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204 The Open Materials Science Journal, 2011, 5, (Suppl 1: M2) 204-214
1874-088X/11 2011 Bentham Open
Open Access
Reality and Future of Rechargeable Lithium Batteries
Haisheng Tao, Zhizhong Feng, Hao Liu, Xianwen Kan and P. Chen*
Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L
3G1, Canada
Abstract: Compared to other types of rechargeable batteries, the rechargeable lithium battery has many advantages, such
as: higher energy density, lower self-discharge rate, higher voltages and longer cycle life. This article provides an
overview of the cathode, anode, electrolyte and separator materials used in rechargeable lithium batteries. The advantages
and challenges of various materials used in rechargeable lithium batteries will be discussed, followed by a highlight of
developing trends in lithium battery research.
Keywords: Rechargeable lithium battery, cathode, anode, electrolyte, separator, review.
1. INTRODUCTION
Energy and environment are the two most challenging
issues faced by our society. With the production of oil
predicted to decline and the number of vehicles and their
pollution impact to increase globally, a safe, low-cost, high-
efficiency and environmentally friendly alternative power
sources have become a most urgent need. Solar energy, H2
energy, fuel cells and batteries are attracting considerable
interest as alternative power sources. Specifically, batteries
are portable and easily replaced, commonly used in
household and industrial applications such as energy storage
and management [1]. Among various existing batteries (Fig.
1), lithium batteries have raised the most interest and have a
high priority on the development of energy projects in many
countries because of their high energy density, long cycle
life, cost-effective, long lasting, and abuse-tolerant
properties [2].
Fig. (1). The energy density of different batteries.
*Address correspondence to this author at the Department of Chemical
Engineering, University of Waterloo, 200 University Avenue West,
Waterloo, Ontario, N2L 3G1, Canada; Tel: (519) 888-4567x35586; Fax:
(519) 746-4979; E-mail: p4chen@uwaterloo.ca
Rechargeable lith ium battery is of present interest. There
are many kinds of rechargeable lithium batteries. They have
different classifications according to the forms of lithium
(e.g. lithium metal as anode directly, we call it lithium
battery. For lithium trans-metal salt as anode, we call it
lithium ion battery. Usually, we simp ly call them lithium
battery). Based on the types of electrolytes (such as lithium-
aqueous battery, lithium-organic battery), and the kinds of
cathodes (such as lithium cobalt oxide battery, lithium nickel
oxide battery, lithium manganese oxide battery, lithium iron
phosphate battery, sulfur battery), we have several other
classifications as well.
However, each battery composes of one or several cells,
and each cell has its own characteristic cell potential (V),
capacity (Ah kg1 or Ah L1), and energy density (Wh kg1 or
Wh L1). These characteristics are determined by the
chemical properties of each component of the cell. To
provide the required voltage and capacity, the cells can be
connected in series or parallel configuration. Three primary
functional components of an individual cell are the anode
(negative electrode), the cathode (positive electrode), and the
electrolyte. The transfer of Li ions between the two
electrodes is facilitated by the dissociated lithium salts in the
electrolyte. To fabricate a complete cell, however, requires
additional components, such as a separator, current collector,
tab and cell can.
In the case of discharge, the transfer of energy and
current of a rechargeable lithium battery occurs when the
electrodes are connected externally to a load (Fig. 2). Li ions
are liberated from the anode, pass through the separator, and
are “inserted” into the cathode. At the same time, the anode
releases electrons, which pass through the external circuit
and then arrive at the cathode. In the case of charge, the
process is reversed. The cycling of a battery depends not
only on the property of individual components: cathode,
anode, electrolyte and separator, but also on the
compatibility among different components. During the
development of rechargeable lithium batteries, the first
breakthrough was the discovery of Li ion intercalation
compounds that enable reversible Li ion
intercalation/deintercalation, which opened the concept of
Reality and F uture of Rechargeable Lithium Batteries The Open Materials Science Journal, 2011, Volume 5 205
rechargeability. A second breakthrough was the application
of nanostructured materials, which continuously improve the
performance of rechargeable lithium batteries [3].
Nanostructured materials can increase the specific surface
area as well as reduce diffusion length for electronic and Li
ion transport, leading to a high charge/discharge rate.
Fig. (2). Schematic illustration of a lithium battery (in the case of
discharge).
As one of the great successes of modern
electrochemistry, the lithium battery has played a key role in
the consumer electronic market. Thanks to the improvement
of the electrodes and electrolytes, the lithium battery is
moving to dominate not only portable battery industry (MP3
player, laptop, cell phone and camera etc.), but also
electronic automotive transpor tation (electric vehicles, full
hybrid electric and plug-in electric vehicles). But the reality
is complex: although scientists have produced numerous
potential battery chemistries, problems of various natures
still prevent the large scale application of lithium batteries
for the electronic automotive transportation. None of them
perform well on all the crucial factors of cost, safety,
durability, power and capacity [4]. To further advance in the
science and technology of lithium batteries, new strategies
must be implemented. This includes modifications of the
electrode and electrolyte components and further
improvements in their safety, environmental sustainability
and energy content.
