© 2001 Macmillan Magazines Ltd
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NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com 359
Rechargeable Li-ion cells are key components ofthe portable, entertainment, computing andtelecommunication equipment required bytoday’s information-rich, mobile society.Despite the impressive growth in sales of
batteries worldwide, the science underlying battery
technology is often criticized for its slow advancement.
This is true whatever the technology considered (for
example, nickel–cadmium, nickel–metal hydride or Li
ion). Certainly, when compared, energy storage cannot
keep pace with the rate of progress in the computer
industry (Moore’s law predicts a doubling of memory
capacity every two years), yet the past decade has
produced spectacular advances in chemistry and
engineering within the emerging technologies of Ni–MeH
and Li-ion batteries. These cells are now supplanting the
well known Ni–Cd batteries.
A battery is composed of several electrochemical cells
that are connected in series and/or in parallel to provide the
required voltage and capacity, respectively. Each cell consists
of a positive and a negative electrode (both sources of
chemical reactions) separated by an electrolyte solution
containing dissociated salts, which enable ion transfer
between the two electrodes. Once these electrodes are con-
nected externally, the chemical reactions proceed in tandem
at both electrodes, thereby liberating electrons and enabling
the current to be tapped by the user. The amount of electrical
energy, expressed either per unit of weight (W h kg–1) or per
unit of volume (W h l–1), that a battery is able to deliver is a
function of the cell potential (V) and capacity (A h kg–1),
both of which are linked directly to the chemistry of the
system. Among the various existing technologies (Fig. 1),
Li-based batteries — because of their high energy density
and design flexibility — currently outperform other sys-
tems, accounting for 63% of worldwide sales values in
portable batteries1. This explains why they receive most
attention at both fundamental and applied levels.
Historical developments in Li-battery research
Before reviewing the present status of research and future
challenges for Li-battery technologies, we present a brief
historical account of developments over the past 30 years, as
personally perceived.
The motivation for using a battery technology based on Li
metal as anode relied initially on the fact that Li is the most
electropositive (–3.04 V versus standard hydrogen electrode)
as well as the lightest (equivalent weight M46.94 g mol–1, and
specific gravity r40.53 g cm–3) metal, thus facilitating the
design of storage systems with high energy density. The
advantage in using Li metal was first demonstrated in the
1970s with the assembly of primary (for example, non-
rechargeable) Li cells2. Owing to their high capacity and
variable discharge rate, they rapidly found applications as
power sources for watches, calculators or for implantable
medical devices. Over the same period, numerous inorganic
compounds were shown to react with alkali metals in a
reversible way. The discovery of such materials, which were
later identified as intercalation compounds, was crucial in the
development of high-energy rechargeable Li systems. Like
most innovations, development of the technology resulted
from several contributions. By 1972, the concept of electro-
chemical intercalation and its potential use were clearly
defined3,4, although the information was not widely dissemi-
nated, being reported only in conference proceedings.
Before this time, solid-state chemists had been accumulating
Issues and challenges facing
rechargeable lithium batteries
J.-M. Tarascon* & M. Armand†
*Université de Picardie Jules Verne, Laboratoire de Réactivité et Chimie des Solides, UMR-6007, 33 rue Saint Leu, 80039, Amiens, France
†Department of Chemistry, University of Montreal, C.P. 6128 Succ. Centre Ville, Montréal, Quebec H3C 3J7, Canada
Technological improvements in rechargeable solid-state batteries are being driven by an ever-increasing
demand for portable electronic devices. Lithium-ion batteries are the systems of choice, offering high energy
density, flexible and lightweight design, and longer lifespan than comparable battery technologies. We present
a brief historical review of the development of lithium-based rechargeable batteries, highlight ongoing
research strategies, and discuss the challenges that remain regarding the synthesis, characterization,
electrochemical performance and safety of these systems
300
200
100
0
En
er
gy
d
en
si
ty
(W
h
l-
1 )
0 50 100 150
Energy density (W h kg-1)

400
Lighter weight
Lead–
acid
200 250
Ni–Cd
Ni–
MH
Li ion
Li metal
('unsafe')
PLiON
S
m
al
le
r s
iz
e
Figure 1 Comparison of the different battery technologies in terms of volumetric
and gravimetric energy density. The share of worldwide sales for Ni–Cd, Ni–MeH
and Li-ion portable batteries is 23, 14 and 63%, respectively. The use of Pb–acid
batteries is restricted mainly to SLI (starting, lighting, ignition) in automobiles or
standby applications, whereas Ni–Cd batteries remain the most suitable
technologies for high-power applications (for example, power tools ).

© 2001 Macmillan Magazines Ltd
structural data on the inorganic layered chalcogenides5,6, and merging
between the two communities was immediate and fruitful.
In 1972, Exxon7,8 embarked on a large project using TiS2 as the
positive electrode, Li metal as the negative electrode and lithium
perchlorate in dioxolane as the electrolyte. TiS2 was the best intercala-
tion compound available at the time, having a very favourable
layered-type structure. As the results were published in readily avail-
able literature, this work convinced a wider audience. But in spite of
the impeccable operation of the positive electrode, the system was not
viable. It soon encountered the shortcomings of a Li-metal/liquid
electrolyte combination — uneven (dendritic) Li growth as the metal
was replated during each subsequent discharge–recharge cycle
(Fig. 2a), which led to explosion hazards. Substituting Li metal for an
alloy with Al solved the dendrite problem9 but, as discussed later, alloy
electrodes survived only a limited number of cycles owing to extreme
changes in volume during operation. In the meantime, significant
advances in intercalation materials had occurred with the realization
at Bell Labs that oxides, besides their early interest for the heavier
chalcogenides10,11, were giving higher capacities and voltages. More-
over, the previously held belief that only low-dimensional materials
could give sufficient ion diffusion disappeared as a framework struc-
ture (V6O13) proved to function perfectly
12. Later, Goodenough, with
LixMO2 (where M is Co, Ni or Mn)
13,14, would propose the families of
compounds that are still used almost exclusively in today’s batteries.
