Composite anodes for lithium-ion batteries

AIMS Materials Science, 3(3): 1054-1106.
DOI: 10.3934/matersci.2016.3.1054
Received: 29 June 2016
Accepted: 24 July 2016
Published: 29 July 2016
http://www.aimspress.com/journal/Materials
Review
Composite anodes for lithium-ion batteries: status and trends
Alain Mauger 1
, Haiming Xie 2,
*, and Christian M. Julien 3,
*
1
Sorbonne Universités, Univ. UPMC, Paris-6, Institut de Minéralogie et Physique de la Matière
Condensée (IMPMC), 4 place Jussieu, 75252 Paris, France
2
Northeast Normal University, National & Local United Engineering, Laboratory for Power
Batteries, 5268 Renmin Str., Changchun, P.R. China
3
Sorbonne Universités, Univ. UPMC, Paris-6, Physico-Chimie des Electrolytes et Nanosystèmes
Interfaciaux (PHENIX), UMR 8234, 4 place Jussieu, 75005 Paris, France
* Correspondence: E-mail: Christian.Julien@upmc.fr; xiehm136@nenu.edu.cn.
Abstract: Presently, the negative electrodes of lithium-ion batteries (LIBs) is constituted by
carbon-based materials that exhibit a limited specific capacity 372 mAh g−1
associated with the cycle
between C and LiC6. Therefore, many efforts are currently made towards the technological
development nanostructured materials in which the electrochemical processes occurs as intercalation,
alloying or conversion reactions with a good accommodation of dilatation/contraction during cycling.
In this review, attention is focused on advanced anode composite materials based on carbon, silicon,
germanium, tin, titanium and conversion anode composite based on transition-metal oxides.
Keywords: composites; anode materials; conversion reaction; alloying; intercalation; Li-ion batteries
1. Introduction
Among all of the power sources, lithium ion batteries (LIBs) have become one of the most
important and efficient devices for storage of electrical energy generated from renewable sources
such as solar and wind energy, and they are currently used to solve the intermittence problem of
these renewable energy sources for their integration in grid-scale stationary storage [1]. However, the
LIBs that are commercialized today cannot yet satisfactorily meet the energy requirements for
HEVs/EVs, and the increase of the energy density without sacrificing the safety is the subject of

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AIMS Materials Science Volume 3, Issue 3, 1054-1106.
intensive research. So far, the most popular material used as the anode of commercial Li-ion batteries
is graphitic carbon. Its theoretical capacity, however, is only 372 mAh g−1
, while the total capacity of
a Li-ion cell is expected to increase significantly with anode capacity up to 500 mAh g−1
approximately. For larger anode capacities, the total capacity reaches saturation because of the
limitation imposed by the cathode [2]. There is thus a strong motivation to find an active anode
material that would substitute carbon to reach a larger capacity. This is a difficult task, however,
because the capacity is not the only pertinent parameter. One advantage of the carbon is the
controlled formation of a stable solid-electrolyte interface (SEI) that is formed during the first cycles.
Also, the volume change during the lithium insertion or delithiation must be controlled to avoid
pulverization or simply breaking of the SEI, which would result in aging of the cell and loss of
capacity upon cycling. In addition, the ionic and electronic conductivities should be as large as
possible to obtain a large power density. Many materials synthesized under different shapes have
been proposed, whether the lithium transport is due to intercalation, alloying, or conversion reactions.
We have recently reviewed them in [3]. Figure 1 illustrates the potential vs. Li0
/Li+
and the
corresponding specific capacity of the next generation of active anode materials. The trend is the
reduction of the size of the active particles to the nanometric range. The advantage is a shorter path
for the Li+
ions and the electrons in the particles, which allows a faster charge and discharge.
Moreover, the effective surface area between the active particles and the electrolyte is enhanced,
which reduces the internal resistance of the cell.
The reduction in size to the nano-range is also needed to accommodate the volume deformation
and prevent the fracture during lithiation and delithiation. Indeed, excellent mechanical properties are
also necessary for high-capacity anode materials the physical mechanisms such as volume
deformation and fracture during lithiation and delithiation are currently the suject of intensive
research. The conditions to avert fracture and debonding of hollow core-shell nanostructures in terms
of the core radius, the shell thickness, and the state of charge have been determined by Zhao et al. [4],
modeled in [5], and simulated by using a non-linear diffusion lithiation model [6]. Optimal
conditions for the full use of inner hollow space were identified in terms of the critical ratio of shell
thickness and inner size and the state of charge by Jiang et al. [7]. Fracture mechanics was used to
determine the critical conditions to avert insertion-induced cracking [8]. A bridge between hardness
and state of charge for electrodes in lithium-ion batteries by introducing electrochemistry-induced
dislocations was proposed by Wang et al. [9]. For Si anodes, the critical sizes of fracture for different
shapes of Si have been found to be 90 nm for nanoparticles, 70 nm for nanowires, and 33 nm for
nanofilms, below which the silicon nanostructures remain undamaged upon lithiation [10]. The
critical sizes of thin films and nanoparticles below which cracking would not occur has been
investigated in [11]. The surface cracking of Si particles has been investigated through calculations
in an elastic and perfectly plastic model [12]. The mechanical properties of Li-Sn alloys have been
published by first-principle studies [13,14] and finite element modeling [15].
The reduction of the size of the particles to the nanometric range is not sufficient to ensure high
capability and high power density; however, all the criteria in terms of ionic and electronic
conductivity and stability upon cycling can hardly be met in any single material. That is why recent
works proposed composites that mix active elements with complementary properties, like carbon for
its high electronic conductivity, and silicon for its high energy density for instance, aiming to obtain

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the best compromise between these different properties. Indeed, the only alternative to carbon anodes
that encountered a success at the industrial scale is the composite Sn/Co/C commercialized by Sony
in 2005 [16]. Since then, many other nanocomposites have been proposed as anode materials for next
generation of Li-ion batteries. The aim of this section is to review them and report on the
state-of-the-art in the investigation of composites as anodes for Li. Bs. Note that the reader is guided
to the review [3] that contains 660 references for any work related to the synthesis and properties of
the single components of the composites. Since then, however, progress has been made for the
synthesis of composite nanofibers by electrospinning and centrifugal forcespinning (FS) for use as
anodes in LIBs. They have been reviewed in [17]. Although electrospinning has been widely used as
a synthesis process of electro-active nanofibers for constructing high performance materials for
Li-ion batteries (see [18,19] for review). FS can produce fibers from melt and solutions without the
need of applying an electric field during processing. Furthermore, the FS method has proven to be
suitable for the mass production of nanofibers. To preserve a reasonable length to this section, we
have chosen to omit any detail on the synthesis. This is a paradox, since the progress in the synthesis
process of the nanocomposites is obviously the reason for the progress in their performance.
However, the synthesis process is usually well described in the publications. Moreover, a significant
progress has been reported on the synthesis and fabrication of composite materials for applications in
LIBs [20] where the voltage limits of the anodes are discussed. The recent progress in enhancing the
LIBs performance of TiO2 with various synthetic strategies and architectures control has been
reviewed in [21,22]. The properties of porous structures and their applications to negative electrodes
for rechargeable lithium-ion batteries have been reviewed in [23]. An overview of several novel
fabrication techniques of the electrodes for improving the electrochemical performance of
silicon-based anode materials and their possible mechanisms are given in [24]. Various approaches to
design materials in the form of 0, 1 and 2D nanostructures and their effect of size and morphology on
their performance as anode materials in LIBs have been reviewed in [25].
Figure 1. The potential vs. Li0
/Li+
and the corresponding specific capacity of the next
generation of active anode materials.

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Taking these considerations into account, in this review, we focus attention on the
electrochemical properties of the composites, although the best electrochemical properties obtained
with the components are reported for the sake of comparison and discussion. The progress has been
so fast that also made the choice to report on the works mainly published the last five years. We
guide the reader to elder review papers on anode materials for prior works [26,27]. Finally, compared
with Li4Ti5O12, niobium based oxides have similar lithium ion insertion/extraction potential and have
a similar good rate performance and promising to be new anode materials with high power
performance. Only Li4Ti5O12 as an anode component has been reviewed here, while Nb-based oxides
as anodes have been reviewed in [28].
2. Carbon-based Anodes
2.1. Carbon Nanotube Composite Anodes
Carbon nanotubes (CNTs) have good thermal and mechanical properties, together with a good
conductivity. They are thus candidates for use as anodes in Li-ion batteries. However, the
performance will depend strongly on the quality of the SEI, which is a function of the quality of the
surface. This is actually a drawback of the decrease of the size of the particles to the nano-range: as
the effective surface in contact with the electrolyte is larger, the SEI effects become more critical.
This has been the motivation for coating the nanotubes or the graphene sheets to protect them. In
particular, The deposition of a 10 nm-thick layer of Al2O3 on the multi-wall carbon nanotubes
(MWCNTs) by atomic layer deposition delivered a reversible capacity of 1100 mAh g−1
in 50 cycles
at the current rate of 372 mA g−1
[29]. This is the best result obtained with MWCNTs so far. The
effect of Al2O3 is the blocking of the electron tunneling to the adsorbed ethylene carbonate (EC)
molecules of the electrolyte, thus decreasing its decomposition [30]. Therefore, the Al2O3 coat acts as
an artificial SEI, which is beneficial for the performance of the MWCNTs. For comparison, the
capacity delivered by commercial MWCNTs is close to 250 mAh g−1
, raising to 400 mAh g−1
after
purification. Some good results have also been obtained with composites of carbon nanotubes and
other active nanostructured materials [31]. For instance, SnSb particles finely encrust within the
mesh-like CNT framework, which consisted of well dispersed CNTs, delivered a reversible capacity
of 860 mAh g−1
during the 40th cycle at a current density of 160 mA g−1
[32]. The uniform Fe3O4
coating of aligned carbon nanotubes by magneton scattering used as an anode delivered 800 mAh g−1
over 100 cycles [33]. Composite electrodes consisting in Fe3O4 nanorods and 5 wt% single wall
carbon nanotubes as a conductive net delivered over 1000 mAh g−1
after 50 cycles at 1C-rate, 800
mAh g−1
at 5C and still 600 mAh g−1
at 10C [34]. One can also cite MoS2/MWCNT composite
electrode that delivered 1030 mAh g−1
after 60 cycles [35]. However, carbon nanotubes did not find a
market to commercial use. Their growth rate is still too low, they are expensive and it is difficult to
prepare them free of impurity. Moreover, the atomic layer deposition used in [29] is a remarkable
process to deposit well-controlled surface layers, but it is also expensive. In addition, the CNTs
suffer from a tendency of embrittlement upon cycling because the morphology of the nanotubes does
not allow the expansion of the graphene sheets in the c-axis or radial direction as in graphite.