The following sections provide a review of rechargeable
lithium batteries in terms of cathode, anode, electrolyte and
separator. Significant changes to the properties and
performance of these battery components brought by the
development of intercalation and nanostructured materials
are demonstrated. The challenges and potentials of
rechargeable lithium batteries are also discussed.
2. CATHODES
The material for an ideal cathode of a rechargeable
lithium battery should have following properties: First, the
material may be capable of reversibly
intercalating/deintercalating Li ions at a large capacity and
high potential. Second, the material should undergo minimal
structural change during Li ion intercalation/ deintercalation,
as required for good cycle performance. Third, the material
should suffer minimal redox potential change during Li ion
intercalation/deintercalation, as required for smooth
charge/discharge curves. Fourth, the material should have
high electronic conductivity, high Li ion diffusion rate and
conductivity, as required for high charge/discharge rate.
Lastly, the material should be chemically stable with the
electrolyte under operating potentials.
The intensively studied cathode materials mainly include
lithium cobalt oxide, lithium nickel ox ide, lithium
manganese oxide, and olivine phases, in particular, lithium
iron phosphate (LiFeO4). Additionally, sulfur based cathode
materials have attracted significant attention recently,
because of their high specific capacity (1670 Ah Kg1). For
lithium cobalt oxide and lithium nickel oxide, both of them
contained layered structure which could offer highly
accessible ion diffusion pathways. The benefit in using
lithium iron phosphate is that, in addition to being naturally
abundant and inexpensive, they are less toxic than cobalt,
manganese, and nickel compounds. There are many merits
making sulfur as a very suitable cathode material, such as the
low equivalent weight, low cost, high capacity, and
environmental friendliness. The advantages and
disadvantages of different cathode materials are discussed
below.
2.1. Lithium Cobalt Oxide and Lithium Nickel Oxide
Lithium cobalt oxide (LiCoO2) is an intercalation
material allowing reversible intercalation/deintercalation of
Li ion. LiCoO2 has a large tap density (2.83.0 g cm3),
large gravimetric capacity (~140 Ah Kg1), excellent
cyclability (500~800 cycles), and high operating voltage (3.6
V). These desirable qualities make LiCoO2 a widely used
cathode material in commercial batteries. However, when the
cell is over-charged or over-discharged, the instability of
LiCoO2 structure could lead to severe material degradation
and even explosion of the cells. This raises serious operation
concerns for commercial operation, but even more so for
personal batter ies. Additionally, cobalt is a relatively rare
and expensive metal, which limits the widespread
implementation of LiCoO2 in power batteries for electric
vehicles.
Besides LiCoO2, there are several other well-known Li
ion intercalation compounds. Lithium nickel oxide (LiNiO2)
is considered a preferred cathode material for its larger
gravimetric capacity (275 Ah Kg1), higher natural
abundance and lower toxicity as compared with LiCoO2. But,
the structure of LiNiO2 is not as stable as that of LiCoO2
[5-
8]. To mitigate the structural instability, partial substitution
of Ni with Al, Ga, Mg or Ti is consid ered. For example,
LiNi0.75Al0.25O2 has shown higher thermal stability and safer
operation during over-charge. Moreover, a novel layered
material of LiNi0.32Mn0.33Co0.33Al0.01O2 with -NaFeO2
structure synthesized by sol-gel method has been used as a
cathode in lithium batteries. The capacity retention of
LiNi0.32Mn0.33Co0.33Al0.01O2 has been improved to 97% [9].
Another problem with LiNiO2 is the capacity fading [10],
which may be solved by partially substituting Ni with Ti and
Mg. A typical example is LiNi1-xTix/2Mgx/2O2, which has
been used as a cathode material at a high capacity of 180 Ah
Kg-1 and good thermal stability [11].
206 The Open Materials Science Journal, 2011, Volume 5 Tao et al.
The high capacity of Ni-rich Li[Ni1-xMx]O2 (M=Co, Mn)
is very attractive, if the structural instability and thermal
properties are improved. Yang-Kook Sun et al. synthesized a
spherical core-shell structure with a high capacity (from the
Li[Ni0.8Co0.1Mn0.1]O2 core) and a good thermal stability
(from the Li[Ni0.5Mn0.5]O2 shell) [12]. This core-shell
structured Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2/carbon cell
has a superior cyclability and thermal stability relative to the
Li[Ni0.8Co0.1Mn0.1]O2 at the 1 C rate for 500 cycles.
Expanding on this idea, a concentration-gradient cathode
material for rechargeable lithium batteries based on a layered
lithium nickel cobalt manganese oxide was developed [13].
This novel high-capacity and safe cathode material with an
average composition of Li[Ni0.68Co0.18Mn0.18]O2, in which
each particle consists of bulk material Li[Ni0.8Co0.1Mn0.1]O2
coated by a concentration-gradient outer layer which the
reactive nickel ions are gradually substituted by manganese
ions. As expected, this concentration-gradient cathode
material achieved a high capacity of 209 Ah Kg1 and also
showed superior thermal stability.
