropriate for ORR are never polarized to the high anodic potentials
required for OER. Several groups have designed aqueous Li/air cells
with this configuration.27, 129 An additional advantage of this configuration in the EDF cell is that the OER electrode can be located between
the IPN and product reservoir, thereby avoiding the overpotential associated with OH− transport through the IPN during charge.129
Figure 32. Activity versus the experimentally measured d-band center relative
to platinum. The theoretical results are shown in black while the experimental
results are shown in red. Reproduced from reference 173.
However, the use of two oxygen electrodes adds mass to the cell
and may significantly lower the system specific energy. It also requires
increased complexity and cost of the electronics used to control the
system.
Computational screening could enable the discovery of suitable ORR,
OER, and bifunctional catalysts.—First-principles modeling of electrocatalytic reactions is fast becoming an indispensable tool for achieving insight into the reaction mechanisms of complex electrochemical
environments.171, 172 Significant efforts in this area have led to detailed interpretations of the experimental data and to the computational design of new catalytically active materials for the ORR in
fuel cells.173, 174 In this work, many authors have extensively used
the concept of “volcano” plots to predict the activity and selectivity
of catalysts for ORR. Volcano plots (shown in Figure 32) illustrate
very well Sabatier’s principle for catalytic materials, which states that
an effective catalytic surface should reach a compromise between
having enough strength to break the bonds of the molecular adsorbates and yet generate weakly-bonded intermediate species that are
not overly stabilized by the surface.175 As an example, Figure 32
(reproduced from 173 and 174) presents the volcano plot for the electrocatalytic reduction of O2 on various Pt-complexes. The peaks in
Figure 32 represent those catalytic surfaces with the largest electrochemical activity for ORR in fuel cells.
The values used to construct such plots are now readily attainable
from first-principles modeling techniques;174 it is therefore conceivable that such computational-based approaches could be used to explore the catalytic activity of various metallic surfaces for both the
ORR and OER. In turn, this possibility provides some hope for a
more versatile design strategy for bifunctional electrodes or bifunctional catalysts for the aqueous Li/air system.
Outlook: Bifunctional electrodes or two-positive-electrode architectures are required.—Unless bifunctional catalysts are discovered
that promote facile kinetics for both the ORR and OER in aqueous air electrodes and are stable over the potential window 0.6 to
2.1 V vs. RHE, the only viable approaches to making aqueous Li/air
systems rechargeable are bifunctional-electrode and two-positiveelectrode architectures. The former design has already been used with
some success in rechargeable Zn/air cells, although it remains challenging to find a combination of catalysts with fast ORR kinetics, stability at the OER potential, and low cost. Two-electrode designs, which
avoid the kinetics-stability compromise, have been implemented in
Li/air cells at the expense of higher complexity and weight.
Summary and Outlook
Lithium/air batteries are an attractive technology, particularly for
long-range electric vehicles, because of their high theoretical specific
R27
Table V. Requirements for durable, high-energy automotive Li/air batteries (N = nonaqueous only, A = aqueous only).
Requirements for durable, high-energy automotive Li/air batteries
Li anode
Robust and flexible containment, including flexible seals
Li protection layer(s)
transport properties
Sufficient conductivity over T range of interest
Negligible electronic conductivity
manufacturability
Sufficiently thin (<50 μm preferable)
Pinhole free
Low cost
mechanical properties
High elastic modulus (to prevent dendrite initiation)
Flexibility required for wound concepts
stability
Stable against O2 , contaminants (CO2 , N2 , trace H2 O)
Stable against Li
Water stable (A)
Air electrode
Continuous gas and electrolyte networks (N)
High surface area (N)
High pore volume (N)
Bifunctional or 2-electrode design
Membrane
Highly selective for OH- (A)
High OH- transport rates (A)
Good CO2 screening
Good H2 O screening (N)
Product reservoir
Promotes uniform LiOH·H2 O precipitation (A)
Flexible (A)
High pore volume (A)
Maintains electrolyte transport pathways (A)
Catalysts
High activity/mass ratio
High activity/cost ratio
Abundant materials
ORR catalyst
Breaks O-O bond (A, and for Li2 O)
Robust to poisoning (A)
OER catalyst
Promotes O-O bond formation (A, and for Li2 O)
Electrolyte
Adequate Li+ conductivity at all temperatures
Good stability at high temperatures
Low viscosity
Chemically inert in presence of O2 over operating voltage and temperature (N)
System
Closed system with tanks and compressors or open system with highly selective gas or anion-exchange membranes
In flow configuration, highly efficient separation of LiOH · H2 O from electrolyte (A)
energy. While researchers have been aware of and worked sparingly
on Li/air batteries for decades, no one has yet demonstrated a Li/air
cell that is reversible and can be cycled over a significant fraction
of its theoretical capacity. The only short-term commercially viable
products are primary Li/air cells that are designed for high specific
energy but not rechargeability.