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2.2. Graphene Composite Anodes
Due to the elasticity, strong mechanical resistance, good electronic conductivity, large surface
area, graphene is an attractive anode material. In addition, the theoretical capacity is 780 mAh g−1
if
Li can be absorbed on both sides up to the chemical formula Li2C6, and 1116 mAh g−1
if Li can be
trapped at the benzene rings up to LiC2. The best performance has been obtained with N- and
S-doped hierarchically porous graphene electrode that showed a high power density (116 kW kg−1
)
and high energy density (322 Wh kg−1
) at current density 80 A g−1
(only 10 s to full charge) [36].
Graphene can thus be used for its intrinsic properties. Moreover, it is also used in composites with
another electroactive anode material for two reasons. First, the grafted nanoparticles prevent the
graphene sheets from re-stacking that is a major cause of capacity fading upon cycling. Second, the
graphene is an elastic support that helps the nanoparticles to accommodate their volume change upon
lithium insertion or delithiation without breaking or pulverization, another source of aging. Third,
such electrodes may not require the need for binders, which reduces the cost of fabrication and
maximizes the content in active materials and thus the capacity. Therefore, many works have been
published on graphene-based composite anodes, which have been reviewed in [37]. We shall report
and discuss the state-of-the-art of such composite anodes in the following sections devoted to the
metals and metal oxide counterparts. The important irreversible capacity during the first cycle is due
the formation of the SEI, and also in some cases due to irreversible reaction in the first cycle.
Therefore, we always report the stabilized reversible capacity after several cycles, which is the
capacity of interest for practical use of a secondary battery.
The incorporation of graphitic carbon nitride into graphene results in a g-C3N4/graphene (CN-G)
hybrid that exhibits improved Li+
storage capacity and rate capability with respect to the pristine
g-C3N4 and graphene [38].
3. Silicon-based Composites
With a huge theoretical capacity (4200 mAh g−1
), Si is a cheap and abundant material that has
attracted a huge interest as a prospective replacement of graphite anode, inasmuch as the material
processing is well known owing to its use in electronics. However, the huge volume change ~400%
between Si and Li22Si5 causes cracking and pulverisation, resulting in poor cycling that hinders its
commercial use. Nevertheless some solutions have been found to overcome this problem, which in
any case require the use of nanosized Si under different shapes.
3.1. Si Films
It is possible to obtain long cycling life up to 3000 cycles with Si films prepared by physical
vapor deposition [39,40] and magnetron sputtering [41]. A 50 nm-thick film of n-doped Si film with
phosphor to improve the electronic conductivity, deposited on to a 30-μm thick Ni foil, maintained a
capacity of 3000 mAh g−1
at rate 12C over 1000 cycles. Even at heavy load at 30C rate, the capacity
was still 2000 mAh g−1
after 3000 cycles [42].
In an attempt to improve these performances that are already remarkable, attempts have been

1059
made to synthesize multi-layer composites. Good results were already obtained more than 10 years
ago with Fe-Si multi-layer composite prepared by alternate deposition using an electron-beam
evaporation method. The layer thickness was 9.5 nm and 7.5 nm for the Si (active) and the Fe
(inactive) films, respectively [43]. After annealing a Fe-Si alloy is formed at the Si/Fe interface,
which acts as a buffer layer for alloy reaction of Si with Li [44]. This material used as an anode
delivered a specific capacity 3000 mAh g−1
over 300 tested cycles at charge and discharge current 30
A cm−2
between 0 and 1.2 V at 30 °C. More recently, another multilayer made of 50 nm-thick Si
films and a soft elastomeric substrate (poly(dimethylsiloxane)) acting as a buffer layer exhibited a
discharge capacity of 3498 mAh g−1
at C/4 current rate with 84.6% capacity retention over 500
charge/discharge cycles [45].
Coating with a metal oxide was chosen to protect direct contact between the Si nanostructure
and the electrolyte. The best results have been obtained with Al2O3 coating. Upon the first lithiation,
this oxide transforms into Al-Li-O glass [46]. This glass is an electrical insulator, but a good ionic
conductor, which are the attributes of a good SEI substitute. Of course, in that case, the thickness of
the coating layer must be small in order to minimize the increase of electrical resistivity induced by
the coating. In practice, the thickness of the Al2O3 obtained by atomic layer deposition (ALD) is
smaller than 10 nm, and has been tested successfully on Si thin films [47,48]. Note that it is difficult
to make a quantitative comparison between the different works on Si thin films, because the
performance depends dramatically of the deposition rate, temperature of the deposition, film
thickness, annealing treatment.
3.2. Nanostructured Si Nanowires and Nanotubes
The shape of the Si material also matters. The nanotubes and nanowires are less prone to
cracking and pulverization than nanoparticles upon cycling. They can also be connected electrically
to the current collector more easily than nanoparticles. That is why many efforts have been made to
prepare them and test them (see the review in [23]). In particular, porous B-doped Si nanowires
recorded 2000 cycles at 18 A g−1
current density, with a remaining capacity still larger than
1000 mAh g−1
[49]. The mass loading, however, was small (0.3 mg cm−2
). This is one major problem
with these Si anodes: high rate capability is obtained only at low mass loading. Figure 2 shows the
HRTEM images of silicon nanowires before (a) and after (b) lithiation (10th cycle at current rate of
0.4 A g−1
) [49].
Even if the porous structure and shape can help in the prevention of cracking, it does not solve
the problem of the SEI. In particular, once the silicon has been lithiated, the delitiation induces a
contraction that can break the SEI layer, which results in a new Si area in contact with the electrolyte,
and more SEI is thus formed. The resulting increase of the thickness of the SEI layer means more
consumption of the electrolyte, an increase of the electrical and ionic resistance, in short aging of the
cell. The strategy to overcome this problem is to coat the silicon with a protective layer. Si nanowires
were coated with carbon to build a mesoporous Si@carbon core-shell nanowires with a diameter of
<6.5 nm [50]. They were prepared by a SBA-15 hard template. The nanotubes were formed by filling
the void space of the template to form an inverse porous structure. This is one example among many
others that illustrates the efficiency of inverse opal nanoarchitecture as lithium ion anode materials,

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to which a recent review has been dedicated [51]. In [50], butyl terminators in the Si-C4H9 precursor
formed a capping that protected Si from reacting with SiO2 during annealing and converted to carbon
shell layers later in the annealing process. These carbon-coated nanowires retained a capacity at
2738 mAh g−1
after 80 cycles. The efficiency of the carbon shell depends on its thickness.
10 nm-thick carbon coating on Si nanowires of 90 nm in diameter increases the initial capacity to
3700 mAh g−1
(compared to 3125 mAh g−1
before coating) and the capacity retention is raised to
86% after 15 cycles [52]. Unfortunately no test has been reported after few hundreds cycles. Si
nanowires were also embedded in a network of carbon nanotubes.
Figure 2. HRTEM images of B-doped silicon nanowires before (a) and after (b) lithiation
(10th cycle at current rate of 0.4 A g−1
). (c) Enlarged HRTEM showing the amorphous Si
structure. (d) SAED pattern showing the crystalline Si in black spots in (b) (from
Ref. [49]).
Replacing the 10 nm-thick carbon coating on the Si nanowires with a Cu coat of same thickness
improves the coulombic efficiency in the first cycle to 90% and the capacity retention to 86% after
15 cycles. This effect can be attributed to the fact that Cu is a better electrical conductor than the
carbon coat. For the same reason, the Cu-coating of Si thin films also improved the performance [53]
The cycling properties of silicon nanowires has also been improved by coating them with a
conductive polymer like (3,4-ethylenedioxythiophene) PEDOT [54].
Cu can be replaced by another metal coating. However, Al coating has a different effect: it does
not improve the coulombic efficiency of the first cycle but it improves the capacity retention of
nanowires [55], mainly because it improves the mechanical structure of the anode [56,57]. Similar
results were obtained by coating the Si nanowires with a 100 nm-thick Ag/PEDOT improving the
capacity retention to 80% after 100 cycles [54].
The coating with a metal was intended to increase the electrical conductivity which is beneficial
to the electrochemical properties because Si is a semiconductor. On another hand, Si nanowires have

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also been coated with Al2O3, to avoid direct contact between Si and the electrolyte, like the Si thin
films previously mentioned. The Al2O3 coating of Si nanowires increased the anode lifetime up to
1280 cycles when the capacity is limited to 1200 mAh g−1
and exhibited a discharge capacity of
1 Ah g−1
at 12 C current rate after 6000 electrochemical cycles [58].
In a different approach, a stiff shell can reduce the mechanical stress of the Si nanostructures
and clamp them. The Si nanotubes with a clamping SiOx layer exhibited a discharge capacity of
1000 mAh g−1
at 12C current rate after 6000 electrochemical cycles [59]. The coat of Si nanotubes
by rigid carbon also proved to be successful to control their growth [59]. In the same spirit, Si
nanotubes were coated with Ge [60,61], SnO2 [62] and TiO2 [61]. Nevertheless, these coatings had
little effect on the initial coulombic efficiency, except Ge coating. The reason is that these coating
materials are almost inactive in the potential window 0.02–1.2 V. On another hand, they all improved
significantly the life of the battery, so that they efficiently avoided the direct contact between Si and
the electrolyte.
More complex structures were elaborated such as aligned carbon nanotubes uniformly coated
with Si and a thin layer of carbon on top of it [63]. Used as an anode, this architecture demonstrated
very good stability up to the 250 cycles that were tested and high capacity of 4200 mAh g−1
. Still this
performance is obtained at low C-rate. Good results have been obtained at high C-rate with
amorphous carbon nanorods with an intermediate layer of aluminum that is finally capped by a
silicon nanoscoop on the very top [64]. This structure maintained a capacity of 412 mAh g−1
over 100
cycles at 40C rate, corresponding to a current density 51.2 A g−1
. The thin layer of carbon at the top
of Si was found essential to obtain the good performance in [63], which suggests that Si particles
cracked and avoided the exposure to fresh Si to the electrolyte only because the carbon could protect
the Si part from any direct contact with the electrolyte. On the other hand, the very good results even
at high C-rate in [64] proves that the aluminum intermediate layer enables the gradual transition of
strain from carbon to silicon so that the integrity of the Si layer is kept during cycling.
3.3. Si Nanoparticles
To facilitate this contact, and keep it along cycling, Si nanoparticles can be dispersed in a more
conductive medium, usually carbon, to form a C-Si composite. The results, however, were not
satisfactory unless the amount of carbon is the order of 50%, i.e. about one order of magnitude larger
than required for any application. One exception is the work of Cho et al. [65] who fabricated
carbon-coated porous Si particles (pore size 40 nm). The carbon coating accounted for 12 wt% of
carbon and such particles delivered 2800 mAh g−1
over 100 cycles at 1C rate.
Recent progress has been made by coating Si nanoparticles with 2–10 layers of graphene,
anchored directly to the Si particle surface at their ends, and lying parallel to the Si surface. These
layers accommodate the volume expansion of silicon via a sliding process between adjacent
graphene layers. When paired with a commercial lithium cobalt oxide cathode, the silicon
carbide-free graphene coating allows the full cell to reach volumetric energy densities of 972 and
700 Wh L−1
at first and 200th cycle [66].
Another strategy to accommodate the huge change of volume during the alloying-de-alloying
reaction was to elaborate composite materials with Si nanoparticles embedded in a hollow structure