2.2. Lithium Manganese Oxides
Lithium manganese oxides yield a high operating voltage
(3.7 V) and are favored over LiCoO2 and LiNiO2 in electric
vehicles because they are safe, cheap and non-toxic. There
are two well known "lithium-rich" manganese oxides, the
spinel LiMn2O4 [14-17] and orthorhombic LiMnO2
[18].
Unfortunately, the capacity of these materials decreases
rapidly during cycling at normal operating temperatures due
to structural instability, but this problem can be solved
through cationic substitution with Cr, Al, Mg, and Fe [15,
19-23]. It was reported that Al-doped LiAl0.1Mn1.9O4
materials using a room-temperature solid-state grinding
reaction followed by calcination showed the obviously
improved cyclability compared with the pristine LiMn2O4
[24]. Another problem is its limited cycling and storage
performance at elevated temperatures (>40 oC), which can be
improved by changing the structure (such as using
synthesizing bi-substituted LiMn2-xAlxO4-zFz) [25, 26], or
modifying the surface (such as acetylacetone, boron oxide
and alkali-hydroxydes solution treatment) [27, 28].
2.3. Lithium Iron Phosphate
In 1996, lithium iron phosphate (LiFePO4) was
discovered and used as a cathode material for rechargeable
lithium batteries [29, 30]. It gained vehicle market
acceptance because of its low cost, low toxicity, excellent
thermal stability, and high gravimetric capacity (170 Ah
Kg1). However, the main application barrier of LiFePO4 was
its intrinsically low electrical conductivity [31]. This may be
overcome by synthesizing LiFePO4 in nanoparticles.
Alternatively, the same improvement in electrical
conductivity can be achieved by carbon coating of LiFePO4
[32-34] or doping LiFePO4 with various metal elements [35-
37]. It was reported that stoichiometric Cu-doped lithium ion
phosphate LiFe1-xCuxPO4/C (x = 0, 0.01, 0.015, 0.02, 0.025)
cathode materials had been synthesized by a solid state high
temperature reaction in an inert atmosphere using Cu(Ac)2 as
a dopant and FePO4 as a precursor. The charge/discharge test
showed that the cathode materials possessed the excellent
charge/discharge capacities, about 150 Ah Kg1 and 297 Ah
L1 at a rate of 0.1 C and more than 127 Ah Kg1 and 252 Ah
L1 at a rate of 2 C [38]. These approaches do not increase
the lattice electronic conductivity or chemical diffusion
coefficient of lithium within the crystal.
The power density (i.e., the charge/discharge rate) of
LiFePO4 is limited by the rate of Li ion and electron
migration through the electrolyte into the electrode [39].
Strategies to increase the charge/discharge rate of LiFePO4
have focused on improving electron transport overall [35] or
simply at the cathode surface [31, 40], along with reducing
the diffusion distance by using nano-sized materials [41, 42].
Moreover, LiFePO4 nanoparticals coated with a lithium
phosphate layer demonstrated an extremely high
charge/discharge rate [43] due to the lithium phosphate outer
layer providing an ultrafast charging and discharging Li ion
tunnel. Another promising cathode material is LiFeSO4F
[44]. By introducing fluorine and by replacing phosphate
group with more electron-withdrawing sulphate groups, this
material does not need nanostructuring or carbon coating.
2.4. Sulfur-Based Materials
The specific capacity of sulfur (S) cathodes is high (1670
Ah Kg1) compared to that of most other materials, e.g.,
LiCoO2, LiNiO2, LiMn2O4, LiMnO2 and LiFePO4. In
addition, S has a low cost and readily available, having a
wide operation temperature range and possibility of long
cycling. However, it has not been widely commercialized
because of the poor electrical conductivity of elemental S
and polysulfide shuttling that occurs during charge/discharge
cycles [45]. To solve these problems as well as large volume
change of S, a nanostructured S-carbon [46] and a
hierarchically structured S-carbon nanocomposite [47] were
developed for cathodes.
Lithium sulfide (Li2S) is also a possible cathode material.
Similar to S, the poor electronic conductivity of Li2S restricts
its practical application, despite a high theoretical capacity of
1166 Ah Kg1 [48, 49]. Metal additives (such as Cu, Co, and
Fe) have been employed to enhance the conductivity of Li2S-
based cathodes [48-50]. But the metal additives result in a
lower output voltage [49, 50]. A nanostructured Li2S-carbon
[47] has been employed as a cathode material to solve the
insulation and solubility of polysulphide anions during
charge/discharge [51].
Both S-carbon and Li2S-carbon nanostructured materials
use electrical conductive micro/mesoporous carbon for
loading S or Li2S. These materials have a high specific
surface area in contact with electrolytes as well as a reduced
diffusion length for electronic and Li ion transport. These
properties lead to a high charge/discharge rate. Furthermore,
the spatial confinement of micro/mesoporous carbon
represses the solubility of polysulphide anions.