To circumvent the safety issues surrounding the use of Li metal,
several alternative approaches were pursued in which either the elec-
trolyte or the negative electrode was modified. The first approach15
involved substituting metallic Li for a second insertion material (Fig.
2b). The concept was first demonstrated in the laboratory by Murphy
et al.16 and then by Scrosati et al.17 and led, at the end of the 1980s and
early 1990s, to the so-called Li-ion or rocking-chair technology. The
principle of rocking-chair batteries had been used previously in
Ni–MeH batteries18,19. Because of the presence of Li in its ionic rather
than metallic state, Li-ion cells solve the dendrite problem and are, in
principle, inherently safer than Li-metal cells. To compensate for the
increase in potential of the negative electrode, high-potential inser-
tion compounds are needed for the positive electrode, and emphasis
shifted from the layered-type transition-metal disulphides to layered-
or three-dimensional-type transition-metal oxides13. Metal oxides are
more oxidizing than disulphides (for example, they have a higher
insertion potential) owing to the more pronounced ionic character of
‘M–O’ bonds compared with ‘M–S’ bonds. Nevertheless, it took
almost ten years to implement the Li-ion concept. Delays were attrib-
uted to the lack of suitable materials for the negative electrode (either
Li alloys or insertion compounds) and the failure of electrolytes to
meet — besides safety measures — the costs and performance
requirements for a battery technology to succeed. Finally, capitalizing
on earlier findings20,21, the discovery of the highly reversible, low-
voltage Li intercalation–deintercalation process in carbonaceous
material22 (providing that carefully selected electrolytes are used), led
to the creation of the C/LiCoO2 rocking-chair cell commercialized by
Sony Corporation in June 1991 (ref. 23). This type of Li-ion cell,
having a potential exceeding 3.6 V (three times that of alkaline
systems) and gravimetric energy densities as high as 120–150 W h kg–1
(two to three times those of usual Ni–Cd batteries), is found in most of
today’s high-performance portable electronic devices.
The second approach24 involved replacing the liquid electrolyte by a
dry polymer electrolyte (Fig. 3a), leading to the so-called Li solid
polymer electrolyte (Li-SPE) batteries. But this technology is restricted
to large systems (electric traction or backup power) and not to portable
devices, as it requires temperatures up to 80 7C. Shortly after this,
several groups tried to develop a Li hybrid polymer electrolyte
(Li-HPE) battery25, hoping to benefit from the advantages of polymer
electrolyte technology without the hazards associated with the use of
Li metal. ‘Hybrid’ meant that the electrolyte included three compo-
nents: a polymer matrix (Fig. 3b) swollen with liquid solvent and a salt.
Companies such as Valence and Danionics were involved in developing
these polymer batteries, but HPE systems never materialized at the
industrial scale because Li-metal dendrites were still a safety issue.
With the aim of combining the recent commercial success enjoyed
by liquid Li-ion batteries with the manufacturing advantages
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360 NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com
Non-aqueous
liquid electrolyte
Positive
(Lix Host 1)
Negative
(Lix Host 2)
Li+
Li+
a
b V
+ -
V
Non-aqueous
liquid electrolyte
Positive
(Lix Host 1)
Negative
(Lithium)
Li+
Li+
+ -
After 100 cycles
Figure 2 Schematic representation and operating principles of Li batteries.
a, Rechargeable Li-metal battery (the picture of the dendrite growth at the Li surface
was obtained directly from in situ scanning electron microscopy measurements71).
b, Rechargeable Li-ion battery.
a b cFigure 3 Schematic representations of polymer
electrolyte networks. a, Pure (dry) polymer
consisting of entangled chains, through which
the Li ions (red points) move assisted by the
motion of polymer chains. b, A hybrid (gel)
network consisting of a semicrystalline
polymer, whose amorphous regions are swollen
in a liquid electrolyte, while the crystalline
regions enhance the mechanical stability.
c, A poly-olefin membrane (Celgard for
instance) in which the liquid electrolyte is held
by capillaries.

© 2001 Macmillan Magazines Ltd
that might be placed on tomorrow’s portable devices, which in turn
places different requirements on the active material chemistry. For
instance, with respect to the lower operating voltages of emerging
electronics, much debate has focused on whether we should develop
a low-voltage active chemistry or rely entirely on electronics
(d.c.–d.c. converters) and persist in searching for high-voltage active
Li chemistry. Finding the best-performing combination of
electrode–electrolyte–electrode can be achieved only through the
selective use of existing and new materials as negative and positive
electrodes, and of the right electrolyte combination, so as to mini-
mize detrimental reactions associated with the electrode–electrolyte
interface — the critical phase of any electrochemical system.
Materials for positive electrodes
The choice of the positive electrode depends on whether we are deal-
ing with rechargeable Li-metal or Li-ion batteries (Fig. 5)27. For
rechargeable Li batteries, owing to the use of metallic Li as the
negative electrode, the positive electrode does not need to be lithiated
before cell assembly. In contrast, for Li-ion batteries, because the car-
bon negative electrode is empty (no Li), the positive one must act as a
source of Li, thus requiring use of air-stable Li-based intercalation
compounds to facilitate the cell assembly. Although rechargeable
Li-SPE cells mainly use Li-free V2O5 or its derivatives as the positive
electrode, LiCoO2 is most widely used in commercial Li-ion batteries,
deintercalating and intercalating Li around 4 V.