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such as a hollow carbon nanotube [67]. Due to the hollow space inside the nanotube, the carbon tube
is not damaged during the volumetric change during cycling. The corresponding electrode delivered
an initial capacity of 969 mAh g−1
. Taking the carbon mass into account, it corresponds to a capacity
of 2061 mAh g−1
for the silicon itself. The capacity retention was 90% after 200 cycles at current
density 1 A g−1
. In the same way a Si@void@C electrode was built, with the property that the Si
nanoparticle expanded without breaking the carbon coating in which it is embedded, owing to the
void space [68]. The capacity was 2833 mAh g−1
for the first cycle at C/10 current rate and stabilized
at ≈1500 mAh g−1
for later cycles at 1C current rate. A hierarchical pomegranate structured Si-C
composite electrode delivered 2350 mAh g−1
at C/20 current rate and over 1160 mAh g−1
after 1000
cycles at a current rate of C/2 [69].
Due to the very good electrical conductivity and mechanical properties, Si
nanoparticles-graphene composites have been investigated, with significant progress in the
electrochemical properties through the years. In 2010, the corresponding electrodes delivered
1168 mAh g−1
and an average coulombic efficiency of 93% up to 30 cycles [70], 708 mAh g−1
at a
current density of 50 mA g−1
after 100 cycles [71], 1350 mAh g−1
at a current density of 1 A g−1
after
300 cycles [72]. Hierarchical three-dimensional carbon-coated mesoporous Si
nanospheres@graphene foam delivered specific capacities of 1104 mAh g−1
and 969 mAh g−1
at
500 mA g−1
and 1 A g−1
after 200 cycles, respectively. Even at a current rate of 10 A g−1
, the electrode
still achieved a capacity of 659 mAh g−1
after 200 cycles [73]. In 2011, Zhao et al. reported
2600 mAh g−1
at 8 A g−1
current rate after 150 cycles [74]. In 2014, the same group reported 1120
and 700 mAh g−1
after 1000 cycles at a current density of 0.5 and 2.1 A g−1
, respectively, due to
sheathed Si particles in graphene sheets, leading to a buffering effect from pulverization and
electrical isolation of the active materials [75]. In this later case, the performance was attributed to
enveloping carbon/SiOx sheathed Si particles in graphene sheets.
The progress made in the performance of Si-based anodes is remarkable, but still Si is barely
competitive with commercial carbon-based anodes that store a charge of 4 mAh cm−2
, owing to their
50-μm thick film on the current collector. Such a thickness is not possible with Si-based anodes,
because of the huge change of volume during cycling. Assuming a Si capacity of 3800 mAh g−1
, an
areal mass of at least 1 mg cm−2
of Si is required to be competitive. If the Si is under the form of
nanowires, their thickness should be 300 nm to fulfill this condition, which would limit the rate to
3C [76]. Si films are closer to the performance required to be competitive, since they are now able to
deliver 1.8 mAh cm−2
. Note, however, that the Si films are usually prepared by physical vapor
deposition of magnetron sputtering that is not cheap synthesis process. With nanotubes, we also meet
the difficulty to combine high charge and power. The overall thickness of the electrode and the
synthesis process are a limiting factor. Nevertheless, two scalable methods to produce Si-based
nanotubes have been proposed recently [63,77], opening the route to their mass production in the
near future.
4. Germanium
The theoretical capacity of Ge is 1624 mAh g−1
. At contrast with Si, Ge is very expensive,
which prevents so far its commercial use, but it also has important advantages over Si: a better

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electronic and ionic conductivity, and a huge theoretical capacity since it can be lithiated to
Li5Ge4 [78]. In addition, contrary to the case of Si, the dilatation/contraction of the lattice upon
lithiation/delithiation is isotropic so that the particles avoid cracking even at high C-rate and big
particles 620 nm in size [79,80]. As a result of these properties, the rate capability of Ge is quite
remarkable [81]. Germanium-graphene composites retained a capacity of 675 mAh g−1
after 400
cycles at current density to 400 mA g−1
; Ge-graphene composite anode for high-energy lithium
batteries with long cycle life [82]. Recently, germanium (Ge) nanoparticles anchored on reduced
graphene oxide intertwined with carbon nanotubes (CNT) delivered a capacity of 863.8 mAh g−1
at a
current density of 100 mA g−1
after 100 cycles and good rate performances of 1181.7, 1073.8, 1005.2,
872.0, 767.6, and 644.8 mAh g−1
at current densities of 100, 200, 400, 800, 1600, and 3200 mA g−1
,
respectively [83]. Germanium 60 nm-thick nanoparticles (Ge-NPs) were synthesized through a
one-step chemical vapor deposition process and were included in a hybrid free-standing SWCNT
electrode [84]. As an anode, this composite delivered a capacity of 983 mAh g−1
versus Li0
/Li+
up to
3 V at low current density 50 mA g−1
, and 780 mAh g−1
up to 1.5 V. The reversible capacity is
maintained at 750 mAh g−1
in cycles up to 1.5 V at current density of 200 mA g−1
.
So far, the results reported on the anodes have been measured on half-cells with lithium
counter-electrodes. Since the major advantage of Ge with respect to the other active anode materials
(except Li4Ti5O12 that will be discussed later) is the high rate capability. It is of interest to test a full
cell where the active element of the positive electrode is also known for its outstanding rate
capability, i.e. carbon-coated LiFePO4. This has been done in [84] with the anode mentioned above.
This full cell demonstrated a good capacity retention almost independent of the C-rate up to 1C.
Other improvements on Ge anodes have been made in 2014–2015, which have not been
reported here, as a recent review on Si anodes has made focus on these two years, and we just guide
the reader to this work. As an example, Figure 3 presents the discharge-charge curves at a rate of
0.1 A g−1
(a) and Nyquist plots (b) of pure Ge, Ge-RGO and Ge-RGO-CNT nano-composite
anodes [85].
Figure 3. Discharge-charge curves at a rate of 0.1 A g−1
(a) and Nyquist plots (b) of pure
Ge, Ge-RGO and Ge-RGO-CNT nano-composite anodes (from Ref. [85]).

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5. Tin-based Composites
With a maximum of 4.4 Li per Sn, the theoretical capacity of Sn is 993 mAh g−1
, but the huge
change of volume (260 % up to full lithiation) looks prohibitive. At least, if makes mandatory the
preparation of composite structures to maintain electrical contact and separate, the Sn
nano-structured elements so that they can accommodate this huge variations of volume. Without any
surprise, the most popular composite element tested for this purpose was carbon, inasmuch as its
reactivity with Sn is small (see [86] for a review dating from 2011). A composite of 3.5 nm Sn
nanoparticles dispersed in nitrogen-doped carbon retained a specific capacity of 660 mAh·g−1
at the
200th cycle and a 630 mAh·g−1
capacity at 400th cycle at 0.2 A g−1
current density [87]. This
illustrates how efficient is the decrease of the size of the active particles to the nano-size to
accommodate their volume change during cycling and thus enhance the cycling life of the batteries.
Among Sn/C composite, Sn particles encapsulated with carbon nanofibers was promising, as it
delivered a capacity of 800 mAh g−1
after 200 cycles [88]. This is a major improvement with respect
to a Sn@C core-shell composite that showed a capacity retention of 400 mAh g−1
after 50 cycles at
the current density of 100 mA g−1
[89]. This illustrates the advantage of the nanofiber morphology of
the carbon additive to enhance the electrochemical performance of anodes. The only Sn-graphene
composite that has been reported to our knowledge [90] delivered a reversible capacity over 100
cycles at current density 100 mA g−1
. Recently, a new scalable technique, forcespinning (FS) was
used to produce binder-free porous Sn/C composite nanofibers [91]. These highly flexible Sn/C
composite nanofibers were directly used as binder-free anodes for lithium-ion batteries. The
produced Sn/C composite nano fibers showed an improved discharge capacity of about 724 mAh g−1
at a current density of 100 mA g−1
for over 50 cycles compared to most nanofiber electrodes
prepared by electrospinning and centrifugal spinning.
Amorphous nano-composites made of an active (Sn) and inactive (transition metal M) along
with some graphitic metal were also proposed with M = Mn [92], Fe [93,94,95], Co [96,97,98],
Ni [99], Cu [100]. The best results were obtained with Sn-Co-C, because cobalt does not react with
carbon and does not form carbides [101]. Sn0.31Co28C0.41 delivered a stable capacity of 500 mAh g−1
up to C/4 rate [102], which means 350 mAh g−1
if we take into account the mass of inactive carbon.
This is comparable to the gravimetric capacity of graphite, but the volumetric capacity is enhanced
by 600% due to the Sn/Co density of 14.3 g cm−3
. Sony has commercialized a 18650 Li-ion battery
with a Sn-Co-C anode and LiCoO2 cathode, named Nexelion, the capacity of which is
3.5 Ah [16,103]. A Cu6Sn5-Cu3Sn-carbon nanotubes composite anode showed a good cycling
performance, with a capacity 513 mAh g−1
after 100 cycles at 1C rate, and a good rate capability,
with a capacity of 411 mAh g−1
at 16C rate [104]. More recently, nanosized Sn particles embedded in
electrically conducting porous multichannel carbon 350 mAh g−1
after 600 cycles, most of them
performed at 2C rate [105].
A composite made of amorphous nanoporous SnO (nanopores distributed randomly on the
surface with a diameter ranging from about 1 nm to over 20 nm) and carbon nanotubes (CNTs) was
synthesized by electrodeposition and anodic oxidation methods. The BET surface area of the
composite was 53.4 m2
g−1
, and the total pore volume is 0.151 cm3
g−1
[106]. This composite as an
anode delivered a reversible capacity of 1608 mAh g−1
,

1065
maintaining at 732 mAh g−1
at the 50th cycle.
Attention has been more focused on tin oxide SnO2. It reacts with Li according to the two steps:
SnO2 + 4 Li+
+ 4 e−
→ Sn + 2 Li2S, (1)
Sn + x Li+
+ x e−
→ LixSn (0 ≤ x ≤ 4.4) (2)
The first reaction is irreversible and results in an irreversible loss of reversible capacity of
711 mAh g−1
. Nevertheless, Li2O has a major advantage: it acts as a buffer and helps to mitigate the
volume changes associated to the second reversible reaction. This is not sufficient, however to solve
entirely the problem, and in this case too, composites have been made to improve the cycle ability.
5.1. Thin Films
In the form of thin films, good results were obtained with the combination Sn-CoO [107] and
Sn-MnO [108]. In the case of Sn-cobalt oxide, the reversible capacity of 734 mAh g−1
increased after
50 cycles to 845 mAh g−1
. This increase of capacity upon cycling is often met with tin-based anodes,
and is due to the better lithium accessibility in the composite during the cycling.
5.2. Coated SnO2
Coating SnO2 was another option and the results improved constantly the last decade.
6–10 nm-thick SnO2 nanoparticles coated with carbon forming a composite with 8 wt% carbon
showed a capacity of 631 mAh g−1
after 100 cycles at current 400 mA g−1
[109]. More recently,
carbon-coated SiO2 nanospheres synthesized via a one-spot hydrothermal method end subsequent
carbonization delivered 1215 mAh g−1
after 200 cycles at current density of 100 mA g−1
. The
reversible capacity is maintained at 520 mAh g−1
at current density 1600 mA g−1
[110]. A
carbon-coated SnO2-NiO multicomposite delivered 529 mAh g−1
and 265 mAh g−1
after 500 cycles
at current density 800 and 1600 mA g−1
, respectively [111]. These results show that it is now possible
to obtain nano-structured SnO2 based composites that maintain the integrity of the structure upon
cycling, even at high rate. A promising result was obtained by coating SnO2 particles not with carbon,
but with HfO2 [112]. This anode was able to deliver a capacity of 853 mAh g−1
after 100 cycles at a
low current density (150 mA g−1
). In this case, HfO2 acts as a passivation layer. SnO2 was also
uniformly decorated with polypyrrole (Ppy) nanowires to obtain an anode that delivered
690 mAh g−1
with 90% capacity retention up to 80 cycles [113].
Hollow geometry proved very efficient, since the voids give some place to maintain the
structural stability during the volume changes experienced during cycling. In particular, SnO2 hollow
nanosphere coated with carbon (carbon content 32 wt%) delivered a stable capacity of 460 mAh g−1
and 210 mAh g−1
over 100 cycles at 0.8C and 4.8C, respectively, with 1C = 625 mA g−1
[114]. SnO2
nanoparticles have also been deposited on the skeleton of inverse opal carbon monoliths [115]. There,
the SnO2 nanoparticles are confined within the mesoporous voids preventing the particles from
disconnecting from the carbon. More recently, porous SnO2 microboxes with a high uniformity and
well-defined non-spherical hollow structure delivered 550 mAh g−1
after 150 cycles at current
density 200 mAh g−1
[116].

