Lithium-Ion Batteries a Look Into the Future

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Cite this: Energy Environ. Sci., 2011, 4, 3287

www.rsc.org/ees PERSPECTIVE

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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.

Energy Environ. Sci., 2011, 4, 3287–3295 | 3287

http://dx.doi.org/10.1039/c1ee01388b






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.



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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.


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,



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


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.



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


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


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



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,


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.


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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Lithium-Ion Batteries a Look Into the Future

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