Initially, the use of layered LiNiO2 was considered
28, as this dis-
played favourable specific capacity compared with LiCoO2. But
expectations were dismissed for safety reasons after exothermic oxi-
dation of the organic electrolyte with the collapsing delithiated
LixNiO2 structure. Delithiated LixCoO2 was found to be more ther-
mally stable than its LixNiO2 counterpart. Thus, substitution of Co
for Ni in LiNi1–xCoxO2 was adopted to provide a partial solution to the
safety concerns surrounding LiNiO2.
Although the reversible delithiation of LiCoO2 beyond 0.5 Li is
feasible, delithiation for commercial applications has been limited to
that value for safety reasons (charged cut-off limited to around
presented by the polymer technology, Bellcore researchers
introduced polymeric electrolytes in a liquid Li-ion system26. They
developed the first reliable and practical rechargeable Li-ion HPE
battery, called plastic Li ion (PLiON), which differs considerably
from the usual coin-, cylindrical- or prismatic-type cell configura-
tions (Fig. 4). Such a thin-film battery technology, which offers shape
versatility, flexibility and lightness, has been developed commercially
since 1999, and has many potential advantages in the continuing
trend towards electronic miniaturization. Finally, the ‘next
generation’ of bonded liquid-electrolyte Li-ion cells, derived from
the plastic Li-ion concept, are beginning to enter the market place.
Confusingly called Li-ion polymer batteries, these new cells use a
gel-coated, microporous poly-olefin separator bonded to the
electrodes (also gel-laden), rather than the P(VDF-HFP)-based
membrane (that is, a copolymer of vinylidene difluoride with hexa-
fluoropropylene) used in the plastic Li-ion cells.
Having retraced almost 30 years of scientific venture leading to
the development of the rechargeable Li-ion battery, we now describe
some of the significant issues and opportunities provided by the field
by highlighting the various areas in need of technological advances.
Present status and remaining challenges
Whatever the considered battery technology, measures of its perfor-
mance (for example, cell potential, capacity or energy density) are
related to the intrinsic property of the materials that form the positive
and negative electrodes. The cycle-life and lifetime are dependent on
the nature of the interfaces between the electrodes and electrolyte,
whereas safety is a function of the stability of the electrode materials
and interfaces. Compared with mature batteries technologies, such
as lead–acid or Ni–Cd, rechargeable Li-based battery technologies
are still in their infancy, leaving much hope for improvement over the
next decade. Such improvements should arise from changes in bat-
tery chemistry and cell engineering. Advances in active chemistry are
left to the solid-state chemists’ creativity and innovation in the design
and elaboration of new intercalation electrodes. At the same time,
they must bear in mind that it is impossible to predict the demands
insight review articles
NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com 361
+
–
3.8V
1.5 Ah Carbon
Cu AI
Li1+xMn204
Li1+xMn204
Separator
Separator
Separator
Cell can
Cell can
Liquid electrolyte
Liquid
electrolyte
Carbon
AI
Cu
Li1+xMn204
Separator
Separator
Cell can
Liquidelectrolyte electrolyte
3–4.1 W
1.1 Ah
3.7 V
0.58 Ah
+
-+
-
AI Mesh
Cu mesh
+Plastic electrode
(Cathode)
– Plastic electrode
(Anode)
Plastic electrolyteCarbon
b
a c
d
Figure 4 Schematic drawing showing the shape and components of various Li-ion
battery configurations. a, Cylindrical; b, coin; c, prismatic; and d, thin and flat. Note the
unique flexibility of the thin and flat plastic LiION configuration; in contrast to the other
configurations, the PLiION technology does not contain free electrolyte.

© 2001 Macmillan Magazines Ltd
4.2 V). Several routes were investigated to circumvent these safety
and capacity issues. Among them was the successful stabilization of
the layered structural framework by an electrochemically inert di-,
tri- or tetravalent cationic substitute for Ni or Co (Al, Ga, Mg or Ti).
This led to LiNi1–xTix/2Mgx/2O2 phases
29, which were claimed to be safe
and which displayed practical capacities of 180 mA h g–1 compared to
only 140 mA h g–1 for LiCoO2. Another line of investigation involved
the synthesis by chimie douce (‘soft chemistry’) of the layered LiFeO2
and LiMnO2 phases to take advantage of the Fe
4+/Fe3+ and Mn4+/Mn3+
redox couples, respectively. In spite of the numerous and diverse syn-
thesis methods, attempts to prepare electrochemically attractive
LiFeO2 phases failed. In contrast, research on LiMnO2 has been more
fruitful30, and the structural instability of the layered phase reversing
to the spinel LixMn2O4 upon cycling has recently been diminished
through cationic substitution by chromium (Li1+xMn0.5Cr0.5O2)
31.
These materials exhibit a capacity of 190 mA h g–1 (larger than that
expected from the full oxidation of Mn3+ to Mn4+) with little capacity
fading upon cycling. It seems that within these materials, the role of
Mn is to stabilize the layered structure of the chromium oxide, and
that the large capacity is nested in the Cr oxidation state that changes
reversibly from +3 to +6. It is therefore unfortunate that Cr presents
major toxicity and pricing issues.
The spinel LiMn2O4, although possessing ≈10% less capacity than
LiCoO2, has an advantage in terms of cost and is perceived as being
‘green’ (that is, non-toxic and from abundant material source).
Additionally, it has long been recognized as a potential alternative
cathode14. Its implementation has been delayed because of limited
cycling and storage performances at elevated temperatures, although
these hurdles were overcome recently by synthesizing dually substi-
tuted LiMn2–xAlxO4–yFy spinel phases
32, and by altering their surface
chemistry33.