1066
5.3. SnO2-Carbon Nanotubes
The composites obtained under the form of nanotubes showed good electrochemical properties,
but at low C-rates only. A uniform layer of SnO2 nanocrystals (crystal size 5 nm) on a multiwalled
carbon nanotube delivered a stabilized capacity of 400 mAh g−1
over 100 cycles at current density of
100 mA g−1
[117]. More recently, better results were reported for such a nano-composite, which
delivered 500 mAh g−1
for up to 300 cycles, owing to a novel ethylene glycol-mediated
solvothermal-polyol route for synthesis [118]. The capacity of a SnO2/multiwalled carbon tube
core/shell structure remained at 720 mAh g−1
over 30 cycles at current density 0.2 mA cm−2
,and the
capacity fading was 0.8% per cycle [119]. A capacity of 540 mAh g−1
almost stable over 200 cycles
at 0.5C rate was reported for porous SnO2 nanotubes with coaxially grown carbon nanotubes
overlayers [120].
Sandwich-like CNT@SnO2/SnO/Sn anodes were prepared by using composite electrodeposition
and anodic oxidation on three-dimensional (3D) Ni foam [121]. The poor coulombic efficiency from
the first cycle to 10 cycles was attributed to the active mechanism of interface layers. But for the next
cycles, the coulombic efficiency raised to 99% and the anode delivered a capacity of 394 mAh g−1
at
the 50th cycle at a current density of 100 mA g−1
. The result suggests that the peculiar architecture of
CNT encapsulated into SnOx with 3D frame space can alleviate the huge volume variation of SnOx.
5.4. SnO2-Graphene Composites
The extensive study of SnO2-graphite composites started in 2010–2011 [122–130]. Typically,
the reversible capacity was maintained in the range 500–650 mAh g−1
for 50 cycles. However, a
storage capacity of 775 mAh g−1
was reported by Zhao et al. [123]. Better results were reported more
recently. A composite made of SnO2 and graphene nanoribbons delivered a reversible capacity of
1130 mAh g−1
, maintained at 825 mAh g−1
after 50 cycles at current density of 100 mA g−1
[131].
Moreover the reversible capacity was 580 mAh g−1
at higher current density of 2 A g−1
. The high
, which is larger than the theoretical prediction for SnO2, was attributed to
the enhancement of the lithium storage by the graphene nanoribbons resulting from edge effects. The
rate capability gives evidence of the ability of the graphene to buffer the volume change of the SnO2
particles. An important improvement was obtained by using a hydrazine monohydrate vapor
reduction method to bind 4–5 nm-thick SnO2 nanocrystals in graphene sheets by Sn-N bonding [132].
Good capacity retention was obtained at 1074 and 417 mAh g−1
for current densities 0.5 and 20 A g−1
,
respectively. The Sn-N bonding was confirmed by the analysis of the L3-edge peak intensity. This
bonding that anchored the SnO2 nanoparticles on the graphene sheets, then avoiding the
agglomeration of the particles plus the small size of the particles, plus the high flexibility of the
graphene explain this remarkable performance during long-term cycling tested over 500 cycles.
Recently, non-doped SnO2-reduced graphene oxide (RGO) composite as an anode delivered
1010 mAh g−1
per mass of the electrode at 0.05 A g−1
with a good capacity retention (470 mAh g−1
)
even at 2 A g−1
(<2C) [133]. SnO2/RGO nanocomposite showed a capacity retention of 500 mAh·g−1
after 50 cycles at 250 mAh g−1
[134]. F-doping of SnO2 was also realized, improving significant
improvement of the electrochemical properties of the SnO2 nanocrystals-reduced graphene oxide

1067
composite as shown in Figure 4 [135]. The chemical diffusion coefficients were calculated from the
Z’ vs. ω−1/2
plots as ~10−16
cm2
s−1
for F-SnO2@RGO and ~4 × 10−17
cm2
s−1
for SnO2@RGO. The
composite delivered a reversible capacity of 1277 mAh g−1
after 100 cycles at current density
0.1 A g−1
. This capacity retention was at least partly attributed to the corrosion resistant properties of
F-doping. The higher rate capability (634 mAh g−1
after 10 cycles at current density 5 A g−1
) was due
to the reduced resistance induced by the F-doping that protects the electrode surface from the
hydrolysis or decomposition of the LiPF6 salt. SnO2/graphene composite with superior cycle
performance and high reversible capacity was prepared by a one-step microwave-hydrothermal
method [136]. The capacity remains 1359 and 1005 mAh g−1
after 100 cycles at current densities of
100 and 700 mA g−1
, respectively. Even at current density of 1 A g−1
, the discharge specific capacity
remained 1057 and 677 mAh g−1
after 420 and 1000 cycles, respectively. Finally, by providing an
in-depth discussion of SnO2/graphene nanocomposites, Deng et al. demonstrated that the
electrochemical performances of SnO2/graphene nanocomposites could be significantly enhanced by
rational modifications of morphology and crystal structures, chemical compositions and surface
features [137].
Figure 4. Nyquist plots of F-SnO2@RGO (a,b) and SnO2@RGO (c,d) electrodes
measured at a fully charge state before (a,c) and after (b,d) charge/discharge. Chemical
diffusion coefficients calculated from the Z’ vs. ω−1/2
plots are ~10−16
cm2
s−1
for
F-SnO2@RGO and ~4 × 10−17
cm2
s-1
for SnO2@RGO (from Ref. [135]).
Composites involving an additional element have also been elaborated. A core-shell structured
Fe2O3-SnO2 uniformly dispersed between layered graphene sheets delivered 1015 mAh g−1
after 200
cycles [138]. SnO2 hollow spheres embedded in graphene sheets and enveloped by
poly(3,4-ethylenedioxythiophene) (PEDOT) delivered 608 mAh g−1
density of 100 mA g−1
, which amounts to 1248 mAh g−1
when only the mass of SnO2 is
considered [139].
These results and the progress that has been made in the last 6 years show that SnO2-based
composites are prospective anodes for Li-ion batteries. The main difficulty at this point is to switch

1068
from the laboratory to the industrial scale, since the synthesis of these nanostructured composites is
long and expensive. Scaling-up their preparation for commercial use at a reasonable price is the main
challenge that needs to be solved for this material as many others to make possible the replacement
of graphite by such composites. SnS2 composites are reviewed in a forthcoming section devoted to
sulfides.
6. Titanium Oxide-based Composites
6.1. TiO2
TiO2 is low cost and safe. Its theoretical capacity is 335 mAh g−1
, according to the reaction:
Ti4+
O2
+ x Li+
+ x e−
↔ Lix(Ti3+
xTi4+
1−x)O2 (x ≤ 1) (3)
This capacity is rather small with respect to other metal oxides reviewed here, but TiO2 has an
advantage over them: its dilatation upon lithium incorporation is smaller (except the “zero-strain”
titanate that will be reviewed hereunder, but titanate has an even smaller capacity). That is why TiO2
has been extensively studied as an anode material for Li-ion batteries.
Good results have been already obtained for TiO2 without the need of another composite
element. 3- and 4-shelled TiO2 hollow microspheres capacity, up to 237 mAh g−1
with minimal
irreversible capacity after 100 cycles is achieved at a current rate of 1C (167.5 mA g−1
), and a
is achieved at a current rate of 10C even after 1200 cycles [140].
Mesoporous TiO2 microspheres assembled from TiO2 nanoparticles with specific surface areas as
high as 150 m2
g−1
delivered a high reversible capacity of <120.1 mAh g−1
when the current rate was
increased from 0.2C to 5C after 50 charge-discharge cycles. Even at 10C, the capacity was still
satisfactory (<100 mAh g−1
), with excellent reversibility [141]. However, better results can be
obtained only in composites.
Like in the case of any metal oxide, composites have been formed with carbon under different
structures. The carbon coating on TiO2 enhances the electronic conductivity and provides space for
suppressing the large volume expansion during cycling, which improved the electrochemical
performance [142]. Carbon nanotubes were conjugated with all the active anode elements because of
their high mechanical properties, low gravity, and high surface area; in addition, in the present case,
they efficiently provide electrons to the nanostructure through the formation of Ti-C bonds, which
helps in the insertion of lithium [143]. Indeed, the anatase TiO2-C nanotubes in this work
demonstrated a superior Li storage capacity of 320 (±68) mAh g−1
.
Many works have been made to prepare composites with graphene. It took some time to obtain
good results, because the strong π-interaction of graphene was an obstacle to the homogeneous
dispersion of TiO2. However, the results obtained the last years are outstanding. A three-dimensional
(3D) foam architecture of TiO2 nanoparticles embedded in N-doped (high-level nitrogen content of
7.34%) graphene networks delivered a capacity of 165 mAh g−1
after 200 cycles at 1C rate,
143 mAh g−1
after 200 cycles at 5C rate [144] and showed excellent rate performance (96 mAh g−1
at
20C) as anode. This performance comes partly from the fact that N doping is favorable to improve
the electronic conductivity, and thus the rate capability [145]. Mesoporous single crystals Li4Ti5O12

1069
grown on reduced graphene oxide (MSCs-LTO-rGO) nanohybrids exhibited high specific capacity
(171 mAh g−1
) with much improved rate capability (132 mAh g−1
at 40C) and cycling stability (85%
capacity retention after 2000 cycles) [146]. TiO2/graphene nanocomposite with well-preserved
flower-like nanostructure formed by TiO2 nanoparticles with a size of several nanometers delivered a
at C/10 (corresponding to a density of 17 mA g−1
), and demonstrates
superior high-rate charge-discharge capability and cycling stability at charge/discharge rates up to
50C [147]. The authors of this work went further by testing a full cell using this TiO2/graphene as the
anode material and spinel LiMnO2 as the cathode material. Good high-rate performance and cycling
stability were observed.
Figure 5. (a) Cyclic voltammograms of TiO2 nanoparticles embedded in N-doped
graphene networks (denoted as UTO/NGF) electrode between 1.0 and 3.0 V at a scan rate
of 0.5 mV s−1
for the first four cycles. Initial charge-discharge curves of UTO, UTO/GF
and UTO/NGF electrodes at a current density of 0.1C (1C = 168 mA g−1
). The
comparative cycling performance of the above three electrodes: (c) 200 cycles at 1C after
5 cycles at 0.1C. (d) 200 cycles at 5C after 5 cycles at 0.1C. (e and f) The corresponding
coulombic efficiency for (c and d), respectively (from Ref. [144]).

1070
TiO2 quantum dots (6 ± 2 nm)/graphene nanosheets (23.5 wt% graphene) retained a capacity of
190 mAh g−1
after 100 charge-discharge cycles at a current rate of 1C [148]. The increase of the
C-rate to 10C leads only to an insignificant drop in capacity to 145 mAh g−1
. At 50C, the capacity
. A composite consisting in 10 nm-thick TiO2 nanotubes built on a graphene
layer delivered a capacity of 350 mAh g−1
at the current density 10 mAh g−1
, 150 mAh g−1
after 50
cycles at the rate of 4 A g−1
, and 80 mAh g−1
after 2000 cycles at the rate of 8 A g−1
[149]. More
recently, well-crystallized TiO2 particles 6 nm in diameter were coated with amorphous carbon are
anchored on reduced graphene oxide owing to post-treatment with UV irradiation [150]. This
composite, as an anode, exhibited reversible capacities of 382 and 73 mAh g−1
at C/10 and 5C,
respectively. The reversible capacity after 100 cycles at C/5 rate was 191 mAh g−1
. Better results,
however, have been obtained on minky-dot-fabric-shaped composite of well-organized porous TiO2
microspheres and reduced-graphene-oxide (rGO) sheets used as an anode delivered a reversible
at 10C for up to 100 cycles [151]. Actually, the comparison between these
different results illustrates that, in addition to the dispersion of the TiO2 particles and their anchoring
on the graphene sheets, the porosity is a crucial parameter that impacts the electrochemical properties
of the TiO2/graphene composites. For instance, mesoporous graphene nanosheets delivered a
reversible capacity of 833 mAh g−1
after 60 cycles [152] owing to the increased effective surface
area associated with the pores.
So far, all the data we have reported were obtained with anatase TiO2. Another polymorph is
rutile TiO2, but very few composites formed with this polymorph have been tested as an anode for
Li-ion batteries. Recently, however, rutile TiO2 mesocrystals/reduced graphene oxide nanosheets
exhibited a large capacity over 150 mAh g−1
at 20C after 1000 cycles, and high rate capability up to
40C [153].
The last polymorph is bronze (TiO2-B), which is attractive, because of its fast kinetics
illustrated by the results obtained with TiO2-B microspheres with 12 nm pore size which retained a
after 5000 cycles [154] at 10C rate. Carbon-coated TiO2-B nanowires
exhibited a capacity of 560 mAh g−1
after 100 cycles at current density 30 mA g−1
and 200 mAh g−1
when cycled at current density 750 mA g−1
[155]. A TiO2-B/N-doped graphene porous
nanocomposite with a high specific surface area of 163.4 m2
g−1
showed a discharge capacity of
220.7 mAh g−1
at the 10C rate with a capacity retention of 96% after 1000 cycles. In addition, it can
deliver a discharge capacity of 101.6 mAh g−1
at an ultra-high rate of 100C, indicating its great
potential for use in high power lithium ion batteries [156]. A simple photocatalytic reduction method
was used to simultaneously reduce graphene oxide (GO) and anchor (010)-faceted mesoporous
bronze-phase titania (TiO2-B) nanosheets to reduced graphene oxide (RGO) through Ti–C bonds.
Owing to this Ti–C bonding, this composite delivered a specific capacity of 275 mAh g−1
at 1C rate
and 200 mAh g−1
even at 40C current. Independent of the current densities, these electrodes
maintained 80% of the initial capacity even after 1000 cycles [157]. Obviously, TiO2-B is the
polymorph that gives the highest rate capability. The specific capacity of TiO2-based composites is
smaller than that of Si-based composites, but the rate capability is much better, so that they both
could find a market as anode materials.
Note, TiO2 does not need to be crystallized to be electro-active, but the results are not good.
Amorphous TiO2 thin films uniformly deposited onto worm-like graphene nanosheets delivered a

