Lithium-Ion Batteries a Look Into the Future
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Cite this: Energy Environ. Sci., 2011, 4, 3287
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Lithium-ion batteries. A look into the future
Bruno Scrosati,*ab Jusef Hassounab and Yang-Kook Sun*b
Received 1st April 2011, Accepted 9th June 2011
DOI: 10.1039/c1ee01388b
A critical overview of the latest developments in the lithium ion batteries technology is reported. We
first describe the evolution in the electrolyte area with particular attention to ionic liquids, discussing
the expected application of these room temperature molten salts and listing the issues that still prevent
their practical implementation. The attention is then focused on the electrode materials presently
considered the most promising for enhancing the energy density of the batteries. At the anode side
a discussion is provided on the status of development of high capacity tin and silicon lithium alloys. We
show that the morphology that is the most likely to ensure commercial exploitation of these alloy
electrodes is that involving carbon-based nanocomposites. We finally touch on super-high-capacity
batteries, discussing the key cases of lithium-sulfur and lithium-air and attempting to forecast their
chances to eventually reach the status of practically appealing energy storage systems. We conclude
with a brief reflection on the amount of lithium reserves in view of its large use in the case of global
conversion from gasoline-powered cars to hybrid and electric cars.
1. Does the actual lithium battery technology complywith the electric road transportation criteria?
Our present energy policy, still mainly based on burning fossil
fuels, inevitably poses a serious concern due to CO2-related
global warming. Accordingly, efforts aimed at ensuring efficient
use of renewable energy sources and replacement of internal
combustion engines with electric motors for the development of
aDepartment of Chemistry, University of Rome Sapienza, Piazzale AldoMoro, 5, 00185 Rome, Italy. E-mail: [email protected]; Fax:+39 06 491769; Tel: +39 06 4991 3530; [email protected]; +3906 4991 3664bDepartment of WCU Energy Engineering and Chemical Engineering,Hanyang University, Seoul, 133-791, Korea. E-mail: [email protected]; Fax: +2 2282-7329; Tel: +2 2220-0524
Broader context
In this review, we discuss the new trends in the lithium battery R &
safety and energy levels higher than those offered by the present tech
renewing the chemistry of the battery, we discuss the approaches un
the anode side, much attention is presently devoted to lithium me
capacity values much higher than conventional graphite. We disc
chemical response may be properly achieved by moving to special m
composites. Electrolyte systems more reliable and stable than th
importance for the progress of lithium battery technology. Goo
accordingly the properties of these solutions and their practical pros
cathode materials, such as sulfur and oxygen, in view of their utiliz
offering energy densities three or four times higher than the comm
This journal is ª The Royal Society of Chemistry 2011
sustainable vehicles, such as hybrid vehicles (HEVs), plug-in
hybrid vehicles (PHEVs) and ultimately, full electric vehicles
(EVs), are in progress worldwide.
Exploitation of alternative, green, energy sources (solar, wind,
geothermal) requires the side support of energy storage systems
that can compensate their intermittent characteristics. It is now
generally accepted that among the various possible choices, the
most suitable are electrochemical batteries. Batteries are portable
devices capable of delivering the stored chemical energy as
electrical energy with high conversion efficiency and without any
gaseous emission. Moreover, batteries offer the most promising
option to power efficiently HEVs or EVs.
In this scenario, particularly appealing are rechargeable batteries
benefiting from high specific energy (high voltage joined with high
specific capacity), high rate capability, high safety and low cost.
D aimed at the development of advanced systems benefiting of
nology. Considering that steps forward may be marked only by
der evaluation for upgrading anode, electrolyte and cathode. At
tal alloys, such as Li4.4Sn and Li4.4Si, since they offer specific
uss the properties of these alloys and show that their electro-
orphological configurations and, in particular, to carbon nano-
e present liquid organic carbonate solutions are also of key
d candidates are solutions of lithium salts in ionic liquids;
pects are discussed. Finally, we examine the use of high capacity
ation for the development of lithium super-batteries capable of
on lithium-ion batteries.
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Fig. 1 Ionic liquids are room temperature molten salts formed by the
combination of a weakly interacting, large cation, e.g. of imidazole type
and a flexible anion, e.g. N,N-bis(trifluoromethane sulphonyl) imide,
TFSI.