Recent investigations of both aqueous and nonaqueous Li/air systems have resulted in a significantly improved understanding of the
main challenges for this technology. Despite several research groups
demonstrating limited cyclability of nonaqueous cells using carbonate
solvents, it is now clear that these solvents participate in the reactions
that consume lithium and oxygen during discharge and are not reversibly generated during charge. Hence, the cycle life of carbonatebased Li/air cells is limited by the quantity of solvent available for
reaction and the buildup of side reaction products. A search for ap-
propriate noncarbonate solvents is underway, but so far adequate reversibility remains elusive.
Once a reversible chemistry is established, the next major challenge that should be addressed for nonaqueous systems is the deposition of electrically resistive products in the air electrode during discharge. Bulk Li2 O2 is electronically insulating and difficult
to solvate; hence it may form an electrically resistive film on the
electrode surface, limiting the achievable capacity far below what
could be obtained by completely filling the pore volume. Appropriate nanostructuring of the air electrode may enable higher capacities in spite of resistive products, while the use of new solvents or additives may enable Li2 O2 precipitation away from active
sites.
Aqueous Li/air chemistry appears to be much more reversible than
nonaqueous Li/air chemistry. LiOH is very soluble (up to 5.25 M at
R28
25◦ C) and precipitates as a monohydrate above its solubility limit
when using pure H2 O as a solvent. The monohydrate can be dissolved reversibly. However, the molar mass of the discharge product,
LiOH · H2 O, is much higher than the discharge product in nonaqueous systems, Li2 O2 , resulting in a comparatively lower theoretical
specific energy. In fact, practical aqueous Li/air cell designs may not
achieve higher energy density than Li/metal-oxide systems. Moreover, breaking and forming of the O-O bond on discharge and charge,
respectively, suggests the need for rate-enhancing ORR and OER catalysts, the stability of which may preclude the use of a bifunctional
electrode.
Controlling the distribution of reaction product is critical for
achieving high capacity and rate capability in nonaqueous and aqueous systems. Provided passivation does not restrict accumulation of
Li2 O2 to the electrode surface in nonaqueous cells, pore blocking
may restrict the accessibility of smaller pores. Poor oxygen transport
could result in higher current density near the gas diffusion layer and
subsequent accumulation of Li2 O2 to block the ingress of oxygen.
Appropriate pore structure and use of both wetting and nonwetting
pores may be necessary to circumvent these issues. LiOH · H2 O tends
to precipitate on the surface of the LTAP separator preferred for aqueous Li/air cells. Polymer coatings on the water side of the separator
can prevent this from occurring. Gravitational effects (i.e., sedimentation) may require special orientation of nonaqueous cells to control
the current density distribution and minimize ohmic and transport
losses.
Massive volume changes at the electrode and cell-sandwich level
may require special cell-design features, including electrolyte or solvent reservoirs, flexible seals, application of pressure, and the use
of components with particular mechanical properties. Recirculating
flow of the catholyte in aqueous cells may alleviate volume change
in the positive electrode. Some components, such as flexible seals for
cells with protected Li metal, have already been developed for primary cells, but reversible cycling must also be demonstrated. Wound
jellyroll designs may be precluded by high volume changes in Li/air
cells.
Another critical system-level issue involves the open nature of
tankless metal-air cells. Air contains contaminants, particularly H2 O
and CO2 , that are very reactive against Li and Li2 O2 . CO2 also reacts with LiOH to from Li2 CO3 . Moreover, evaporation of the solvent
from the positive electrode compartment can occur in an open system. Several membranes have been proposed to avoid contamination
of the cell and evaporation, but there are no reports yet on total effectiveness or the long-term stability of these membranes. In case a
membrane solution is not adequate, a tank and compressor solution
seems feasible, at the expense of some specific energy and energy
density.
Li metal is itself one of the most challenging components of the
Li/air cell, as it tends to roughen and develop dendrites with cycling.
Application of pressure and the use of stiff solid-electrolyte separators can diminish this morphology development, but this results in
an additional challenge for the Li/air system. The separator must be
chemically stable against both electrodes and provide sufficient ionic
conductance over all operating conditions. It should not significantly
reduce the specific energy and energy density of the system (i.e., it
should preferably be thin), but it should also be mechanically robust to thousands of cycles with significant electrode volume change.
It seems lilkely that a multi-layer composite will be the ultimate
solution for Li/air cells, particularly aqueous cells. Several composites meet some of these requirements, but manufacturability remains
uncertain.
Table V summarizes the requirements for the various components
in Li/air cells. Those requirements that apply only to nonaqueous or
aqueous systems are labeled N and A, respectively.
Our prognosis for the rechargeable Li/air system is that it will
primarily remain a research topic for at least the next five years.
Even if the challenges discussed in this review are successfully addressed in that time period, the stringent requirements of durable and
safe operation in an automotive environment will further delay com-
mercialization of Li/air EVs. However, as a long-term solution to
the daunting challenge of low-cost, high-range electromobility, Li/air
batteries remain one of the few and most promising, and funding of
research to accelerate their introduction into the marketplace should
be a top priority for government, academic, and industrial research
institutions.
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A Critical Review of Li/Air Batteries

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