1071
stable capacity of ~140 mAh g−1
after 100 cycles at a specific current of 100 mA g−1
and accounted
for a sustainable 95 mAh g−1
capacity at a specific current of 1200 mA g−1
[158].
Due to the smaller volume change during cycling, TiO2 has been mixed with another metal
oxide hoping to find a compromise taking advantage of the stability of TiO2 upon cycling and the
large capacity of the other component. Unfortunately, low cycle retention was witnessed for these
TiO2-based dual oxide anodes, e.g. TiO2/Fe2O3 (capacity retention after 100 cycles at current density
200 mA g−1
≈ 68%) [159], TiO2/MnO2 (≈60%) [160] or TiO2/SnO2 (≈52%) [161]. One exception is
TiO2/Fe3O4 that will be discussed in the section devoted to iron oxides.
In any cases, the process to make these composites is expensive and difficult to prepare at the
industrial scale, and indeed, the only new anode that emerged on the market so far is another titanium
oxide such as Li4Ti5O12.
6.2. Li4Ti5O12
All the metal oxides have in common the property of a huge volume change during cycling,
with one exception: Li4Ti5O12 (LTO), which explains the tremendous interest in this material
considered as the “zero-strain” anode material, despite its much smaller theoretical capacity of
175 mAh g−1.
The rather high operational voltage of 1.55 V vs. Li0
/Li+
that limits the tap density, but
it also mitigate the formation of the SEI provided the voltage is maintained above 1 V, so that the
cycling tests of LTO are usually performed in the voltage range 1.0–2.5 V. The structural stability is
very good, and the LTO anode is safe.
Figure 6. (a) Typical XRD patterns of C-Li4Ti5O12 composite and (b) HRTEM image of
a 90 nm sized LTO particle 1.5 nm carbon coating (from Ref. [163]).
The electrical conductivity is small (10−12
–10−13
S cm−1
), so that carbon-coating the LTO of any
kind (particles, nanorods, hollow spheres) improves the electrochemical properties. 5 wt% C-coated
100 nm-sized LTO particles delivered a capacity of 165 mAh g−1
with 99% retention after 100 cycles
at 1C rate [162]. Similar particles (same thickness 1.5–3.0 nm of the carbon coat, particles 90 nm in
size (see Figure 6) were tested in a full 18650 cell with C-LiFePO4 cathode [163]. The cell displayed
a charge capacity of 650 mAh at low C-rate and retained more than 80% of rated capacity at 60C

1072
charge rate. As shown in Figure 7. The reduction of the thickness of the carbon coat to 1 nm
improves the diffusivity of the lithium through the coat and thus improves the rate capability as a
at 20C rate has been reported for such a C-LTO composite [164].
Figure 7. (a) Typical plateau of the discharge-charge curves of C-Li4Ti5O12/1 mol L−1
LiPF6 in EC-DEC/Li cell at C/4 rate. (b) Charge curves of C-Li4Ti5O12/1 mol L−1
LiPF6
in EC-DEC/Li cell at various C-rates (from Ref. [163]).
One strategy consisted in coating LTO with N-doped carbon instead of un-doped carbon, to take
benefit of the doping effect on the electrical conductivity already mentioned in this review. The best
results were obtained with 7 wt% N-doped carbon coated LTO spherical particles. At 2C rate, the
initial capacity was 150 mAh g−1
, with a capacity retention of 83% after 2200 cycles [165]. Recently,
10 wt% hollow graphitized nano-carbon was used as a conductive agent, uniformly distributed on the
surface of 50–100 nm-thick LTO particles providing 162 and 105.4 mAh g−1
at the rate of 15 mA g−1
and 1.5 A g−1
, respectively, and the discharge capacity retention rate after 500 cycles is 91.2% at
5C [166].
Another strategy to improve the performance was to use porous LTO to increase the effective
surface area. C-coated porous LTO particles (pore diameter 4.3 nm) delivered 158 and 100 mAh g−1
at 1C and 50C rate, respectively, with good capacity retention [167]. These results are quite
comparable with the prior results above mentioned for un-doped C-LTO, but the particles were much
bigger: 0.5–1 μm instead of 90–100 nm. With smaller particles, even without carbon coating, spheric
porous LCO particles of size 660 ± 30 nm, primary particle size of 20–100 nm, specific surface area
of 15.5 m2
g−1
, capacities of 179 mAh g−1
at 0.5C and 109 mAh g−1
at 10C were observed, with a
capacity retention of 97.8% over 100 cycles [168].
Carbon nanotubes/LTO composites also gave good results [169,170]. Better results can be
obtained by adding carbon. For a total 6 wt% carbon plus nanotubes, capacities of 163 and
143 mAh g−1
were reported at 0.5C and 10C rates, respectively. The capacity was stable at
146 mAh g−1
over 100 cycles at 5C rate [171]. The same capacity retention of 146 mAh g−1
over 100
cycles was observed, but at 10C-rate, in a LTO/multiwalled carbon nanotube (MWCNT)
composite [172]. 50 nm-thick LTO particles anchored on MWCNTs for an amount of MWCNTs of
10 wt% delivered a capacity of 171 mAh g−1
and 90 mAh g−1
at 1C and 30C rate, respectively, stable

1073
over the 30 cycles that have been tested [173]. Note, however, that this is comparable to the
performance of the particles coated with n-doped carbon mentioned above, according to a synthesis
process that seems more scalable. Again, still better results can be obtained with porous LTO.
MWNT/LTO core/25 nm-thick sheet coaxial nanocables providing a surface area of 80 m2
g−1
exhibited a capacity of 90 mAh g−1
over 100 cycles at 40C rate [174].
Only few works are related to LTO/graphene composites. A graphene-embedded LTO composite
with 1wt% graphene gad a reversible capacity of 110 mAh g−1
at 22C rate. After 1300 cycles, a
was maintained [175]. More recently, well-dispersed mesoporous LTO
particles onto rGO delivered a reversible capacity of 193 mA h g−1
at 0.5C, 168 mAh g−1
at 30C
between 1.0 and 2.5 V, owing to the addition of the improved electronic conductivity and buffering
effect of the graphene, and the increased surface lithium storage capability brought by the
porosity [176].
In an attempt to find a compromise between Si that has a very large capacity and LTO that has a
very good rate capability, LTO/Si composites have been explored. with moderate Si contents
(LTO:Si = 50:20) were the best compromise, delivering stable capacity (100 mAh g−1
) with good
cycling performance, even at a very high current density of 7 A g−1
[177]. For the same reason,
LTO/anatase TiO2 composites have also been synthesized by the hydrothermal process [178,179,180].
Such a nano-composite with particles 30 nm-thick exhibiting rich hierarchical pores and a specific
surface area of 91.88 m2
g−1
at 20C, and capacity retention of 151.4 mAh g−1
after 100 cycles at 1C [180].
7. Sulfides and Nitrides
7.1. Sulfides
7.1.1. SnS2-based Composites
SnS2 operates by a conversion reaction followed by alloying/de-alloying process (Eqs. 1–2).
Only the second reaction is supposed to be reversible, but as usual in nanostructured materials, the
experimental capacity can be larger because the surface offers new active sites resulting in higher
than 4.4 Li per tin atom. For instance, layered SnS2 nano-sheet arrays consisting of 1–5 atomic layers
synthesized on Sn foil delivered 1050 mAh g−1
[181]. An Economical synthetic route for production
of large-amounts (gram scale) of two-dimensional (2D) layered SnS2 nanoplates showed good
electrochemical performance, but was tested only for 30 cycles [182].
Composites composed of acetylene black adorned porous SnS2 secondary microspheres that are
assembled from a number of nanosheets with thicknesses of 20–50 nm (specific surface area of
129.9 m2
g−1
) delivered 525 mAh g−1
over 70 cycles [183].
Mesoporous carbon anchored with SnS2 nanosheets (14.3 wt% carbon) delivered a stable discharge
capacity of 428.8 mAh g−1
with a retention of 64.4%
comparing with the first reversible capacity [184]. SnS2 nanoflakes decorated multiwalled carbon
nanotubes delivered 518 mAh g−1
between 5 mV and 1.15 V
versus Li0
/Li+
. A stable reversible capacity of 510 mAh g−1
is obtained for 50 cycles [185]. These