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Promising candidates are Li-ion batteries that today exceed at least
by a factor of 2.5 any competing technology thanks to the high
value of energy density, i.e. 150 Whkg�1 and 650 Whl�1.1
Due to their unique features, lithium-ion batteries are the
power sources of choice for the portable electronic market
(including popular products such as cellular phone, laptop
computers, mp3’s, etc), are aggressively entering the power tool
equipment market and, in particular, the emerging sustainable
vehicle market.2 However, the present Li-ion batteries, although
a commercial reality, are not yet at such technological level as to
meet the requirements of efficient hybrid or electric vehicles.
Reduction in cost, enhancement in safety and, especially,
improvement in energy density, are mandatory requirements
Conventional Li-ion batteries rely on a graphite anode,
a lithium cobalt oxide cathode and liquid, organic carbonate
electrolytes.3 At extreme operational conditions, such as at the
upper limit of the charge process, oxygenmaybe released from the
layered LiCoO2 cathode and, in the event of local overheating, it
may reactwith the flammable organic liquid electrolyte, giving rise
to thermal runaway effects, if not even to explosions. Various fire
incidents have been in fact reported during batterymanufacturing
and/or for battery-operated devices. Considering their very
Bruno Scrosati
Bruno Scrosati is Senior
Professor of Electrochemistry at
the University of Rome. He
received the title of Doctor in
Science ‘‘honoris causa’’ from
the University of St. Andrews in
Scotland and the honorary
doctor degree from Chalmers
University of Technology in
Sweden. He was President of the
Italian Chemical Society and of
the Electrochemical Society. He
is European Editor of the
‘‘Journal of Power Sources’’ and
member of the Editorial Boards
of various other international
journals. Bruno Scrosati is author of more 450 publications; 30
books and chapters in books and 22 patents. His H factor is 43.
Jusef Hassoun
After graduating in Chemistry,
Jusef Hassoun worked in
a company for 3 years before
returning to the University of
Rome ‘‘La Sapienza’’ to obtain
the PhD degree in Material
Science in the field of advanced
lithium ion batteries. He is
currently a Postdoctoral
Researcher and has more than
40 publications in international
journals.
3288 | Energy Environ. Sci., 2011, 4, 3287–3295
limited number in respect to the level of production (a few
hundred cases versusbillions of cells per year), these incidents have
not considerably affected the consumer electronic market;
however, they cannot be tolerated in the vehicle market: a fire in
a battery-powered car would be very delecterious, if not fatal, for
the success of the entire electric transportation business. Hence,
full safety achievement is a mandatory goal for Li-ion batteries
directed to HEV or EV use. This is not an easy task. The substi-
tution of LiCoO2 with a chemically more stable material, e.g.
olivine LiFePO4 (the PO4 group has stronger covalent bonds than
the CoO2 one) is not a final solution, due to the reactivity of the
other components and, in particular, the instability and flamma-
bility of the liquid organic electrolyte.
2. Ionic liquids: exciting, safe electrolyte media foradvanced batteries
Much attention is presently devoted to the investigation of
electrolyte systems more reliable and stable than the present
liquid organic carbonate solutions.
Yang-Kook Sun
Yang-Kook Sun obtained his
PhD from Seoul National
University, Republic of Korea,
and is now a Professor of Energy
Engineering at Hanyang
University. His research inter-
ests include metalfluoride-
coated cathodes, lithium transi-
tion metal oxides, olivine-related
cathodes and core-shell with
concentration-gradient mate-
rials for advanced lithium ion
ies, and lithium-metal-free
lithium sulfur and lithium air
batteries.
This journal is ª The Royal Society of Chemistry 2011
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Particularly promising are solutions of lithium salts in ionic
liquids. Ionic liquids (ILs) are room temperature molten salts
formed by the combination of large organic cations (e.g. imida-
zolium or pyrrolidinium cations) and high-charge delocalized
anions (e.g., N,N- iso(trifluoromethane)sulfonimide (TFSI)
anion),4 see Fig. 1. The basic science and electrochemical char-
acteristics of ILs are widely reported and discussed in compre-
hensive reviews4 to which the reader is referred for details.