In the search for improved materials for positive electrodes, it has
been recognized recently that NaSICON (a family of Na super-ionic
conductors) or olivine (magnesium iron silicate) oxyanion scaffolded
structures (Fig. 6), built from corner-sharing MO6 octahedra (where
M is Fe, Ti, V or Nb) and XO4
n– tetrahedral anions (where X is S, P, As,
Mo or W), offer interesting possibilities34. Polyoxyanionic structures
possess M–O–X bonds; altering the nature of X will change (through
an inductive effect) the iono-covalent character of the M–O bonding.
In this way it is possible to systematically map and tune transition-
metal redox potentials. For instance, with the use of the phosphate
polyanions PO4
3–, the Fe3+/Fe2+ and V4+/V3+ redox couples lie at higher
potentials than in the oxide form. One of the main drawbacks with
using these materials is their poor electronic conductivity, and this
limitation had to be overcome35,36 through various materials process-
ing approaches, including the use of carbon coatings, mechanical
grinding or mixing, and low-temperature synthesis routes to obtain
tailored particles. LiFePO4, for example, can presently be used at 90%
of its theoretical capacity (165 mA h g–1) with decent rate capabilities,
and thus is a serious candidate for the next generation of Li-ion cells
(Fig. 7). As expected in the light of these promising results, polyoxyan-
ionic-type structures having XO4
n– entities (where X is Si, Ge) are now
receiving renewed attention with respect to their electrochemical
performance as electrode materials.
Although numerous classes of insertion–deinsertion materials
were synthesized over the past 20 years, no real gain in capacity was
achieved. One possible way to achieve higher capacities is to design
materials in which the metal-redox oxidation state can change
reversibly by two units (Mn+2/Mn), while preserving the framework
structure, and having molecular masses similar to those of the
presently used 3d metal-layered oxides (for example, LiCoO2). Such
an approach is feasible with W-, Mo- or Nb-based metal oxides37, but
there is no overall gain in specific energy with these heavier elements.
Inserting more than one Li ion per transition metal is also feasible
with a few V-based oxides (V5+ is reduced to an average state of 3.5 in
‘v-Li3V2O5’ (ref. 38) or to 3.67 in Li5V3O8). In principle, except for
coordination number requirements, there is no obvious reason why
this should not happen with other early transition metals and, in this
respect, the recent finding of the reversible Cr6+/Cr3+ redox couple in
a 3d metal-layered compound provides encouragement.
Tuning the morphology or texture of the electrode material to
obtain porous and high-surface-area composite electrodes
constitutes another exciting, although less exploited, route to
enhance electrode capacities39. Indeed, V2O5 aerogels, which are
insight review articles
362 NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com
3d-Metal oxides
Capacity (A h kg-1)

1000 3,800 4,0000 400200 600 800
0
1
2
3
4
Nitrides LiMyN2
Positive material:
of Li ion
limited RT cycling
of Li metal
Li1-xCo1-yMyO2
Li1-xMn2-yMyO4
Polyanionic compounds [Li1-xVOPO4 , LixFePO4]
LixMn1-yMyO2 [M=Cr, Co,...]
Vanadium oxides
[V2O5 , LiV3O8]
Li1-xNi1-y-zCoyMzO4 [M=Mg, Al,...]
MnO2
[Sn(O)-based]
of Li ion
of Li metal
( limited cycling)
Negative material:
P
ositive m
aterials
N
egative m
aterials
Li metal
Li-ion
potential
Li-metal
potential
Composite alloys
[Sn(M)-based]
Graphite
Carbons
P
ot
en
tia
l v
er
su
s
Li
/L
i+
(V
)
Figure 5 Voltage versus capacity for positive- and negative-electrode materials
presently used or under serious considerations for the next generation of rechargeable
Li-based cells. The output voltage values for Li-ion cells or Li-metal cells are
represented. Note the huge difference in capacity between Li metal and the other
negative electrodes, which is the reason why there is still great interest in solving the
problem of dendrite growth.

© 2001 Macmillan Magazines Ltd
mesoporous materials in which nanometre-sized domains are net-
worked through a continuous, highly porous volume of free space,
were reported recently to have electroactive capacities up to 100%
greater than polycrystalline non-porous V2O5 powders and superior
power rate capabilities40 compared to usual V2O5 powders. Such extra
capacity apparently derives from the onset of a pure capacitance,
associated with the large surface area and high-porosity aerogel
matrix, which adds to the existing faradic component. Conductive
oxide aerogels such as V2O5 and MnO2 therefore have the potential to
boost the field of energy storage once the capacity penalty (in terms of
W h l–1) attributable to their poor tap density (.0.2 g cm–3) is over-
come. Tailor-made nanostructured materials, such as aerogels, create
new opportunities not only at the applied level, but also at the funda-
mental level where some elementary questions, such as the exact
mechanism governing these large capacities, remain unanswered.
A radically different approach41 takes advantage of the facile and
reversible redox cleavage of the sulphur–sulphur bond to give
lithium thiolate: –SS–&2Li+&2e– Û –SLi&LiS–. Depending on the
electron, the voltage withdrawing power of the moieties attached to
the sulphur can be up to 3 V (sulphur itself works at 2.4 V). Although
promising in principle in terms of capacity and cost, these systems
presently suffer from the relative low density of the reactants and
solubility of the resulting thiolates in the electrolyte, leading to self
discharge.