1074
results are an improvement with respect to pristine SnS2 nanoflakes, pointing to the advantage of the
composite, owing to the flexibility and the conductivity of the carbon nanotubes.
Balogun et al. demonstrated an effective strategy to improve the first reversible capacity and
lithium storage properties of SnS2 by growing SnS2 nanosheets on porous flexible VN substrates [186].
The 3D porous VN coated SnS2 nanosheets yielded a reversible capacity of 75% with high specific
capacity of about 819 mAh g−1
at a current density of 0.65 A g−1
, decreasing to 791 mAh g−1
after 100
cycles. A capacity of 349 mAh g−1
was still retained after 70 cycles at current density of 13 A g−1
.
SnS2/graphene nanosheets have been the subject of intense research these last years. Ce-SnS2
crystal particles with a flower-like structure (particle sizes of each petal are in the range 50–100 nm)
distributed on the graphene sheets delivered 707 mAh g−1
at C/2 after 50 cycles [187]. SnS2
nanosheets linked with each other and dispersed uniformly on reduced graphene oxide surface
after 40 cycles at C/10 and 657 mAh g−1
after 40 cycles at 1C rate [188].
Few-layer SnS2/graphene composite delivered a stable reversible capacity of 920 mAh g−1
at the 50th
cycle at 100 mA g−1
, 600 mAh g−1
at 1 A g−1
[189]. Better results have been reported: SnS2/graphene
nanosheets delivered a capacity of 1114 mAh g−1
after 30 cycles at current density 100 mAh g−1
, and
870 mAh g−1
at current density 1 A g−1
[190]. A SnS2@RGO nanocomposite consisting of 93.2 wt%
SnS2 and 6.8 wt% RGO delivered 564 mAh g−1
after 60 cycles at C/5 and 503 mAh g−1
at 2C [191].
SnS2 nanosheets uniformly coating on the surface of graphene delivered a capacity of 570 mAh g−1
after 30 cycles at C/5 rate [192]. SnS2 nanoplates with a lateral size of 5–10 nm are anchored on
graphene nanosheets maintained a capacity of 704 mAh g−1
in the 100th cycle with charging at
1.6 mA cm−2
and discharging at 0.2 mA cm−2
[193]. Better results were obtained with a one-step
hydrothermal route to prepare SnS2/reduced graphene oxide nanocomposites (TSG) with hexagonal
SnS2 nanoplates (lateral size and thickness 140 nm and 25 nm) and nanocrystals (6 nm-thick) [194].
As an anode, this composite delivered 1005 mAh g−1
after 200 cycles at the current density of
100 mA g−1
and 612 mAh g−1
at the rate of 2 A g−1
. This result is due to the fact that this composite
takes benefit not only on the synergistic effect between SnS2 and graphite, but also of the porosity
(average pore size 5.37 nm, surface area 77.8 m2
g−1
. SnSn2 nanocrystals (lateral size of 3–4 nm)
decorated on flexible reduced graphene oxide delivered 1034 mAh g−1
after 200 cycles at C/10,
737 mAh g−1
after 200 cycles at 1C, and the capacity after 450 cycles was maintained at
570 mAh g−1
(3C) and 415 mAh g−1
(5C), respectively [195]. SnS2 nanoplates ca.
7 nm-thick/graphene composite exhibited a reversible capacity stable at ca. 668 mAh g−1
after 10
cycles at a rate of 100 mA g−1
and 480 mAh g−1
at 1600 mA g−1
[196]. The best result, however, was
reported on 3D porous graphene networks anchored with Sn nanoparticles (5–30 nm) encapsulated
with graphene shells of about 1 nm. As an anode, this composite exhibited very high rate
performance. The capacity reached 682 mAh g−1
at the rate of 2 A g−1
and remained approximately
96.3% (657 mAh g−1
) even after 1000 cycles [197].
Among SnS2/graphene aerogels, the best result reported a capacity of 656 mAh g−1
with a
coulombic efficiency of over 95% after 30 cycles and 240 mAh g−1
at the rate of 1000 mA g−1
[198].
Other Sn-based composites have also been reviewed in [199].
A Cu6Sn5-Cu3Sn-CNTs composite anode showed a good cycling performance, with a capacity
513 mAh g−1
after 100 cycles at 1C rate, and a good rate capability, with a capacity of 411 mAh g−1
at 16C rate [200].

1075
7.1.2. Titanium Sulfides
TiS2 has a high conductivity, low cost and light weight. Unfortunately, it suffers irreversible
changes including passivation of the surface and distortion of the chemical structure. Nevertheless, it
has excellent cycling performance between 3.0 and 1.4 V. Particle pulverization and formation of
Li2S lead to performance degradation because of the irreversible dissolution of Li2S into the
electrolyte below 0.5 V [201]. TiS2-multi-walled carbon nanotubes (MWCNTs) (1:1) composite
delivered an initial capacity of 450 mAh g−1
with 80% capacity retention after 50 cycles owing to the
synergetic effect between TiS2 and the MWCNTs [202]. However, TiS2 like MoS2 suffers from the
rather high working potential (in the 2–V range) with respect to the Li0
/Li+
redox potential, which
reduces significantly the energy density of the battery. As a consequence, TiS2 and MoS2 are
considered as so that they are now placed in the category of 2-volts cathode materials, not as
promising anode elements [203].
7.1.3. Nickel Sulfides
Nickel sulfides NiS, NiS2, Ni3S2, and Ni3S4 have gained special attention as electrode materials
for lithium-ion batteries and super-capacitors [204]. In particular, NiS has attracted attention, owing
to its high theoretical capacity of 590 mAh g−1
. As usual, various carbonaceous materials to support
NiS anodes to buffer the volume change of NiS and increase the electrical conductivity.
Nickel sulfide-carbon core-shell composite powders were prepared by a one-step spray
pyrolysis, where the nickel sulfide particles were of submicron size, and crystallized in the Ni7S6
phase. As an anode, this composite delivered a discharge capacity of 472 mAh g−1
at a high current
, even after 500 cycles [205].
NiS-carbon fiber composite electrodes coated with Li2S-P2S5 solid electrolytes by pulsed laser
deposition (PLD) delivered a capacity of 300 mAh g−1
at the 50th cycle at current density
3.8 mA cm−2
corresponding to approximately 1C rate [206]. This coating was thus able to form
avorable lithium ion and electron conduction paths for the NiS active materials. Another NiS (50 nm
diameter)-carbon fiber composite retained a discharge capacity of 520 mAh g−1
and a
charge-discharge efficiency of approximately 100% after 30 cycles in the voltages of 0–4.0 V vs.
Li0
/Li+
under a constant current density of 1.3 mA cm−2
[207].
Camphoric carbon wrapped NiS powders were prepared in a morphology showing a network of
interconnected nanoscale units with rod like profiles, which terminated into needle-like apexes
spanning diameters of about 50–80 nm with BET surface area of 32 m2
g−1
[208]. As an anode, this
composite delivered a specific capacity reduced from 290 mAh g−1
to 225 mAh g−1
(20%) after the
100th cycle from 3.2 V to 0.5 V at current density 5 mA g−1
.
The morphology, size and phase control of NiS nanoflowers on a graphene substrate was
investigated in [209]. The best result was obtained with graphene supported NiS nanorod-assembled
nanoflower that delivered a capacity of 887 mAh g−1
after 60 cycles at 59 mA g−1
, much larger than
the theoretical capacity of NiS (590 mAh g−1
). These good results show that prevention of graphene
agglomeration could be preserved during repetitive cycling. A NiS/graphene (NiS/G) nanohybrid was
synthesized by a facile in situ one-pot hydrothermal route in a sheet-on-sheet structure, where

1076
ultrathin NiS sheets (below 5 nm) are anchoring on few-layer (below 8 layers) graphene sheets [210].
This composite cycled between 0.05 V and 3.0 V at 50 mA g−1
retained a charge capacity of
481 mAh g−1
after 100 cycles. At a current density of 800 mA g−1
(1.5C rate) NiS/G still yielded a
charge capacity of 386 mAh g−1
.
Ni3S2 nanobowls with an average size of 250 nm and shell thickness of 30 nm wrapped by
reduced graphene oxide sheets 43 mAh g−1
at 0.5C after 500 cycles (theoretical capacity of Ni3S2 is
462 mAh g−1
) [211]. The improved cycle stability of the composite could be ascribed to the bowl
structure with both exposed interior and exterior arch surfaces which could stand much more
lithiation-delithiation than quasi-1D hollow chains.
Activated cotton textile (ACT) with porous tubular fibers embedded with NiS2 nanobowls and
wrapped with conductive graphene sheets (ACT/NiS2-graphene) retained a capacity of 645 mAh g−1
at 1C after 100 cycles recovered to <1016 mAh g−1
at C/10 after 400 cycles [212]. Sphere-like NiS2
particles/graphene composite delivered a capacity of 1200 mAh g−1
at the 120th cycle current density
100 mA g−1
and about 810 mAh g−1
. After 1000 cycles at a current
this NiS2/graphene composite still preserved a capacity of 810 mAh g−1
,
indicating its excellent cyclic stability, an evidence that the reduced graphene oxide efficiently
restrained NiS2 from agglomerating during hydrothermal synthesis [213].
With its high theoretical capacity (704.5 mAh g−1
), safety and low cost, Ni3S4 is also a promising
candidate as an anode. Ni3S4-nitrogen doped graphene composite showed outstanding performance
with 98.87% capacity retention from 2nd (1338.4 mAh g−1
) to 100th (1323.2 mAh g−1
) discharge
cycle at 0.2C in the range of 0–3 V [214]. A capacity of 793.9 and 558.2 mAh g−1
stable over 100
cycles was observed at 2C and 4C rate, respectively. The authors attributed this performance to the
fact that nitrogen-doping process introduced disordered carbon and lithium intercalated active sites.
Also the annealing at 250 °C increased the grafting of the Ni3S4 particles on the graphene, thus
increasing the conductivity, and increased the percentage of pyridinic nitrogen, which is more
effective to enhance the reversible capacities of graphene than other types of nitrogen [215]. This
attribution of the improvement of the electrochemical properties to the graphene and its N-doping is
confirmed by the good performance of NiS1.03-nitrogen doped graphene composite prepared in the
same conditions, which showed 95.94% capacity retention from 2nd (1084 mAh g−1
) to 100th
(1040 mAh g−1
) cycle discharge at 0.2C rate.
7.1.4. Cobalt Sulfides
Cobalt sulfides also have promising potential as a high capacity, high rate and long life anodes.
The growth of a carbon shell on hollow nanospheres of mesoporous Co9S8 improved the reversible
capacities at high rates: 896 mAh g−1
after 800 cycles at 2 A g−1
[216]. Carbon-coated Co9S8
nanodandelions delivered a specific capacity of 520 mAh g−1
(1.8C)
during the 50th cycle (against 338 mAh g−1
without carbon coating) and a 10th-cycle capacity of
373 mAh g−1
(10.9C) [217].
Graphene-wrapped CoS nanoparticles delivered a delivered a specific capacity of 749 mAh g−1
after 40 cycles at 62.5 mA g−1
or 550 mAh g−1
after 100 cycles at 312.5 mA g−1
[218].

1077
7.1.5. Iron Sulfides
FeS2 has been less studied than other sulfides, although its theoretical capacity is high
(894 mAh g−1
). Promising results were obtained with carbon-coated FeS2 particles composed of
rod-like second particles with diameters of 0.2–1μm and lengths of 0.5–3 μm, the rod-like second
particles being made up of small first nanoparticles with 30–100 nm in diameters. The pores between
the nanoparticles are 5–30 nm in size [219]. The specific reversible capacity of this FeS2/C
composite after 50 cycles is 495 mAh g−1
at 0.05 C and 371 mAh g−1
at 0.2C, respectively.
FeS2 decorated sulfur-doped carbon (FeS2@S-C) fibers have been successfully synthesized by a
facile bio-templating method. As an anode, this composite retained a high reversible specific
after 100 cycles at current density of 0.1 A g−1
[220]. After 10 cycles at
2 A g−1
the capacity was 400 mAh g−1
. A composite of reduced graphene oxide and FeS2
microparticles maintained a capacity of 1001 mAh g−1
over 60 cycles at a current rate of
100 mA g−1
[221]. Nevertheless, FeS2 severs from irreversible reactions elucidated in [222], which
results in aging of the material and reduces the capacity along cycling. In addition, two charge plateaus
at about 1.8 V and 2.4 V are clearly shown at the first charge process, followed by two discharge
plateaus at about 1.55 V and 2.05 V, due to the conversion of metallic Fe to FeS and oxidization of Li2S
through lithium polysulfide to sulfur, respectively. The following discharge plateaus are their
corresponding reverse processes. According to these rather high voltages, FeS2 can be considered
either as a cathode of the 2-V family [223] rather than as an anode with a limited energy density.
7.2. Nitrides
Transition metal nitrides possess fair electrical conductivity but superior chemical stability, which
qualify them for candidates for supercapacitors or electrodes for Li-ion batteries. In particular, Nb4N5
nanochannels electrode with an ultrathin carbon coating exhibits an areal capacitance 243.6 mF cm−2
at 0.5 mA cm−2
, with nearly 100% capacitance retention after 2000 CV cycles [224]. Due to the fast
diffusion of Li ions as a result of the Li vacancies after the conversion reaction, metal nitrides also offer
the possibility to reach relatively high power densities [225]. Except LiMoN2 that is a cathode material
in a voltage range of 2.7–4.2 V, the newly developed lithium layer nitrides were anodes because their
Li intercalation voltage plateau lies below 2 V. A recent review on metal hydrides as high performance
anode materials for lithium-ion batteries can be found in [226], where the reader will find their
intrinsic properties. We are interested here on the improvement obtained by their integration in
composite materials.
7.2.1. Carbon and Silicon Nitrides
Attempts are currently made to use carbon nitride to enhance the stability of the SEI and also act
as a buffer layer for the volume change of the active materials during cycling, since strong covalent
bonds between carbon and nitride leads to materials highly resistant against the corrosion by the
electrolytes while the high electrical conductivity is maintained by the conjugative π structure which is
similar to graphite. In addition, Li+
uptake for 10% graphitic carbon nitride is similar to a graphite