Indeed, ILs are characterized by many favourable features,
including high conductivity, environmental compatibility and,
especially, high thermal stability.4 In contrast to organic
carbonate solutions that catch fire very easily, IL-based solutions
are not flammable and generally stable up to 300–400 �C. Inaddition, ILs are basically composed of organic ions which may
undergo almost unlimited structural variations because of the
easy preparation of a large variety of their components and this
provides a unique source of materials that can be designed and
selected to fulfil the requests of any desired device.5
Considering these features, ILs are in principle very appealing
lithium ion battery electrolytes. Effectively, ILs are presently
under testing in many industrial and academic laboratories with
the aim of establishing their practical feasibility.6–9 However, the
route to implementation of these systems is still a long one. A lot
remains to be done before ionic liquid-based solutions can find
their successful application as practical lithium ion battery elec-
trolytes. Many basic aspects are yet to be clarified. For instance,
the structure of the IL/electrode interface is still unknown despite
the fact that its full understanding is of key importance in view of
battery applications.4 In addition, IL-based solutions are
apparently not stable at low, reducing voltages, e.g., when in
contact with highly negative electrodes, such as lithium metal
and/or graphite anodes. There is still a debate on whether this
instability has to be assigned to reduction of liable groups in the
IL cation’s structure and/or, more simply, to residual impurities.
Therefore, there is an urgent need to devote more attention to
these aspects by carrying out a series of systematic studies on the
fundamental processes occurring at the solid electrode–ionic
liquid interface. Both experimental (e.g., detailed and systematic
AC impedance studies) and theoretical (e.g. computing models)
approaches may be appropriate for reaching the goal of shedding
some light on the basic electrochemistry of ionic liquids.
Fig. 2 TEM images of a Sn–C composite (a) and of a single Sn particle
(b). Taken from ref. 14 with permission from Wiley-VCH STM
Copyright.
3. The challenge of practical implementation ofsuper capacity tin and silicon lithium alloys
Another key requirement for ensuring profitable use of Li-ion
batteries in electric vehicles is an increase in energy density. It
may be estimated that for ensuring a 150 km driving range with
a single charge for a common, sub-compact passenger car, the
weight of the battery, if based on presently available Li-ion
technology having an average energy density of 150 Whkg�1,
would be over 160 kg, this largely exceeding any practically
acceptable limit. Clearly, advanced batteries, having levels of
energy density two or three times higher than those offered by
conventional systems, are urgently needed. The achievement of
this goal requires the development of electrode materials having
a much higher specific capacity than the conventional ones, still
maintaining the same voltage levels.
This journal is ª The Royal Society of Chemistry 2011
With this in mind, a lot of attention is presently devoted to tin
and silicon, since they are abundant (cheap), environmentally
benign and, most importantly, their alloys with lithium, i.e.,
Li4.4Sn and Li4.4Si, offer specific capacity values much higher
than conventional graphite, namely 990 mAhg�1 and
4200 mAhg�1, respectively, versus 370 mAhg�1. The potentialities
of these materials have been known for some time10,11 but their
exploitation has been until recently prevented by a serious issue
associated with large volume expansion-contraction changes
experienced during the lithium alloying-de-alloying electro-
chemical process that in turn induce cracks and, eventually,
pulverization of the electrode, finally leading it to dying in the
round of a few cycles. The advent of nanotechnology has helped
to control this issue. In fact, by reducing the metal particle size to
a nanometric level, the volume change may be controlled and the
lithium diffusion length greatly reduced, thus improving the
performance of the electrode in terms of both life and rate
capability.12,13 Nanostructured electrodes are not, however,
practically feasible since their large surface area reflects on high
reactivity and on reduction of tap density, with associated
increase in safety hazard and decrease in volumetric energy
density. All these aspects are discussed in detail in recent and
comprehensive reviews12,13 to which the reader is referred for
more comprehensive information.
The breakthrough in this field has been achieved by the
development of cleverly designed carbon-metal composites. By
an oversimplified way, these composites may be described as
formed by low-size metal particles dispersed within a carbon
matrix. The carbonmatrix, while maintaining in its core the nano
sized metal particle configuration that helps in containing the
volume stress, supplies an overall compact structure which
ensures stability and provides high tap density.