Materials for negative electrodes
As a result of numerous chemical (pyrolitic processing) or physical
(mechanical milling) modifications, carbon negative electrodes42
display electrochemical performances that are improving continu-
ously. Reversible capacities of around 450 mA h g–1 are now being
reached, compared with a practical value of 350 mA h g–1 for graphite
(372 mA h g–1 for the end compound LiC6). In parallel, ongoing
research efforts are focused on searching for carbon alternatives in
the hope of finding materials (Fig. 4) with both larger capacities and
slightly more positive intercalation voltages compared to Li/Li+, so as
to minimize any risks of high-surface-area Li plating at the end of fast
recharge, which are associated with safety problems. Such an effort
resulted in the emergence of Li transition-metal nitrides as a new
potential class of anode materials43, owing to the large, stable and
reversible capacity (600 mA h g–1) displayed by one family member,
Li3–xCoxN. This result triggered worldwide interest, although perfor-
mances of the other newly reported Li-based nitrides unfortunately
display inferior electrochemical performances compared to the Co
phase. Furthermore, use of Li3–xCoxN is constrained by the restrictive
manufacturing requirements for handling such moisture-sensitive
negative electrodes.
Throughout the search for carbon alternatives, much effort has
been devoted to the use of Li alloys. The first commercial cell was
insight review articles
NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com 363
FeO6
LiO6
PO4
[TeOc2] layer
Li+
a
b
Figure 6 The crystal structure of olivine LiFePO4 in projection along [001]. On the left,
expanded view of the framework built on FeO6 octahedra and PO4 tetrahedra, with Li
ions in red. The FeO6 octahedra are linked together through corner sharing in the
(b, c) plane. On the right, restricted view of Li, Fe and P distribution between two
distorted, h.c.p. (hexagonal close packed) oxygen-dense layers (PTd[LiFe]oct.O4). LiO6
octahedra share edges and Li ions may diffuse along [010] and [001].
200
150
100
50
0
C + (mA h)
C – (mA h)
C
ap
ac
ity
(m
A
h
g
-1
)
0 50 100 150
No. cycles
C/10, 55°C
0 0.2 0.4 0.6 0.8 0
4.2
3.9
3.6
3.3
3
2.7
V
ol
ta
ge
(V
v
s
Li
+ /
Li
)
x in LixFePO4
Figure 7 Cycling behaviour at 55 7C of an optimized LiFePO4/C composite electrode
(83% of active material) at a scan rate of C/10. Fine particles of LiFePO4 were obtained
from annealing at 500 7C a solid intimate mixture resulting from evaporation of an
aqueous solution containing Li, Fe(III) and P precursors. The composite electrode was
obtained from ball-milling LiFePO4 with carbon SP. From ref. 36.

© 2001 Macmillan Magazines Ltd
introduced in the 1980s by Matsushita; this was based on Wood’s
metal (a low-melting alloy of Bi, Pb, Sn and Cd), whose cycling
performances were found to deteriorate with increased depth of
discharge. While attractive in terms of gravimetric capacity, Li alloys
suffer from cyclability issues resulting from large Li-driven volume
swings (up to 200%), which cause disintegration and hence a loss of
electrical contacts between particles. Although a reduction in alloy
particle size clearly benefits the cyclability by increasing tolerance to
stress cracking, so far the gains are not sufficient44. However, it
became clear that any physical or chemical means of overcoming the
problem of reactant expansion should be beneficial, hence the use of
composite negative electrodes. The basis behind this concept is the
use of a ‘buffer matrix’ to compensate for the expansion of the reac-
tants, so preserving the electrical pathway45. Initially, such a buffer
action was achieved by mixing two alloys that reacted at different
potentials so that the electrochemically active phase was imbedded in
a non-electrochemically active matrix.
A similar approach held considerable promise in 1997, when Fuji
announced the commercialization of a new Li-ion technology
(STALION) using an amorphous tin composite oxide (ATCO) as
negative electrode. This reacts reversibly with Li at about 0.5 V, and
has a specific capacity twice that of graphite46. In situ X-ray diffrac-
tion studies of the ATCO electrode led to the conclusion that the Li
reactivity mechanism in these composites was based on oxide
decomposition by Li through an initial irreversible process to form
intimately mixed Li2O and metallic Sn, followed by a Li alloying reac-
tion to form nanodomains of Li4.4Sn embedded within the Li2O
matrix47. However, the STALION cell was never commercialized, in
spite of its announcement at the end of 1998. This was most likely due
to poor long-term cyclability, the huge and irreversible capacity loss
during the first cycle, which was reported by many groups, and the
necessity of finding a convenient source for the two initial Li ions
needed for the SnO reduction process.
Besides ATCO, other investigations, such as those pursued by
Dahn et al.48 on the –Sn–Fe–C system, have also revealed an appealing
low-voltage reversible reactivity in composite materials developed as
negative electrodes (in spite of initial irreversibility and short-lived
capacities). The best experimental proof of the beneficial aspect of
the buffer matrix arises from the ability to obtain several hundred
cycles on a composite made by precipitating Sn metal at the grain
boundaries of electrochemically inactive SnFe3C grains
49. However,
the cycling performance was improved at the expense of the electrode
electrochemical capacity.
A new approach to alleviate the problems of alloy expansion,
proposed by Thackeray et al.50, involved selecting intermetallic alloys
such as Cu6Sn5, InSb and Cu2Sb that show a strong structural
relationship to their lithiated products, Li2CuSn and Li3Sb for the Sn
and Sb compounds, respectively. InSb and Cu2Sb electrodes are
particularly attractive candidates because they operate through a
reversible process of lithium insertion and metal extrusion, with an
invariant face-centred-cubic Sb array (that is, this array provides a
stable host framework for both the incoming and extruded metal
atoms). In the ternary LixIn1–ySb system (0*x*3, 0*y*1), the Sb
array expands and contracts isotropically by only 4%, whereas the
overall expansion of the electrode is 46% if the extruded In is taken
into account. This expansion is considerably more favourable than the
expansion of binary systems such as LiAl, which expand by ~200%
during the phase transition of Al to LiAl. InSb and Cu2Sb electrodes
provide reversible capacities between 250 and 300 mA h g–1. Despite
the new and elegant concept behind the design of these intermetallic
electrodes, they still suffer from poor cyclability, particularly upon the
initial cycle; nevertheless, the approach deserves further study.