There are a number of commercial cathode materials
available. However few of them can meet all the necessary
requirements. Considering of these, ternary materials [Such
as LiCoxMnyNi1-x-yO2 (0<x<0.5, 0<y<0.5)] and sulfur are the
possible cathodes to be commercialized. Table 1 summarizes
the properties of commonly used materials in rechargeable
lithium batteries.
Reality and F uture of Rechargeable Lithium Batteries The Open Materials Science Journal, 2011, Volume 5 207
3. ANODES
The materials for the ideal anode of rechargeable lithium
batteries should have following properties: First, the anode
materials may have the ability to reversibly
intercalate/deintercalate Li ion, at a large capacity and low
potential. Similar to cathode materials, the anode materials
should suffer minimal structural and redox potential change
during the Li ion intercalation/deintercalation. These two
requirements are important for good cycle performance and a
smooth charge/discharge curve. Second, the anode materials
should have high electronic conductivity, high Li ion
diffusion rate and conductivity. Lastly, the anode materials
should form a good solid-electrolyte interface with
electrolytes at the first circle, and remain chemically stable
with the electrolyte in the subsequent cycles.
Carbon-based materials are generally used in commercial
Li ion batteries as the anode. However, based on the
limitation of the theoretical gravimetric capacities of these
materials (372 Ah Kg1, LiC6), many efforts have been
carried out to develop higher capacity anode materials, such
as Li-based materials, Tin-based materials, Transition-metal
oxides and silicon. The advantages and disadvantages of
different types of anode materials are discussed below.
3.1. Li-Based Materials
Lithium is a most electropositive (3.04 V versus
standard hydrogen electrode) as well as the lightest metal in
the elemental table [3]. As an anode material, it has a
theoretical capacity of 3860 Ah Kg1. However, lithium can
react with organic electrolytes, resulting in an insulating
layer or gas evolution, thus increasing the internal pressure
in the cell. Meanwhile, lithium dendrites form on the lithium
electrode surface during charge/discharge, bringing in two
serious results: a gradual fading of capacity due to the
fracture of dendrites, and internal short circuiting of cells due
to the penetration of dendrites through the separator. Ionic
conductors, lithium phosphorus oxynitride, lithium nitride,
lithium superionic conductors (LISICON) and different
electrolytes have been used to restrain the formation of
lithium dendrites [52-60].
In order to improve the safety of lithium batteries, Li3-
xCoxN (x=0.1~0.6) was used as a new material to substitute
the existing lithium anode. Despite the large and reversible
capacity (760 Ah Kg1 or 1500 Ah L1), moisture sensitivity
can restrict the practical use of this material [61].
3.2. Carbon-Based Materials
In commercial Li ion batteries, soft carbon (such as
nature graphite) and hard carbon (such as pyrolytic carbon
from polymer) are widely used as the anode, because of
theirs low and smooth charge/discharge potential, and good
cycle performance. It has some shortcomings, however, such
as low capacity (372 Ah Kg1 or 830 Ah L1), bad
performance under high charge/discharge rates (lithium can
deposit on the surface of graphite), and cointercalation of
organic solvent into graphite [62, 63]. Many current research
efforts are focused on searching for new materials to
substitute graphite anodes.
Graphene is a two-dimensional aromatic monolayer of
carbon atoms. It has been proposed that Li ion could be
adsorbed on both sides of graphene sheets, which leads to
two layers of lithium for each graphene sheet, with a
theoretical capacity of 744 Ah Kg1 through the formation of
Li2C6
[64, 65]. It was reported that the electrochemical
performances were supposed to be greatly enhanced if
combined with Fe3O4 nanoparticles and graphene [66]. The
Fe3O4 nanoparticles were dispersed on graphene sheets via
microwave irradiation synthesis. As anode materials for Li
ion batteries, they showed high reversible capacities, as well
as significantly enhanced cycling performances (about 650
Ah Kg1 after 50 cycles) and high rate capabilities (350 Ah
Kg1 at 5 C), which might be attributed to graphene sheets
not only acted as electron conductors, but also buffers which
accommodated the strains of Li ion insertion/extraction and
relieved the strain associated with the volume variations
during cycles.
Table 1. Comparison of Typical Cathode Materials
LiCoO2 LiMn2O4 LiFePO4
Theoretical Capacity (Ah Kg1) 145 148 170
Commercial Capacity (Ah Kg1) 135~ 140 100~ 110 140~160
Tap Density (Kg L1) 2.6~3.0 1.8~2.4 0.8~1.4
Discharge Plateau (V) 3.6 3.7 3.3
Cycle Life (Cycles) 500-800 1000-1500 > 3000
Working Temperature (oC) 20~55 20~50 20~60
Advantages
1. Simple process
2. High volumetric capacity
1. Cheap
2. Simple process
1. Cheap
2. Eco-friendly
3. Safe
Disadvantages
1. Expensive
2. Toxic
1. Capacity fades
at elevated
temperature
1. Low conductivity
2. Complex process
3. Low volumetric
Capacity
Applied Areas Portable Devices Electric Vehicles Electric Vehicles
208 The Open Materials Science Journal, 2011, Volume 5 Tao et al.
3.3. Tin-Based Materials
Lithium will form alloys with some metals under certain
conditions, in which Li ion can reversibly
intercalate/deintercalate. Metals and alloys have been
investigated for anode application since 1970, and have a
much larger capacity than that of graphite. A commercial
battery based on a low-melting alloy (an alloy of Bi, Pb, Sn
and Cd) for anodes was introduced in the 1980s [67, 68].