It is now widely assumed that the nanocomposite approach is
the most promising one to lead to electrode materials having
practical relevance. The validity of this assumption and of the
nanocomposite concept is supported by two convincing experi-
mental examples. One is given by the tin–carbon, Sn–C
composite, the morphology of which is shown in the TEM
images of Fig. 2.14 The images clearly show that the tin particles
are kept at nano size dimensions (about 10–30 nm) and that they
are evenly dispersed within the carbon matrix. The latter has
a two-fold, critical action, namely it provides enough free volume
to accommodate the tin’s expansion–contraction, this ensuring
cycling stability, and, at the same time, acts as a protecting shell,
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Fig. 3 Characteristics of a Sn–C composite electrode. (a) Capacity
versus cycle number in a lithium cell; (b) in situXRD analysis of a Li/Sn-C
cell from its pristine state and after a full charge–discharge cycle; (c) XRD
evolution of Sn–C composite powder exposed to ambient atmosphere.
Taken from ref. 14 and 15 with permission fromWiley Wiley-VCH STM
Copyright.
Fig. 4 Morphological and electrochemical characteristics of a Si–C
composite. (a) SEM image of the composite particles; (b) cross-section
SEM image of a composite sphere; (c) high resolution TEM image of the
composite; (d) charge–discharge cycle of the Si–C composite in a lithium
cell. Courtesy of Prof. Lee from Kangwon National University, South
Korea, taken from ref. 21 with permission from Elsevier (license #
2647500596262).
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this warranting chemical stability and safe handling of the
electrode.
The above two beneficial actions are clearly demonstrated by
Fig. 3. Fig. 3a shows how the composite can be efficiently cycled
in a lithium cell with a charge (lithium-tin alloying process: SnC
+ 4.4Li / L4.4Sn + C) - discharge (lithium-tin de-alloying
process: L4.4Sn + C / SnC + 4.4Li) coloumbic efficiency
approaching 100%. The long and stable cycle life combined with
high efficiency is a very convincing evidence that indeed the
volume stress is here successfully buffered. Obviously, the
3290 | Energy Environ. Sci., 2011, 4, 3287–3295
delivered specific capacity is depressed with respect to that
associated with pure tin, namely about 500 mAhg�1 versus
990 mAhg�1, due to the weight contribution from the carbon
matrix that is 50% in the case of the Sn–C composite here dis-
cussed. This penalty in capacity is largely counterbalanced by the
excellent electrochemical response of the composite. Note that
the capacity delivered by the composite electrode is still much
higher than that obtainable by a conventional graphite anode.
Fig. 3b, which reports in situ XRD analysis of the Li/Sn-C cell at
its pristine state and after a full charge–discharge cycle,15
demonstrates that the electrochemical lithium alloying–de-
alloying process is fully accomplished and this further confirms
the reversibility of the electrode. Finally, Fig. 3c shows the time
evolution of the XRD patterns of a Sn–C composite sample kept
in open air at room temperature for over a month. No evidence
of peak modification nor appearance of peaks related to tin oxide
was detected after long air exposure or even after heat treat-
ment.14 This result demonstrates the stability of the composite,
which in turn reflects in easy handling, a very important
advantage in view of practical applications.
The interest in Li–Si alloys is even larger than that in Li–Sn
alloys and this is understandable considering that the former
offers the highest theoretical specific capacity so far known for
lithium battery electrode materials. The volume stress issue,
already discussed for tin, also holds for silicon, possibly even at
a higher extent. Indeed, the volume change expected for silicon
upon full alloying with lithium to form Li4.4Si is of the order of
300% while that for tin at the same Li4.4Sn composition is about
250%. Therefore, similar to the tin case, extensive work has been
devoted in the last few years to the design of electrode configu-
rations capable of buffering the volume changes such as to make
also the silicon electrode viable for practical applications.
This journal is ª The Royal Society of Chemistry 2011
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Fig. 5 Scheme and electrochemical process of a lithium–sulfur cell.
Courtesy of Prof. K. Kim, Gyeongsang National University, South
Korea.
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Morphological modifications aimed at developing nanometric
structures, such as nano-sized particles,16 sub-micrometric
pillars17 and nanowires18 led only to limited improvement in
terms of cycle life. Beside, some of these morphologies, e.g.,
silicon nanowires, are dangerous for the human health.
Some advancements have been obtained by using methyl
cellulose CMC as a binder for the Si electrode blend19 and by
exploiting 3D porous Si particle configuration.20 However the
cycle capability and the tap density of these electrodes are both
low, this resulting in a limited life and modest volumetric energy
density, respectively.
As in the case of tin, also for silicon the real breakthrough can
arrive with the development of appropriate carbon composites.