1078
electrode, indicating that it does play a role in determining the storage capacity of graphitic carbon
nitride-based composites as anode materials for Li-ion batteries [227]. Nanowire arrays of CuO
followed by deposition of ultrathin carbon nitride (CNx) films through radio-frequency sputtering of a
graphite target in a high-purity N2 gas formed 3D CuO/CNx core-shell nano-architectures that
[228]. Graphitic
carbon nitride (g-C3N4) uniformly coated on SnO2-TiO2 nanoparticles displayed a capacity of
380.2 mAh g−1
at 0.2C rate after 20 cycles and 330 mAh g−1
at 0.5C [229]. Let us recall the good
performance of a hybrid of graphitic carbon nitride with reduced graphene oxide [38]. Instead of
carbon nitride, silicon nitride was also used as a coating material. In particular, it was shown that a
silicon nitride coating of silicon thin films has a positive effect on both the cycling stability and high
rate performance [230].
7.2.2. Vanadium Nitride
Vanadium nitride (VN) nanowires possess obvious pseudocapacitive characteristics with a very
low redox potential (0.01–3V vs. Li0
/Li+
), and can get further gains in energy storage with the
assistance of graphene so that it is a promising anode material [231]. In particular, by employing such
VN/reduced graphene oxide composite and porous carbon nanorods with a high surface area of
3343 m2
g−1
as the anode and cathode, respectively, a novel hybrid Li-ion was fabricated, with an
ultrahigh energy density of 162 Wh kg−1
at 200 W kg−1
, which also remains 64 Wh kg−1
even at a high
power density of 10 kW kg−1
. The speciﬁc capacity was 640 mAh g−1
at 0.1 A g−1
[231]. Vanadium
nitride and nitrogen-doped graphene nanosheet (G) hybrid materials showed an initial coulombic
efficiency 74.6%, while that of the pristine VN was only 55.8% [232], illustrating that the combination
of such binder free materials and metal nitrides increases the conductivity of metal nitrides [233].
7.2.3. Titanium Nitrides
Titanium nitride nanowires on carbon textiles supported Si nanofilms investigated as anodes
without using binders and conductive additives delivered a capacity of 3258.8 mAh g−1
with a
discharge capacity retention of 94.7% after 200 cycles at a current density of 1 A g−1
, an important
improvement with respect to the performance obtained with TiO2 instead of TiN [234]. TiN-coated
Li4Ti5O12 nanocomposites with different mass ratios exhibited markedly improved electrochemical
propertieswith respect to pristine Li4Ti5O12 as it exhibited a capacity of 130 mAh g−1
at a
charge/discharge rate of 20C and a capacity retention 85% after 1000 cycles at 10C [235]. A result
comparable to the VN/N-doped graphene composite in [233] was reported for another binder free
graphene/titanium nitride hybrid material [236]. This G/TiN hybrid anode exhibits a reversible
capacity as high as 325 mAh g−1
at current rate 2000 mA g−1
, much higher than that of pure graphene at
98 mAh g−1
after about 35 cycles.
7.2.4. Other Transition Metal Nitrates Composites
A NbN nanoparticle/nitrogen-doped graphene composite was explored as a new anode with a

1079
high energy density of 123 Wh kg−1
[237]. 81.7% capacity retention after 1000 cycles at 0.5 A g−1
was
obtained.
Zheng et al. modified the surface of Fe3O4 nanoparticles with Fe3N [238]. A reversible capacity
over 739 and 620 mAh g−1
was obtained after each 60 cycles at a current density of 50 and
200 mA g−1
, respectively for this Fe3O4@Fe3N composite.
In the same way, nanocoating of Mo2N on the surface of hollow-nanostructured MoO2 is highly
effective in improving the conductivity of MoO2 compared to the bare MoO2 [239]. At a current
, this composite delivered a capacity of 815 mAh g−1
after 100 cycles. At very
high current density of 5 A g−1
, the capacity was still 415 mAh g−1
(three times the capacity in absence
of Mo2N coating). In addition, these results were obtained without any binder, i.e. Mo2N is the binder.
Mn3N2 has been proposed as a novel anode in 2012 [240]. The irreversible electrochemical
reaction mechanism results in an unavoidable capacity loss and low columbic efficiency of 70% in
the first cycle. Nevertheless, at 160 mA g−1
, the discharge capacity was 365.4 mAh g−1
after the
300th cycle. The main process in all the charge processes can be characterized by one slope plateau
at around 1.1 V. These results show that this nitride is a promising anode material. Since then, carbon
coated MnO@Mn3N2 core-shell composites (MnO@Mn3N2/C) were synthesized in a simple
approach by calcinating MnO2 nanowires with urea at 800 °C under an ammonia atmosphere. Urea
derived carbon nanosheets were partially coated on pure phase MnO@Mn3N2 core-shell
composites [241]. After 60 cycles with current density constantly changing, the reversible capacity of
MnO@Mn3N2/C still remained above 626 mAh g−1
with a current density of 100 mA g−1
, exhibiting a
significantly improved performance in comparison with the MnO/C and Mn3N2/C samples.
8. Conversion Anode Composites
The conversion reaction in transition metal oxides involves the formation of Li2O according to
the reaction:
MO + 2 Li+
→ M + Li2O, (4)
with M = Mn, Fe, Ni, Co, Cu. The first discharge reaction results in the amorphization of the lattice
followed by the formation of nanoparticles of metal M embedded into the Li2O matrix. In the charge
reaction, Li2O decomposes, and MO is reformed.
8.1. CoO-based Composites
Due to its low electrical conductivity and important volume changes upon cycling, good results
with CoO as an anode can be obtained only in composites. Among the hybrid coated CoO particles,
an unsealed hollow porous CoO shell with a Co metal core with avoid between the shell and the core
[242]. The same
result was found for a carbon-decorated CoO composite, but at current density 100 mA g−1
[243].
Better results were obtained with CoO/NiSix core-shell nanowire arrays, which delivered a reversible
at 1C rate, and 400 mAh g−1
at 44C rate [244]. The deposition of CoO onto
Cu nanaorods form nano-structures electrodes that delivered a reversible capacity of 900 mAh g−1

1080
(0.3C) [245]. In a Co-Li2O@Si core-shell nanowire
array, the coated Si shell could be electrochemically active, while the Co-L2O nanowire core could
function as a stable mechanical support and an efficient electron conducting pathway during the
charge-discharge process. The Co-Li2O@Si core-shell nanowire array anodes exhibited good cyclic
stability and high power capability compared to planar Si film electrodes [246].
CoO/graphene composites were studied because the graphene nanosheets function as a robust
matrix increasing the conductivity of the CoO composite, but also buffering the large volume swings
of the active CoO composite during the charge-discharge process. However, to avoid the aggregation
of CoO nanoparticles, the practical applications of graphene-based materials in energy fields often
require the assembly of 2D reduced graphene oxides sheets into 3D architectures [247]. After 100
discharge/charge cycles at 50 mA g−1
, the reversible capacities of a CoO/3D graphene nanocomposite
remained 860 mAh g−1
. At 500 mA g−1
, the composite electrode was capable of delivering a stable
capacity of about 391.2 mAh g−1
[248]. In another work, CoO/graphene composites with a mass ratio
9:1 exhibited a capacity of 1400 mAh g−1
over 60 cycles at current density of 100 mA g−1
, and still
500 mAh g−1
at 8 A g−1
[249]. This result is better than the performance exhibited previously by
CoO/graphene nanosheets [250]. However, CoO quantum dots (3–5 nm) on graphene nanosheets
were able to deliver 1592 mAh g−1
and 1008 mAh g−1
and 1000 mA g−1
, respectively [251]. At last, 5 nm-thick CoO particles densely anchored on
graphene sheets maintained a stable capacity of 1015 mAh g−1
for 520 cycles [252].
8.2. NiO-based Composites
Due to low cost, safety and low toxicity, NiO is considered as a promising anode for Li-ion
batteries. The anode proceeds by the reversible conversion reaction:
NiO + 2 Li+
+ 2 e−
→ Ni0
+ Li2O, (6)
providing a theoretical specific capacity of 718 mAh g−1
.
The first lithiation occurs at 0.6 V, the subsequent ones slope from 1.0 to 1.4 V. However, the
larger delithiation potential ranging from 1.5 to 2.2 V makes NiO less attractive since higher
potential means less energy density. Nevertheless, it can be coupled with a high voltage cathode such
as LiNi0.5Mn1.5O4 electrode [253]. Like the previous materials already reviewed, it gives poor results,
unless it is nanostructured; the porosity also plays an important role. The best result obtained for NiO
used as an anode was reported for three-dimensional “curved” hierarchical mesoporous NiO
nano-membranes, which delivered a capacity of 721 mAh g−1
at rate 1.5C and a lifetime of 1400
cycles [254]. Nanoporous NiO films directly grown on the foam Ni delivered a capacity of
543 mAh g−1
after 100 cycles at rate C/5, and showed a reversible capacity of 280 mAh g−1
at 10C
rate [255]. Composites have been built to avoid the barrier that a uniform coat of NiO with a metal
could rise for lithium transport. In particular, a composite NiO/Co-P composed of 200 nm-thick NiO
particles and 30 nm-thick granular particles of Co-P delivered capacities of 560 and 270 mAh g−1
at
current density 200 and 1000 mA g−1
[256].
Good results have been obtained with NiO: graphite composites, but the results depend very
much on the architecture of the electrode, and also the amount of carbon. For instance a sandwich of

1081
graphene nanosheets/NiO nanosheets delivered a capacity of 1030 mAh g−1
after 50 cycles at 0.5C
rate, and retained 492 mAh g−1
at 5C; however the C/NiO was very high: 77.2:22.8 [257]. Improved
rate capability was obtained with NiO/graphene nanosheet hierarchical structure prepared by
electrostatic interaction between negatively charged graphene oxide and positively charged NiO in
aqueous solution with pH = 4 [258], with a capacity of 615 mAh g−1
at a current density of 1.6 A g−1
(5.6C rate). In 2012, for almost the same current density 1.5 A g−1
, the capacity was raised to
727 mAh g−1
with reduced graphene oxide and nanosheet-based NiO microsphere composite [259].
In 2014, a core-shell Ni/NiO nanocluster-decorated graphene delivered 863 mAh g−1
after 300 cycles
at current density 1.5 A g−1
, and 700 mAh g−1
[260]. Finally,
in 2016, hierarchical NiO/Ni nanocrystals covered with a graphene shell were obtained with the
hollow ball-in-ball nanostructure intact [261]. This NiO/Ni/graphene composite delivered a high
reversible specific capacity of 1144 mAh g−1
, 962 mAh g−1
after 1000 cycles at 2A g−1
and an
average capacity of 805 mAh g−1
. This illustrates the fast improvement of
the electrochemical performance of NiO-graphene composites through the recent years.
8.3. MnO-based Composites
MnO has a high theoretical capacity of 755 mAh g−1
, high density (5.43 g cm−3
), and is
abundant. The application as an anode, however, is still hindered by the same characteristics as any
conversion anode elements: large volume change during cycling, low electrical conductivity.
Carbon-coated MnO prepared by ball-milling with sugar and pyrolysis at 600 °C exhibited a capacity
retention of 650 mAh g−1
over 150 cycles at 0.08C. [262]. Increasing the sintering temperature
whenever it does not damage the electrochemical-active material is a good idea because the
conductivity of the carbon will be improved. 20 nm-thick MnO particles coated with carbon at
700 °C delivered a capacity of 939 mAh g−1
after 30 cycles at C/10 rate [263]. Porosity also helps, as
usual with all the metal oxides. Porous carbon-coated MnO microspheres prepared at the same
temperature delivered 700 mAh g−1
at current density of 50 mA g−1
over 50 cycles, and 400 mAh g−1
at 1.6 A g−1
[264]. A breakthrough has been achieved with new porous carbon-coated MnO spherical
particles of size 300 nm [265]: this composite delivered a capacity of 1210.9 mAh g−1
after 700
cycles at 1.0 A g−1
.
Hollow structures also helped to accommodate the volume change during cycling of MnO.
Hollow MnO/C microspheres delivered 700 mAh g−1
100 mA g−1
[266]. Decreasing the size of the hollow spheres to the nano-range, and adding the
porosity of the shell, the capacity could be raised to 1515 mAh g−1
100 mA g−1
, and 1050 mAh g−1
as shown in
Figure 8 [267]. MnO nanocrystals embedded in 100–200 nm-thick carbon nanofibers obtained with a
porous structure delivered 1082 mAh g−1
and
575 mAh g−1
[268].
MnO/graphene composites were also synthesized, delivering a capacity of 2014 mAh g−1
after
150 cycles after 150 cycles at current density 200 mA g−1
, 843 mAh g−1
density 2 A g−1
with only 0.01% capacity loss per cycle [269].