Based on the peculiar behaviour (that is, large capacity at low
potential) of the transition-metal vanadates M–V–O, first proposed
by Fuji Co.51 and later studied by several groups, Poizot et al. 52
reinvestigated the reactivity of Li-metal oxide. Surprisingly, they
found a Li electrochemical activity for well known oxides, but these
did not react with Li according to the classical processes of Li
insertion–deinsertion or Li alloying, the catchwords of the past
20 years. For instance, MO-type compounds (where M is Co, Ni, Fe,
Cu or Mn), having a rocksalt structure and containing metal
elements (M) that do not alloy with Li, exhibited capacities two to
three times those of carbon with 100% capacity retention for up to
100 cycles. The mechanism of Li reactivity in such materials differs
from the classical processes, and is nested in the electrochemically
driven, in situ formation of metal nanoparticles during the first
discharge, which enables the formation and decomposition of Li2O
upon subsequent cycling53. Remaining issues relate to a problem of
surface, with the chemical reactivity being enhanced as the particle
size becomes smaller. These findings open new avenues of research
aimed at capitalizing on the beneficial effect that particle-size
confinement could have within the field of electrochemistry. These
and related nanocluster systems under development hold much
promise for future developments.
Polymer and liquid electrolytes
Besides the electrodes, the electrolyte, which commonly refers to a
solution comprising the salts and solvents, constitutes the third key
component of a battery. Although the role of electrolyte is often con-
sidered trivial, its choice is actually crucial, and is based on criteria
that differ depending on whether we are dealing with polymer or
liquid-based Li-ion rechargeable batteries54. For instance, working
with a highly oxidizing (>4V versus Li/Li+) positive electrode
material for Li-ion batteries requires electrolyte combinations that
operate well outside their window of thermodynamic stability
(3.5 V). This is the reason why early workers in the field ignored very
positive cathode materials. But fortunately this electrolyte stability is
kinetically controlled, enabling the use of non-aqueous electrolytes
at potentials as high as 5.5 V (ref. 55). Similarly, the use of a polymer
rather than a liquid electrolyte adds further selection criteria linked
to the electrochemical stability of the polymer56. There are numerous
liquid solvents available, each with different dielectric constants and
viscosity, and we can select specific solvents to favour the ionic
insight review articles
364 NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com

2.7 3.12.9 3.53.3 4.34.13.93.7
103 K/T
Lo
gσ

(
S
c
m
-1
)
- 3
- 4
- 5
- 6
- 2
- 1
4
3
2
5
1
10
T (°C)

b -Alumina
100 80 60 40 20 0 -20 -40
0.18P
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6
8
7
9
Figure 8 Arrhenius plot of conductivity for various solid electrolytes. 1, First-
generation PEO-LiCF3SO3; 2, new solutes with high-dissociation PEO-Li[(CF3SO2)2N];
3, low-Tg combination polymer; 4, plasticized polymer electrolyte
PEO-Li[(CF3SO2)2N]&25% w/w PEG-dimethylether (molecular weight, 250); liquid
crystalline polymer electrolytes; 5, heating curves; 6, cooling curve64; 7, gel-type
polymer (X-linked PEO-dimethacrylate-Li[(CF3SO2)2N]-PC 70%); 8, liquid electrolyte
PC/DME LiCF3SO3; 9, liquid electrolyte EC/DMC-LiPF6 at low temperature
61; 10, gel
electrolyte P(VDF-HFP)/EC/DMC-LiPF6 (ref. 61).

© 2001 Macmillan Magazines Ltd
conductivity of the electrolyte. In contrast, there are only a few
Li-based salts or polymers to choose from, the most commonly used
ones being based on polyethylene oxide (PEO). The results from
research efforts aimed at counterbalancing this deficit have led to the
present level of research and development on electrolytes.
Guided by general concepts of viscosity and dielectric constants,
optimizing the ionic conductivity of a liquid electrolyte almost
becomes a field-trial approach with the hope of finding the key
ingredients. For instance, only ethylene carbonate can provide the ad
hoc protective layer on the surface of graphite that prevents further
reaction (continuous electrolyte reduction and self-discharge).
Ethylene carbonate is therefore present in almost all commercial
compositions, thinned with other solvents owing to its high melting
point. Why the homologous propylene carbonate is unsuitable for
this protective layer remains an open question, reminding us that
chemistry has its secrets.
In contrast, achieving high ionic conductivity in Li-based
polymer electrolytes requires a better understanding of the
fundamentals of ion dissociation and transport. Both the nature of
the polymer–salt interaction and the precise structure of highly
concentrated electrolyte solutions have always resisted rationaliza-
tion. Nevertheless, a principal goal has been to search for new, highly
conductive salts with a large electrochemical window, which form a
eutectic composition with PEO that melts at the lowest possible tem-
perature57. The concept of non-coordinating anions with extensive
charge delocalization was achieved with the perfluorosulphonimide
Li+[CF3SO2NSO2CF3]
– salt (abbreviated as LiTFSI)58. Figure 8 shows
the marked improvement when passing, with simple PEO, from a
‘conventional’ LiCF3SO3 salt (curve 1) to the imide salt (curve 2),
where an order of magnitude is gained, not ignoring the larger elas-
tomeric domain towards low temperature. The polymer architecture
has a role independently of dissociation. Attaching the side chains of
the solvating group to the polymer increases the degrees of freedom
as a result of dangling chain ends; this improves conductivity (Fig. 8,
curve 3), but compromises the mechanical properties.