Li22Sn5 has a high theoretical capacity of 990 Ah Kg1 or
7200 Ah L1. However, the volume and composition of this
kind of alloy change during the electrochemical reaction,
leading to fragmentation of the alloy [69, 70].
Intermetallic compounds MM´ (M is an “active” element,
can form an alloy with lithium, and M´ is an “inactive”
element, cannot form an alloy with lithium) have been used
as anode materials, and the cycling performance of these
compounds can be improved significantly if the active alloy
element is finely dispersed completely in an inactive matrix
[71]. Therefore, it is believed that the inactive species
provides structural stability and combats the expansion of the
alloy composite. The reversible reactivity of intermetallic
compound SnFe with Li ion has been explored for an anode
material [72-74]. The benefit arises from the alloy formation
between Li ion and Sn atoms at the grain boundaries of
SnFe3 particles [75]. However, the cycling performance was
improved at the expense of the capacity. Another approach
to improve the cycling performance was that Li ion could be
intercalated into the intermetallic compound Cu6Sn5 to yield
the product Li13Cu6Sn5 with the volumetric capacity of 2964
Ah L1. In addition, Li ion does not alloy or react with the
“inactive” component (i.e., Cu), which further increases the
stability of the anode. Note that Cu6Sn5 has relatively small
irreversible capacities compared to tin oxides [76].
Tin-based amorphous oxides, SnMxOy (M is B, P or Al,
x1), have a high volumetric specific capacity of more than
2200 Ah L1. In these compounds, Sn forms the
electrochemically active center for Li ion intercalation, and
the other metal group provides an electrochemical inactive
network of -(M-O)- bonding, to confer high reversibility in
Li ion storage and release [77, 78]. This type of the material
has not been commercialized because of poor long-term
cyclability and the fact that a large amount of the capacity is
irreversibly lost during the first cycle (because many Li ion s
have reacted to form Li2O and solid-electrolyte interface
film).
Tin-transition metal-carbon (Sn-TM-C) alloys have been
used to replace graphite as the anode for Li ion batteries [79].
The Sony Corporation launched in 2005 a Li ion battery that
uses a “tin-based amorphous anode” comprising tin, cobalt
and carbon as the anode [80]. The Sn-Co-C system has been
proposed to be the best choice among Sn-TM-C (TM=
transition metal) for anodes in Li ion batteries since a
nanostructure consisting of amorphous CoSn grains in a
carbon matrix is formed during sputtering or during
mechanical milling [81-83]. It was reported that Sn30Co30C40
had good capacity retention for at least 100 cycles at around
425 Ah Kg1, and Sn30Co15TM15C40 also showed good
capacity retention for at least 100 cycles ranging from 270
Ah Kg1 for samples with TM=Ni to 500 Ah Kg1 for
samples w ith TM=Ti, which might be attributed to the
desired nanostructured-type XRD pattern [84].
3.4. Transition-Metal Oxides
Transition-metal oxides (MO) were proposed as anode
materials for their large capacity at low potentials [85, 86].
The reaction mechanism between MO (M is Co, Ni, Cu or
Fe) and Li ion differs from the classical Li ion
intercalation/deintercalation or Li-alloying processes [87]. It
involves the composition and decomposition of Li2O,
accompanied with the reduction and oxidation of the
transition metal, respectively. The capacities of these
transition-metals are all greater than 700 Ah Kg1, with high
capacity retention and high recharging rate. These systems
hold much promise for future development.
Among MO materials, the capacities of Ti-based oxides
are less than half that of graphite (175 Ah Kg1 for
Li4Ti5O12). However, these materials have many advantages,
such as outstanding stability, rapid charge rate, and wide
operating temperature (range from 50 °C to 75 °C). The
combination of these advantages results in ultra long
durability (around 20 years) and cycle life (9000 cycles). In
addition, these batteries do not explode or result in thermal
runaway under harsh conditions [88, 89].
3.5. Silicon
Among all the compounds proposed to replace the
graphite anode, silicon is very promising because it has a
theoretical specific capacity of 4200 Ah Kg1 (for Li22Si5)
[90, 91]. Moreover, silicon is the second most abundant
element on earth and already has a mature industrial
infrastructure in existence. It is an attractive material when
considering commercial applications. But the biggest
holdback preventing the commercial application of silicon is
the large inherent change in specific volume (up to about
410%) during the intercalation/deintercalation of Li ion. This
causes crumbling, and a loss of electrical contact between
the active material and the current collector [69, 92-95].
Recently, Si nanowires were developed as the anode material
to accommodate the large volume change and to avoid
capacity loss during cycling [96-102]. In addition, nanowires
form direct chemical bonds with the current collector for
good adhesion and electron transport, which makes the
binding polymer and conducting graphite unnecessary. The
observed specific capacity was about 2800 Ah Kg1 [84].