Illustrative in this respect is the case of a recently developed
spherical silicon–graphite–carbon composite.21 The morphology
of this material and its electrochemical response in a lithium cell
are illustrated in Fig. 4. We can see that the composite is formed
of spherical Si particles of 30 nm average size, surrounded by
amorphous carbon that may act as a buffering agent to account
for the volume stress (Fig. 4a–c).
This particular configuration is beneficial in terms of optimi-
zation of the electrochemical response: in fact, Fig. 4c which
illustrates a typical charge–discharge cycle of the composite
electrode in a lithium cell, shows that the voltage profiles are
those expected for the lithium alloying process in silicon,
xLi + Si / LixSi and, importantly, that the initial irreversible
capacity is low. The value of the delivered specific capacity is of
the order of 800 mAhg�1. Considering that the amount of silicon
in the composite is expected to be 30% and that the contribution
of the graphite to the overall capacity may be estimated as
200 mAhg�1, one may assume that for this particular composite
electrode, the Li alloying process is not proceeding to completion
(i.e., up to x ¼ 4.4). The capacity delivery upon cycling and the
coulombic efficiency values, however, are very stable and high,
this confirming that the microstructure of this nano-Si–graphite
composite is effective in buffering the volume expansion–
contraction upon the electrochemical cell. There are also indi-
cations that the amount of alloyed lithium, and thus the value of
the capacity, can be enhanced by optimizing the morphology and
the composition of the composite particles.
The results reported for both tin and silicon confirm that
moving to composite configuration is the right way to bring the
related lithium alloy electrodes to commercial stage, and that the
search for high capacity lithium metal alloy anodes may be
considered successfully completed. In fact, their use is now seri-
ously considered by battery manufactures. An example is
provided by the Sn–C–Co ternary composite that is presently
exploited by a Japanese manufacturer to produce a battery that
goes under the commercial name of Nexelion.22
4. The Holy Grail of batteries
One has to consider, however, that anodes are only part of
a battery system, and that quantum jumps in overall energy
density require also upgrading at the cathode side. So far, Li-ion
battery chemistry has been based on intercalation-type electrodes
that may accept a maximum of one lithium ion equivalent per
mole of the host compound. This necessarily limits the specific
capacity and thus to enhance energy density, new approaches
This journal is ª The Royal Society of Chemistry 2011
must be undertaken. One is to pass from intercalation to
conversion chemistry, namely to electrochemical processes that
can ensure the exchange of much more lithium and thus, to offer
orders of magnitude higher specific capacities.
Along this line, a very promising system is the one provided by
the lithium–sulfur, Li–S cell, characterized by an electrochemical
process involving a two-electron reaction: 2Li + S /Li2S to
which a specific capacity of 1675 mAhg�1(S) and an open circuit
voltage of 2.23 V are associated, this leading to a theoretical
energy density of 3730 Whkg�1, i.e. almost one order of magni-
tude higher than that of conventional, intercalation chemistry-
based Li-ion batteries. The structure of the lithium/sulfur battery
and its related electrochemical process are schematized in Fig. 5.
The concept of this battery is not new. However so far, the
effective development of this high energy system has been pre-
vented by a series of issues. A major one is associated with the
high solubility of the cell reaction products in common lithium
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ion organic carbonate electrolytes. As schematically shown in
Fig. 5, the electrochemical discharge process of the Li/S cell
proceeds through the sequential formation of polysulphides
LixSy which easily dissolve in the liquid carbonate electrolyte
solutions and eventually diffuse to the lithium anode with severe
corrosion effects.23,24 All of these events result in loss of active
materials, low utilization of the sulphur cathode, low overall
coulombic efficiency and, ultimately, in severe capacity decay
upon cycling. Another basic issue is the low electronic conduc-
tivity of S, Li2S and the intermediate Li-S products, which
severely affects the rate capability of the battery. A final problem,
often overlooked, is the use of lithium metal as the preferred
anode, which is known to cause serious safety risks due to uneven
deposition upon charge, which may result in the shortage of the
cell, with associated thermal runaway and, eventually, fires or
explosions.