Although efforts aimed at enhancing the ionic conductivity of
polymer electrolytes have been insufficient to permit operation at
room temperature, they have benefited liquid-based electrolyte
systems in terms of cost and safety, so that battery manufacturers of
Li-ion cells are eager to see the further development of organic anion-
based salts able to operate at voltages greater than 4.5 V. LiTFSI is an
example of this cross-fertilization. Although extremely resistant to
oxidation itself, the electrochemical use of such a salt is limited to 4 V
in presence of an Al collector, because a stable and soluble Al salt can
be formed as a consequence of the robustness of the anion bonds.
With the less stable coordination anions (LiPF6), decomposition
occurs immediately and is accompanied by formation of protective
AlF3. However, owing to its high conductivity in any medium, its
safety and lack of toxicity, LiTFSI is being used increasingly in Li-ion
batteries, the corrosion problem having being solved by simple
addition of a passivating coordination-type salt. A wide range of
anion-forming systems now exists, especially in the imide family,
and these are viewed as candidates for high conductivity and Al
passivation.
Having exploited most of the possibilities offered by ‘dry’
polymers to improve conductivity (ability, amorphous state and
lowest possible glass-transition temperature Tg controlling the ion
mobility), a remaining option was to use additives, known in
polymer science as plasticizers, to act as chain lubricants, so leading
to the development of ‘hybrid’ polymer electrolytes59. Indeed,
suitable plasticizers are chosen between the same polar solvents as for
liquid electrolytes60, such as propylene carbonate, g-butyrolactone
or polyethylene glycol ethers, or are formed from short-chain PEO
(4–25 monomer units). A lightly plasticized material (10–25%
additive) improves conductivity by an order of magnitude (Fig. 8,
curve 4). Gels, on the other hand, contain 60–95% liquid electrolyte,
and are only 2–5 times less conductive than their liquid counterpart61
(Fig. 8, curves 7–10). Interestingly, when the gelling agent is a
polyether, most of the solvation still takes place through the polymer
chains rather than the carbonate solvents, the latter being less prone
to donate electron pairs. Understandably, the lightly plasticized
systems can be used in a Li-metal configuration, as much of the
resilience of the pristine polymer is retained, whereas the much softer
gels require a Li-ion configuration.
It is surprising that in spite of the direct link between their ionic
conductivity and their degree of amorphicity, very little is known
about the structural chemistry of polymer electrolytes. In contrast to
the well established dynamic view of ionic conductivity on these
materials, Bruce et al.62 recently proposed a structural view,
highlighting the importance of aligning or organizing the polymer
chains in order to enhance the levels of ionic conductivity. Similarly,
Wright and co-workers63 and Ingram64 focused on the liquid
crystalline state to force the solvating polymer into a conformation
that was dictated by the liquid crystal part. The result is a partial
decoupling of the conductivity from the glass-transition tempera-
ture of the polymer. The conductivity of such liquid crystalline chain
polymers is low at room temperature, but reaches liquid-like values
at high temperature or when kept under polarization, and remains so
upon cooling to room temperature (Fig. 8, curves 5 and 6), without
appreciable activation energy64. As these new perspectives generate
renewed interest in the design of polymer electrolytes, it is hoped that
solutions may eventually be found to the problems of ionic
conductivity afflicting this class of materials at ambient or
subambient temperature.
The addition of nanoparticle fillers (10% w/w), such as Al2O3 or
TiO2, to simple PEO compounds increases the conductivity several-
fold at 60–80 7C, and prevents crystallization for at least several weeks
at room temperature65. Two important advantages of these systems
are an increase in the apparent Li transport number, from a low of
≈0.3 (common to polymer, liquid and gels) to ≈0.6, and the forma-
tion of a stable, low-resistance interface in contact with Li. Because
these materials obey different conduction mechanisms, they are
presently the focus of many studies, both practical and theoretical66.
Technologies based on either solid polymer or ‘hybrid’ polymer
electrolytes offer great advantages that will be necessary to meet the
flexible, shape-effective requirements dictated by today’s electronic
miniaturization, while at the same time providing a larger autonomy.
Current Li technologies rely on liquid-jellyroll or prismatic-cell
configurations. Neither fits well in a multiple-cell configuration.
This is in marked contrast with the recent thin, plate-like plastic
(PLiON) technology that enables excellent packing efficiency, as
multiple plates can be densely packaged in parallel within one cell
while preserving the flexibility of the overall package. Future technol-
ogy improvements should focus on better chemical engineering of
the bonded laminates, so as to obtain even thinner cells. Similar
attributes can be provided by the solid Li-polymer technology, which
in addition exhibits extra capacity and is free of electrolyte leakage.
This currently operates at 80 7C. Although warm temperatures may
be an advantage for the large batteries required by the transportation
sector, problems of conductivity have to be solved for electronic
applications, as emphasized earlier.
The electrode–electrolyte interface
The Li-ion cell density can be improved through a selective use of
appropriate existing or new materials for negative and positive
electrodes. However, optimizing an electrode material is only the first
step in the process leading to its implementation in a practical cell.