Yushin et al. conducted a large-scale hierarchical bottom-up
assembly route for the formation of Si on the nanoscale-
containing rigid and robust spheres with irregular channels
for rapid access of Li ions into the particle bulk. Reversible
capacities of the C-Si nanocomposite reached up to 1,950 Ah
Kg1, which is over five times higher than that of the
theoretical capacity of graphite [103, 104].
The capacity of different anode materials is shown in Fig.
(3). Despite many candidates for anode materials, graphite is
the only widely used commercial anode material. Other
anode materials, such as Li4Ti5O12 and Li22Sn5, are currently
on a small scale application. Looking into the future, the
trend of anode materials may shift to high capacity lithium or
silicon materials.
Reality and F uture of Rechargeable Lithium Batteries The Open Materials Science Journal, 2011, Volume 5 209
Fig. (3). The capacity of different anode materials.
4. ELECTROLYTES
Electrolyte is one of the key components of a battery,
which is commonly referred to as a solution comprising
solvents and salts (such as LiPF6, which is well known for its
rapid dissolution in carbonate solvents, lower cost and good
conductivity [105, 106]). The choice of electrolytes is very
crucial, and depends on the choice of the anode and the
cathode. For instance, graphite anodes operate in a highly
reducing voltage range (<1.2 V vs lithium), at which point
most electrolytes are thermodynamically reduced. To ensure
reversible behavior, the deposition of an efficient passivating
layer at the graphite surface is necessary during the first
cycle. The electrolyte was probably oxidized when the
lithium oxide cathode materials were charged up to more
than 4 V vs lith ium [7, 107]. Electrolyte oxidation leads to
irreversible loss of capacity, because of the generation of
new chemical species, which deposit on the electrode surface
as an insulating layer or evolve as a gas, thus increasing the
internal pressure in the cell. Electrolyte oxidation is believed
to be the main failure mechanism for rocking-chair
technology [108]. Thus, minimizing electrolyte oxidation is
a major requirement in enhancing the cycle life and
improving the performance of lithium batteries at elevated
temperatures. Fortunately, the electrolyte oxidation reaction
is limited. Most of the electrolytes can be used beyond the
voltage range of their thermodynamic stability [108].
4.1. Traditional Organic Liquids
There are numerous liquid compounds available to be
selected as electrolytes. Viscosity, dielectric constants and
ionic conductivity of an organic liquid should be considered
first to determine a suitable electrolyte. Most liquid
electrolytes are composed of ethylene carbonate (EC) and
dimethyl carbonate (DMC). EC is present in almost all
commercial compositions, because of its low cost, good
electrochemical stability, and high dielectric constant which
permits better ionic dissociation of the salt and improves the
ionic conductivity. Furthermore, it can provide a protective
layer on the surface of graphite that prevents further reaction
[109]. However, a pure EC-based electrolyte was not used
because of its high freezing point (35-38 oC), which is not
compatible with practical application. DMC, commonly
known as a thinning solvent, is used with EC to reduce the
viscosity. Note that a pure DMC-based electrolyte is not
compatible with graphite anodes, since no passivation layer
can build up during cycling [108]. In general, the traditional
liquid electrolytes have several disadvantages, such as
flammability and a narrow range of operating temperatures
[110]. These problems could be solved by novel electrolytes
such as ionic liquids, organic solid electrolytes or inorganic
solid electrolytes.
4.2. Ionic Liquids
Ionic liquids as electrolytes for lithium batteries have
been studied in recent years [111-113]. The ionic liquids are
nonflammable as they contain no volatile compound. In
addition, they show a broad electrochemical stability
window (generally>4 V). Early attempts to cycle Li ion
batteries using electrolytes on the basis of ionic liquids failed
because of electrolyte reduction occurring at the low
potential [114]. One type of good ionic liquid utilized as an
electrolyte is an aluminum chloride (AlCl3) based solution.
However, AlCl3 is toxic and difficult to process [115-117].
N-methyl-N-propylpiperidinium-bis
(trifluoromethylsulfonyl) imide [111] and n-
hexyltrimethylammonium-bis (trifluoromethylsulfonyl)
imide [118] (AlCl3-free ionic liquids) were successfully
developed and have been used as electrolytes. Highly
reversible and stable cycling have also been obtained using
1-ethyl-3-methylimidazolium-bis (trifluoromethylsulfonyl)
imide as the electro lyte [119]. A solvent-free, ternary
polymer electrolytes based on a novel poly
(diallyldimethylammonium) bis (trifluoromethanesulfonyl)
imide polymeric ionic liquid (PIL) as polymer host and
incorporating PYR14TFSI ionic liquid and LiTFSI salt are
reported (Ternary polymer electrolytes containing
pyrrolidinium-based polymeric ionic liquids for lithium
batteries) [120]. The PIL-based polymer electrolytes
exhibited room temperature ionic conductivity above 104 S
cm1, and the Li/PIL-LiTFSI-PYR14TFSI /LiFePO4 solid-
state batteries are capable to deliver above 140 Ah Kg1 at 40
oC. Over all, still only a few ionic liquid electrolytes have
been found suitable for lithium batteries because of their
high viscosity issues [121].