These hurdles have for some time cooled down the interest in
lithium/sulfur battery. However, the attention towards this
system rose again in the last years due to a series of important
technological breakthroughs. Optimization in the fabrication of
the cathode, passing from simple sulfur–carbon mixtures to
sulfur–carbon composites, proved to be a successful approach
for improving the conductivity of the electrode, as well as for
controlling the solubility of the discharge products.24–26
Another step forward in this technology was obtained by
changing the cathode from the common sulfur–carbon
composite to a lithium sulfide–carbon composite.27 In this way
the cathode becomes the lithium ion source allowing to replace
the reactive lithium metal with any other, more reliable lithium-
accepting material thus obtaining a metal-free, lithium-ion sulfur
battery. This concept was first demonstrated by using a tin-
carbon anode28,29 and was further confirmed with a silicon-
carbon anode.30
These results consistently contributed to improve the tech-
nology of the lithium/sulfur battery; however, some residual
issues still prevent the full practical exploitation of this high
energy battery system. Most of the recent work relies on
conventional liquid organic carbonate solution as the preferred
electrolyte: although the sulfur and/or lithium sulfide are shielded
into a carbon matrix, either mesoporous25 or spherical,31 this in
principle preventing the direct contact with the electrolyte, it is
not yet fully established whether the issue of the solubility of the
polysulfides has been successfully addressed.
Possible steps forward may be achieved by moving from
conventional liquid electrolytes to gel-type electrolyte
membranes of suitable composition. A valid example is provided
by a membrane formed by trapping a lithium conducting organic
solution, added by a dispersed ceramic filler, in a poly(ethylene
oxide)-lithium trifluoromethanesulfonate, PEO-LiCF3SO3
matrix.28,29 One may simply describe this composite electrolyte as
a membrane consisting in liquid zones contained within a poly-
mer envelope. It is assumed that the polymer shell acts as
a physical barrier to prevent contact of the polysulphide cathode
products with the internal liquid solutions. Experimental results
have in fact confirmed the validity of this approach.28,29
Even a more effective action to block the dissolution of the
reaction products is expected to be provided by the use of totally
solvent-free, solid-state, lithium conducting membranes, such as
those formed by poly(ethylene oxide), PEO-lithium salt
3292 | Energy Environ. Sci., 2011, 4, 3287–3295
complexes.32 These membranes have been effectively tested as
separators in lithium–sulfur cells by Jeong et al.33 and more
recently in our laboratory.34 The results are quite encouraging,
demonstrating that full capacity can be obtained by solid-state,
PEO-based polymer Li/S-C batteries.34 An apparent issue is that
these batteries have to operate around 70–90 �C. i.e. in the
temperature range where the conductivity of the solid-state
membranes reaches useful values.32 However, this cannot in
effect be a dramatic penalty if the battery is used for applications
where a moderately high temperature operation can be tolerated,
such as in the electric vehicle area.
Oxygen is in principle a cathode material even more exciting
than sulfur. Its use may lead to the development of lithium-air
batteries having a theoretical energy density reported as high as
that provided by gasoline, although this assessment is somewhat
questionable. It is this astonishing energy value, largely greater
than that of any known battery, that has triggered worldwide
frenzied interest in the lithium-air battery. There have been
papers and press reports enthusiastically announcing that this
battery is the power source of the future, i.e. the only device that
may ensure the establishment of the electric vehicle market,
defining it as the ‘‘holy grail of batteries’’.
Accordingly, research in lithium-air is heavily funded in
various countries and a large number of academic and industrial
laboratories are engaged in the lithium-air battery race.
However, so far no real convincing evidence of the effective
practical relevance has been reported. Severe issues, ranging
from the reactivity of the lithium anode to the poor reversibility
and efficiency of the oxygen electrode, have so far limited the
performance of lithium-air battery prototypes to few charge–
discharge cycles and to low rate capability.
There are mainly two approaches, basically varying by the
type of electrolyte used, that are presently exploited for the
development of lithium-air batteries. One approach sees the use
of an aqueous electrolyte in combination with a lithium metal
electrode protected by a water-repulsive Li+ conductive glass-
ceramic of the Nasicon type35–37 or by an anion exchange
membrane,38 see Fig. 6a. Here the overall electrochemical process
is: O2 + 4Li + 2H2O / 4LiOH to which an estimated energy
density of the order of 5000 Whkg�1 is associated. This lithium-
air aqueous battery operates preferentially in the primary mode.
The remaining issues to be solved before achieving full practical
exploitation are in the mechanical instability of the protecting
film, in its high interfacial resistance and in the solubility of the
reaction products.