Indeed, while the capacity of a cell is nested in the structural or
electronic behaviour of its electrode, poor cell lifetimes are rooted
mainly in side reactions occurring at the electrode–electrolyte inter-
face. Thus, mastering the chemical stability of any new electrode
material with respect to its operating liquid or polymer electrolyte
medium, which requires a control of the electrode–electrolyte
interface through surface chemistry, is as important as designing new
insight review articles
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© 2001 Macmillan Magazines Ltd
materials. Tackling interfacial issues is both tedious and complex. We
should remember that, despite many years of research devoted to the
mechanism by which the solid electrolyte interphase forms on Li or
carbonaceous materials, its composition and nature are still the sub-
ject of much controversy. In contrast, the positive electrode interface
has received little attention over the years, despite its equally crucial
role. Its importance is amplified with the Li-ion technology, where
high voltages exceed the electrochemical resistance of the electrolyte
oxidation, and even favour its catalytically driven decomposition.
Thus, it is critical to control the electrode surface so as to modify its
catalytic activity towards electrolyte decomposition. The strategy
developed to address this issue uses coatings that encapsulate,
through chemical or physical means, the electrode grains with either
an inorganic or an organic phase. This concept, successfully applied
to the spinel LiMn2O4, is based on minimizing the surface area of the
active material in direct contact with the electrolyte33. The coating
must allow easy diffusion of Li ions and, although insulating in
nature, must be thin enough to allow the electrons to tunnel through.
Equally relevant is the unexplained role of filler additives in polymer
electrolytes65, which markedly reduce the interfacial impedance in
contact with Li.
Thirty years after its initial observation, the key issue of Li den-
drite growth, which was thought to be governed mainly by current
densities, remains highly topical, especially in light of recent promis-
ing results obtained by Aurbarch’s and Bates’ groups. Revisiting
Exxon’s solvent 1-3-dioxolane, Aurbarch and co-workers67showed
that the use of LiAsF6 salt led to a completely different Li morphology
from that obtained from an ethylene carbonate–dimethyl carbonate
(EC–DMC) electrolyte. They explained this in terms of the reactivity
of dioxolane with lithium, which forms an elastomeric coating
endowing the Li surface with plasticity and flexibility, thereby reduc-
ing dendrite growth. These findings were implemented in Li/MnO2
commercial Tadiran cells that, under well defined cycling conditions,
are claimed to be safe. The bulk polymerization of the cyclic ether,
initiated at the positive electrode on overcharge, acts as a thermal
shutdown. Even more spectacular are recent reports by Bates et al.68
who succeeded in cycling LiCoO2/Li thin-film batteries for more
than 50,000 cycles using a glassy electrolyte in ≈1-mm-thick films
obtained by sputtering techniques. By controlling the uniform Li
stripping–plating mechanism, the same authors demonstrated the
feasibility of a Li-free, rechargeable, thin-film battery — that is, cells
constructed in the discharged state with no Li metal initially
present69. Such findings, whether resulting from low-current density
or the use of solid electrolyte, show that the problem of dendrite
growth can be solved, at least with special cell configurations. Visco
and co-workers70 recently showed that a glassy nanometric layer
deposited on Li metal completely insulates it from its environment,
even in the presence of liquids, and that this coating can be applied at
a high production rate. With further work devoted to the implemen-
tation of these findings to large-size Li batteries, the development of a
Li-free rechargeable battery remains a realistic goal for the future.
The principal challenge for Li-based rechargeable batteries, or
indeed for any battery, lies in gaining better understanding and
control of the electrode–electrolyte interface in the hope of designing
new solid–solid or solid–liquid interfaces. For example, the nature of
the secondary reactions occurring at high temperature, which cause
cell failure, remains an unanswered question that must be addressed
to ensure the practical success of these technologies. In this case,
however, the main difficulty stems from a lack of available techniques
to probe the evolution of the electrode–electrolyte interface at a local
level. We have so far relied (with the exception of X-ray diffraction)
on post-mortem rather than in situ studies to determine how the
electrodes or interfaces age with time either under cycling or storage
conditions, thereby missing key information. But introduction of the
plastic Li-ion-type technology has created new opportunities to
perform a wide variety of in situ characterization techniques. These
include X-ray absorption near-edge structure, nuclear magnetic
resonance and Mössbauer spectroscopies, or even scanning electron
microscopy observations that allow real-time visualization of
dendrite growth at an interface71. Efforts aimed at developing new
characterization tools must be vigorously pursued so as to create a
comprehensive database on the electrode–electrolyte interface.
Conclusion
Consumers are in constant demand for thinner, lighter, space-
effective and shape-flexible batteries with larger autonomy. Such
demand will continue to generate much research activity towards the
development of new cell configurations and new chemistries. In this
review we hope to have conveyed the message that the field of energy
storage is advancing faster than it perhaps has ever done in the past.
The benefits, in terms of weight, size and design flexibility provided by
today’s state-of-the-art Li-ion configurations, which owe much to the
design engineers’ striving to develop efficient, economical microtech-
nologies, are a good illustration. The Li-based battery chemistry is
relatively young, and as such is a source of aspirations as well as
numerous exciting challenges. The latter are not limited to solid-state
chemists. The effort should be highly multidisciplinary with strong
roots in the fields of organic and inorganic chemistry, physics, surface
science and corrosion. Through materials design we can expect
significant improvements in energy density. And although designing
new materials can be intuitive or based on chemical concepts,
coupling these efforts with those of theorists who are able to perform
band-structure calculations on envisioned compounds will prove to
be highly beneficial. Of equal importance is a better understanding of
the electrode–electrolyte interface to facilitate design of new
interfaces. Here the goal is well defined, although we must diverge
from the empirical approach used so far, and make full use of the
recent progress achieved by in situ characterization. As Li-recharge-
able batteries enter their teenage years, scientists and engineers predict
an even brighter future lies ahead. nn
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Acknowledgements
The authors thank their colleagues, both in academic institutions and industry, for
sharing the gratifying dedication to this field of progress, and P. Rickman for help drawing
the figures.
insight review articles
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