4.3. Organic Solid Electrolytes
Many efforts have been dedicated to develop all-solid-
state Li ion batteries [2]. Organic polymers (rubbery
electrolytes) are the promising candidates. Polymer
electrolytes are commonly composed of a lithium salt (LiX)
and a high-molecular-weight polymer such as polyethylene
oxide (PEO). However, PEO crystallization below 60 oC is a
challenge for electrolyte application at lower temperature. As
a result, PEO-LiX electrolytes work only at temperatures
above 60 oC. The most common approach for lowering the
operational temperature is adding liquid plasticizers (such as
propylene carbonate or polyethylene glycol ethers) or gels
(contain 60-95% liquid) [122]. But this method promotes the
deterioration of the electrolyte’s mechanical properties and
increases its reactivity towards the lithium anode. A series of
“polymer-in-salt” materials were developed as electrolytes,
in which lithium salts were mixed with small quantities of
the polymers, e.g., polypropylene oxide and polyethylene
oxide. The glass transition temperature of these materials is
low enough to remain rubbery at room temperature with
good Li ion conductivity and high electrochemical stability
210 The Open Materials Science Journal, 2011, Volume 5 Tao et al.
[123]. When using TiO2 or Al2O3 nanoparticles as solid
plasticizers in PEO, a solid-state polymer electrolyte has
been developed [124]. The conductivity of these electrolytes
increased and the crystallization was well prevented. Li ion
doped plastic crystalline matrixes are stable over a potential
of 5 V and very attractive for battery applications in
combination of possible structural variations of plastic
crystal matrixes and conductivities [125]. Huang et al. have
prepared a novel solid-state composite polymer electrolyte
based on poly (ethylene oxide) (PEO) by using LiClO4 as
doping salts and inorganic-organic hybrid poly
(cyclotriphosphazene-co-4, 40-sulfonyldiphenol) (PZS)
microspheres as fillers [126]. Compared with traditional
ceramic fillers such as SiO2, PZSMS in PEO-based polymer
electrolytes leads to higher enhancement in ionic
conductivity.
4.4. Inorganic Solid Electrolytes
Other promising candidates for solid electrolytes are
inorganic materials (brittle superionic glass electrolytes). For
inorganic solid electrolytes, lithium superionic conductors
(LISICON) are very importance with respect to achieving an
all solid-state lithium battery. This technology may solve the
safety problems of the rechargeable Li ion batteries using
nonaqueous liquid electrolytes. In 1978, Li14Zn(GeO4)4, a
type of LISICON was found [127], which attracted attention
for its potential application as a solid electrolyte [128].
A new solid system based on lithium germanium sulfide
and lithium silicon sulfide was found [129, 130], named
“thio-LISICON”. This is the first example of crystalline
ionic conductor with a high ionic conductivity and high
decomposition potential at room temperature. Sulfide-based
electrolytes generally have a higher Li ion conductivity, by
several orders of magnitude, compared with oxide-based
electrolytes. For example, the Li ion conductivity of thio-
LISICON is around 10-3 S cm1 for Li3.25Ge0.25P0.75S4, four
orders of magnitude higher than Li14Zn(GeO4)4, a typical
oxide LISICON.
The thio-LISICO N has high electrochemical stability,
which is important for all solid-state lithium batteries. Many
studies focused on the binary Li2S-P2S5 system [131].
70Li2S-30P2S5 (mol%) glass was prepared by a quenching
melt techniques [132]. The obtained glass-ceramic showed
high Li ion conductivity of 2.1103 S cm1 at room
temperature. Glass-ceramic Li2S-P2S5 electrolytes were
prepared by a single step ball milling process at 55 oC [133].
The produced crystalline glass-ceramic materials exhibit
high Li ion conductivity over 10-3 S/cm at room temperature
with a wide electrochemical stability window of 5 V.
Ideally, liquid electrolytes will be replaced by solid state
electrolytes that can perform similarly without excessive
safety issues [134]. The current barriers with solid state
electrolytes include inferior charge/discharge rate, ionic
conductivity, interfacial stability and mechanical strength
[135, 136]. Efforts have been carried out to find a solid state
electrolyte that can outperform liqu id electrolytes.
5. SEPARATORS
A separator is an important component of a battery cell,
as it prevents short circuit by separating the anode from the
cathode, as well as providing passages for Li ion [137]. In a
Li ion battery, the separator is required to be capable of
battery shutdown at the temperature below that at which
thermal runaway occurs, and the shutdown should not result
in loss of mechanical integrity. Otherwise, the electrodes
could come into direct contact and the resulting chemical
reactions cause thermal runaway [111]. Shutdown is an
important trait of a good separator for the safety of lithium
batteries. The promising separators are those with high
electrolyte permeability and mechanical strength, as well as
good thermal, chemical, and electrochemical stability.