The second approach considers the use of non-aqueous elec-
trolytes.39 The battery represented in Fig. 6b by the classic
scheme drawn by Bruce and co-workers,40 comprises a lithium
metal anode, a separator embedded of the non-aqueous elec-
trolyte, the carbon-supported air electrode (with or without
catalyst) and a gas diffusion layer. This is the most appealing and
studied version since its basic process 4Li + O2 / 2Li2O is the
one that provides the highest theoretical energy density.
Although the practical value is obviously affected by the weight
of the ancillary battery components, e.g., air compressor and
flowing control, the energy expectations for the non-aqueous
electrolyte lithium-air battery remain substantial. Unfortunately,
the implementation of this non-aqueous system is compromised
by a number of issues that exceed those previously discussed for
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Fig. 6 Lithium-air battery. (a) Aqueous electrolyte, protected anode design (courtesy of Dr Mark Salomon, Max Power Inc, USA); (b) non-aqueous
electrolyte, unprotected lithium design (courtesy of Prof. Bruce, St. Andrews University, UK); (c) non-aqueous electrolyte, protected cathode design
(courtesy of Dr Mark Salomon, Max Power Inc, USA). Taken from ref. 36 and 40 respectively, with permission from Elsevier (# 2647501083527) and
Wiley Wiley-VCH STM Copyright.
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the aqueous case. A major one is sensitivity to moisture: even
traces of water must be avoided since their contact with the
lithium metal electrode can seriously affect the reliability, safety
and the cycle life of the battery. In open systems this requires the
addition of an air scrubbing apparatus. As an alternative,
cathode-protected designs exploiting a water-blocking
membrane, can be used, see Fig. 6c. Obviously, these modifica-
tions may affect the performance of the battery in terms of cost
and energy density (weight of the additional ancillary parts), as
well as of rate capability (large iR drop due to the limited ionic
conductivity of the protective membrane). It is to be recalled here
that high rate capability is a key requirement for batteries
designed for electric vehicle applications.
Another issue is the instability of non-aqueous electrolytes with
respect with the electrochemical reaction species. In early studies,
This journal is ª The Royal Society of Chemistry 2011
typical lithium-ion organic carbonate solutions were used. It is
now ascertained that these electrolyte solvents are rapidly
decomposed by the reaction products41,42 thus making the
discharge dominated by electrolyte decomposition rather than the
expected Li2O2 formation. Therefore, alternative, more stable
electrolyte systems must be employed. Possible candidates are
di-methoxy ethane DME-based or ionic-liquid-based solutions,
although their use may be affected by overpressure (in the DME
case) or by cost (in the IL case).Amore promising choice is offered
by polymer electrolytes, especially those based on poly(ethylene
oxide)-lithium salt, e.g., PEO-LiCF3SO3 complexes.43 The well
documented resistance of the PEO ether linkage combined with
the stability towards nucleophiles of the LiCF3SO3 salt, is
expected to provide amedium particularly suitable for lithium/air
battery studies, as in fact experimentally demonstrated.37,43
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Finally, there are still thermodynamic and kinetics aspects that
need to be clarified. In addition, there is some confusion in the
values of the specific capacity and of the related gravimetric
energy density, that can be obtained by the system.39 Considering
the quoted electrochemical process occurring in the lithium-air
cell, i.e. 4Li + O2 / 2Li2O and properly taking into account
both O2 and 4Li as the active materials, one obtains a specific
capacity of 1790 mAhg�1 and, assuming a potential of 2.91 V,39
an energy density of 5200 Whkg�1. In some cases, however, the
two parameters are reported as related to lithium only, excluding
O2, thus leading to impressive, but questionable, 3682 mAhg�1
and 11, 420 Whkg�1 values. Probably, the most realistic
approach is that assuming only O2 as the active material since
this is in fact the specie that contributes to the electrochemical
process; in this case the theoretical values of capacity and energy
density are 3350 mAhg�1 and 9,715 Whkg�1, respectively.
However, this choice poses some practical problems since
temperature, volume and oxygen pressure are needed to estimate
the number of moles, and thus the weight of the reacting oxygen,
in any specific volume. Considering these difficulties, the most
commonly adopted method to estimate capacity is that of
referring it to the electrode mass, either carbon, catalyst or both.
However, also the values so obtained are not totally valid since,
depending on the electrode surface and on the oxygen flux, they
are not easily reproducible from one laboratory to another.
Finally, results are also quoted for cells using Li2O as the
working electrode. In this case the electrochemical process
evolves according to the scheme: Li2O /1/2O2 + 2Li and the
related specific capacity and energy density values, unambigu-
ously referring to the Li2O weight, are 1787 mAhg�1 and 5182
Whkg�1, respectively.44,45
Another point of uncertainty has for some time been on the
mechanism of the oxygen reduction process. Recent results43–45
have clarified that this process evolves through a series of steps,
including the formation (via an intermediate O2�� radical anion
specie) of lithium superoxide LiO2 that subsequently transforms
into lithium peroxide Li2O2 and eventually, to lithium oxide
Li2O. The radical anion is a very strong base with high depro-
tonation activity that explains why conventional carbonate esters
Fig. 7 Li-O2 cell discharge/charge profiles of carbon (black,
85mA g�1carbon) and PtAu/C (red, 100 mA g�1
carbon) in the third cycle at
0.04 mA cm�2electrode. Adapted with permission from ref. 45. Copyright
2011 American Chemical Society.
3294 | Energy Environ. Sci., 2011, 4, 3287–3295
cannot be used as electrolytes in lithium-air cells. Note also that
Li2O2 and Li2O are electrically insulating products that, besides
mechanically clogging the electrode owing to insolubility, impair
the diffusion of oxygen and of electrolyte, thus originating an
extra practical difficulty.
Due to all the above described unsolved issues, the state-of-
the-art lithium–oxygen (and not yet lithium-air!) batteries are far
from being of real practical interest. A study reported in a recent
paper46 shows that, on the basis of the presently achieved
performance level of these batteries, to obtain the full automotive
power of 100 kW required for EVs, a cell area of several thou-
sand of square meters would be needed, an obviously unac-
ceptable value. Considerable improvements are needed; in
particular, charge and discharge rates have to be raised at the 10
mAcm�2 level, namely an order of magnitude higher than the
presently available one.46
An obvious way to increase the electrode kinetics is that of
exploiting the aid of a catalyst for promoting the oxygen
reduction process. Many materials have been proposed,
including a series of manganese, iron, nickel, cobalt and copper
oxides.42,45 However, the activity of the catalysts may be
compromised by various negative effects intrinsic to the battery
medium, such as precipitation on the catalyst surface of Li2O2
formed during discharge and/or dissolution of the nanometric
catalyst particle into the electrolyte. In addition, in view of the
complex reaction mechanism, see above, it cannot be given for
granted that both the reduction and the re-oxidation process can
be influenced by the same catalyst compound. Indeed, recent
work by Gasteiger and co-workers has shown that 75% efficiency
of the battery processes can only be achieved with the use of
bifunctional Pt/Au catalyst, see Fig. 7.45 Unfortunately, the cost
barrier may exclude practical use of this material, and more
realistic options have to be identified.
Lithium-air is undoubtedly a most fascinating energy storage
system and this accounts for the large attention and considerable
funding presently devoted to its investigation and development.
The route, however, towards making the lithium-air battery
feasible for vehicle applications is still long, especially consid-
ering that so far most of the studies have been limited to oxygen
fuelled systems, while cost effectiveness and practical feasibility,
obviously require the use of air cathodes.
5. Are we running out of lithium?
We may conclude this brief overview on the lithium systems by
tackling the question of the availability of the metal in view of the
amount needed for ensuring large road diffusion of electric
transportation. Doubts have been raised on whether the lithium
in the earth’s crust can be sufficient to satisfy this request. The
doubts are groundless since lithium is indeed greatly abundant.
Lithium carbonate reserves in the salt mines in South America
comprise several billion cubic meters of brine. Recently a new
deposit, discovered in Afghanistan, has been estimated to have
a value of thousand billions of dollars to the point that Afgha-
nistan has been identified as one of the possible main lithium
supplying countries, although the present situation makes it
difficult to foresee an easy operation of these reserves. Resources
of lithium are also available in sea water although in a lower
content than sodium. Altogether, the availability of lithium
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amounts to several hundred thousand of megatonnes, and thus
its content appears to be more than sufficient to meet the
demand, even in the improbable case of total conversion of the
gasoline-powered cars into hybrids or electric cars.
Acknowledgements
This research was supported in part by the Italian Institute of
Technology in the framework of the SEED Project ‘‘REALIST’’
(Rechargeable, advanced, nano-structured lithium batteries with
high energy storage) and by the WCU (World Class University)
program through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology
(R31-2008-000-10092)
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