5.1. Organic Separators
In commercially available lithium batteries, microporous
membranes fabricated from polyethylene (PE) and
polypropylene (PP) are used as separators [138-140]. These
polyolefin separators are suitable for batteries, since they can
be used for hundreds of cycles without any chemical or
physical degradation. The collapse of the pores occurs when
the temperature approaches the melting point of the material,
which forms a nonporous insulated film and results in a
sharp increase in impedance. Since the impedance of a PP
separator increases less than that of a PE separator, the
impedance of the PP separator may not be large enough for
complete shutdown, and thermal runaway could still happen.
Therefore, PE is the preferred separator material for most Li
ion batteries [141, 142]. Robust mechanical properties of the
separator are expected even above the shutdown temperature
because the battery temperature may increase continuously
after shutdown. When the separator undergoes a meltdown,
the mechanical properties of the separator could deteriorate
greatly and the cell may experience an internal short circuit,
resulting in a hazard. The shutdown temperature of a
separator should be lower than its meltdown temperature. A
sandwiched separator containing one porous PE layer
between two porous PP layers (PP-PE-PP trilayer) has been
made to maintain the robust mechanical properties. The PE
layer offers a lower shutdown temperature; whereas the PP
provides mechanical stability above the shutdown
temperature (such as Celgard® [143]). The shutdown
temperature and meltdown temperature were increased to
142 °C and 155 °C respectively, when the separator was
coated with diethylene glycol dimethacrylate [144]. When
silica nanoparticles were added to the separator, the
meltdown temperature increased to 170 °C [145]. Polyolefin
membrane separators, however, have several drawbacks,
such as large thermal shrinkage near its melting/softening
temperature, low porosity and low wettability in electrolyte
solutions. Silica-composite nonwovens using polyolefin fiber
and nanosize silica powder showed not only better
wettability than the polyolefin-based membrane and
nonwoven, but also thermal shrinkage of ~3% at 160 °C
under air atmosphere and thermally stable at 150 °C in the
liquid electrolyte [146].
5.2. Inorganic Separators
Separators can also be made of inorganic sub-micron
sized particles and a small amount of polymer binder, which
have dimensional stability at a high temperature as well as
wettability [147]. This type of separator is highly desirable
for the developmen t of large-size lithium batteries, especially
those installed in electric vehicles and power tools.
Nonwoven support materials can improve the mechanical
Reality and F uture of Rechargeable Lithium Batteries The Open Materials Science Journal, 2011, Volume 5 211
strength of inorganic separators [146]. Recently, nonwoven
supported inorganic separators prepared via a sol-gel coating
method were commercialized for Li ion batteries [148, 149].
Kim et al. prepared an inorganic separator by coating
inorganic submicrometer sized Al2O3 particles on a
nonwoven matrix Aramid fiber followed by an E-beam
irradiation treatment [150]. The mechanical and thermal
properties of the separator were greatly enhanced by the
simple curing under E-beam irradiation. Remarkable
improvements of the separators with respect to the battery
safety have been demonstrated by a series of abuse tests.
However, the practical application of the inorganic particle
coated nonwoven separators using polymeric binders has not
been realized, because of insufficient mechanical strength to
withstand the roll-to-roll manufacturing process [150].
CONCLUSIONS
The performance of rechargeable lithium batteries
depends on the properties of cathodes, anodes, electrolytes
and separators. The discovery of intercalation materials has
played a significant role in increased performance. Th e
intercalation materials, such as the graphite anode and the
LiCoO2 cathode, impact the large scale commercialization of
rechargeable lithium batteries. Besides intercalation
materials, the application of nanostructured materials results
in substantial improvemen ts of r echargeable lithium
batteries. Currently, lithium cobalt oxide, lithium manganese
oxide and lithium iron phosphate are the most used cathode
materials. Other promising cathode materials, such as sulfur
and lithium sulfide, are under development. In the area of
anode materials, graphite is the most widely used. However,
the tin-based anode material technology is maturing, and the
lithium or silicon-based materials are also becoming next
generation anode materials. In the past, lithium was chosen
as the anode material of lithium batteries due to its largest
theoretical energy density. However, the market for lithium
anodes has dropped due to safety concerns and battery
stability . As shown in Fig. (1), the highest energy density of
existing Li ion batteries is only about 150 Wh Kg1 or 300
Wh L1, and the maximum energy density of the ideal Li ion
batteries is about 580 Wh kg1 or 1810 Wh L1. The ideal
lithium-sulfur (Li-S) battery has a much higher energy
density of 2500 Wh kg1 or 2800 Wh L1. In this regard, the
Li-S battery may be the rechargeable lithium battery of the
future.
From potable electronics to electric vehicles or hybrid
electric vehicles, power sources that have a high
charge/discharge rate, high power density, long cycle life,
and safe operation are in constant demand. The successful
design of new assembly technology, discovery of new
materials, and development of new theories will promote the
development of next generation rechargeable lithium
batteries.
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Received: January 18, 2011 Revised: May 17, 2011 Accepted: June 23, 2011
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