1082
Figure 8. Comparison between discharge-charge curves of hollow MnO2, Mn3O4 and
MnO nanospheres as anode materials for lithium ion batteries. Cycling was carried out at
current density 100 mA g−1
(from Ref. [267]).
We can thus conclude that there are now porous nanostructures of MnO either carbon-coated or
anchored on graphite that have a long cycling life high capacity and good rate capability. The problem
of deficiency of regularity in the structure and the limited elasticity of the shell material that were at the
origin of the aging of the MnO-based composites [270] is definitely solved.
8.4. MnO2-based Composites
MnO2 has a higher theoretical capacity of 1232 mAh g−1
. It is thus logical that many efforts have
been made to make MnO2/C composites in parallel to the work made on MnO/C composites.
Nano-urchin structure consisting of spherical onion-like C and MnO2 nanosheets could deliver a
reversible capacity of 404, 263, 178 and 102 mAh g−1
at rates of 200, 500, 1000, and
2000 mA g−1
[271].
MnO2 nanoflakes coated on carbon nanohorns exhibited a capacity of 565 mAh g−1
at current
density 450 mA g−1
[272]. Caterpillar-like nanoflaky MnO2/carbon nanotuber delivered 801 mAh g−1
(<1000 mAh g−1
can be attributed to the MnO2 porous layer alone) for the first cycle without capacity
fade for the first 20 cycles [273]. Mesoporous γ-MnO2 particles/single-walled carbon nanotubes
composites maintained a capacity of 934 mAh g−1
over 150 cycles [274].
Graphene-MnO2 nanotube as anode delivered a reversible specific capacity based on electrode
composite mass of 495 mAh g−1
at 100 mA g−1
after 40 cycles; the capacity was 208 mAh g−1
at
1.6 A g−1
[275]. Graphene-Wrapped porous MnO2-Graphene Nanoribbons maintained a specific

1083
capacity of 612 mAh g–1
after 250 cycles at 400 mA g−1
[276]. MnO2/3D porous graphene-like
(62.7 wt% MnO2, effective surface area 58 m2
g−1
, total volume 0.2 cm3
g−1
) delivered a discharge
at the second cycle and the discharge capacity remained at 836 mAh g−1
at
the 200th cycle at current rate 100 mAh g−1
and the capacity remained at 433 mAh g−1
at current
density 1.6 A g−1
[277].
8.5. Mo Oxide-based Composites
Two Mo oxides have been investigated as anodes for Li-ion batteries: MoO2 and MoO3. Both of
them have a good theoretical capacity that exceeds that of graphite: 838 mAh g−1
for MoO2 and
1111 mAh g−1
for MoO3. They share with the other metal oxides the same feature, i.e. the large
volume change during cycling, and lo electrical conductivity, implying the use of composites to
buffer the volume change, and increase the conductivity. In addition, they suffer from the fact that
the conversion reaction is very slow, which limits the rate capability, reinforcing the need to decrease
the size of the material to the nano-range.
Carbon-MoO2 composites have improved the electrochemical properties with respect to bare
MoO2. A composite consisting in MoO2 nanofibers coated with a 3 nm-thick carbon layer exhibited
capacities of 762.7 mAh g−1
at 50 mA g−1
and 430.6 mAh g−1
at 200 mA g−1
with a columbic
efficiency of nearly 100%, steadily retained after 50 cycles [278]. MoO2/C nanospheres with
diameters of about 15–25 nm were interconnected to form a cage-like architecture delivered a
capacity of 692.5 mAh g−1
after 80 charge-discharge cycles at a current density of 200 mA g−1
[279].
The specific capacity of MoO2/C hybrid nanowires at current density of 1 Ah g−1
, was retained
at 327 mAh g−1
, i.e. 92% of that in the first cycle, after 20 cycles of discharge and charge [280]. An
improvement of the rate capability has been obtained with MoO2-Ordered Mesoporous Carbon
Nanocomposite where ordered mesoporous carbon aggregates consisted of micrometer-sized rod-like
particles. As an anode, this composite attained capacity of 401 mAh g−1
at a current density of
2 A g−1
[281]. Porosity can also compensate for an increase of the size of the structure from nano to
micro range. Porous 2 μm sized MoO2/Mo2C/carbon sphere composites with BET surface areas the
order of 155 m2
g−1
delivered discharge capacities of 650 mAh g−1
500 mAh g−1
[282].
MoO2/graphene composites were also an improvement. A MoO2/graphene oxide composite
and 560 mAh g−1
after 30 cycles at a current density of
100 mA g−1
and 800 mA g−1
, respectively [283]. MoO2 nanocrystallites of ca. 30–80 nm were
wrapped in graphene layers and assembled into the secondary rods [284]. For this composite, the
capacity reaches 407.7 mAh g−1
after 70 discharge and charge cycles at current density 2A g−1
. In
another case, MoO2-graphene electrode with graphene content 10.4 wt% showed a capacity of
1009.9 mAh g−1
after 60 charge/discharge cycles at a current density of 100 mA g−1
in the potential
range 0.01–2.5 V [285]. When the current density is raised to 500 mA g−1
, the electrode can still
deliver a charge capacity of 519 mAh g−1
up to 60 cycles. Note that these results are comparable with
those obtained with layered MoS2/CN-G composite anode that delivered a capacity of 800 mAh g−1
with great cycling stability [286].
MoO3 has been investigated for many applications. For instance, redox Active

1084
Polyaniline-h-MoO3 hollow nanorods [287], ordered mesoporous α-MoO3 with iso-oriented
nanocrystalline walls [288], and ultrathin MoO3 nano-crystals self-assembled on graphene
nanosheets [289] were proposed as new supercapacitors. As an anode material, MoO3 not only has a
superior theoretical specific capacity of nearly 1111 mAh g−1
but also has a very stable
one-dimensional (1D) layered structure. Recently, single-crystalline α-MoO3 nanobelts prepared in
5 mol L−1
HNO3 delivered a capacity of 175 mAh g−1
at the first cycle at 2C rate, falling to
150 mAh g−1
after 10 cycles, while commercial MoO3 delivers a stable reversible capacity of
72 mAh g−1
at 2C rate [290]. To overcome the degradation of MoO3 nanorod anodes in lithium-ion
batteries at high-rate cycling, coating of the nanorods by a protective layer has been proposed. The
capacity of α-MoO3-In2O3 core-shell nanorods maintains 1114 mAh g−1
and 443 mAh g−1
after 50
cycles at 0.2C and 2C rate, respectively [291]. Conformal passivation of the surface of MoO3
nanorods by HfO2 using atomic layer deposition (ALD) was proposed with success [292]. After 50
charge/discharge cycles at high current density1500 mA g−1
, HfO2-coated MoO3 electrodes exhibited
a specific capacity of 657 mAh g−1
. MoO3 nanoparticles were electrodeposited on Ti nanorod arrays
that showed enhanced rate capability (300 mAh g−1
at 100 A g−1
) and excellent durability (retaining
300 mAh g−1
after 1500 cycles at 20 A g−1
) [293]. α-MoO3/MWCNT nanocomposite delivered
490 mAh g−1
owing to the high conductivity of the carbon
nanotubes [294]. Porous MoO3 grafted on TiO2 nanotube array was found to improve the specific
capacity with respect to MoO3 alone and the TiO2 nanotubes alone by a factor three [295]. A
tremendous increase of capacity was achieved with SnO2/MoO3 core-shell nanobelts: 3104, 2214,
2172, 2110 and 2031 mAh g−1
at the 1st, 2nd, 10th, 20th and 30th, respectively, at C/10 rate. The
reversible capacity after 50 cycles maintains 1895 mAh g−1
(C/10) and 1530 mAh g−1
(C/2) [296].
This result was explained by the fact that the Mo nanoparticles, generated together with Li2O during
the conversion reaction, increase the electrochemical reactivity and make the conversion between Li+
and Li2O reversible.
According to these results, MoO3 shows good electrochemical properties for use as an anode for
Li-ion batteries. The price to be paid to obtain such performance, however, is the reduction of the
size of the particles to the nano-range, and the synthesis of nano-composites. Such synthesis
processes, however, are not scalable, and expensive, which explains that the anode of the commercial
Li-ion batteries is still graphitic carbon, although much better results are obtained since years at the
laboratory scale, not only with MoO3 but also with silicon and many metal oxides. In an attempt to
make MoO3 available as an anode for commercial Li-ion batteries, a facile scalable and not
expensive method that improves the properties of the commercial MoO3 was proposed [297]. The
composite was formed of stoichiometric layered MoO3, orthorhombic oxygen deficient phases
MoO3−δ with δ = 0.25 (γ-Mo4O11) and α-ZrMo2O8. This ZrMo2O8-decorated composite delivered a
at 2C rate, twice the capacity observed for the MoO3 particles alone.
8.6. Iron Oxide-based Composites
Fe2O3 and Fe3O4 have drawn particular attention because of their high theoretical capacity of
≈1000 mAh g−1
, non-toxicity, and low processing cost. A recent review on the critical role of
crystallite size, morphology, and electrode heterostructure of the pristine iron oxides on the

1085
performance of the electrochemistry can be found in [298]. Again, the best results have been
obtained in hybrid compounds.
8.6.1. Carbon-coated Iron Oxides
Carbon-coated Fe3O4 nanospindles delivered a capacity of 530 mAh g−1
after 80
charge/discharge cycles [299]. Better results have been obtained with carbon-coated Fe3O4 nanorods
that delivered 808 mAh g−1
after 100 charge/discharge cycles at a current density of
924 mA g−1
[300]. In addition, the synthesis of the composite was made in a one-step process.
150–200 nm-thick Fe3O4 nanospheres coated with a thin layer of amorphous carbon with a uniform
thickness of <6 nm forming a Fe3O4@C composite with specific surface area of <29 m2
g−1
delivered
a capacity of 712 mAh g−1
retained after 60 charge/discharge cycles at a current rate of
200 mAh g−1
[301].
Carbon-coated Fe2O3 hollow nanohorns on carbon nanotubes delivered a capacity of
820 mAh g−1
within 100 cycles at current density of 500 mA g−1
and the capacity retained at current
density 3 A g−1
[302]. Other nano-composites with carbon nanofibers did not
give better data [303]. The best result has been obtained recently with a mesoporous composite made
of carbon-coated FeOx particles (20 ± 14 nm-thick)/carbon nanotube (CNT) with high specific
surface area (66 m2
g−1
). As an anode, this composite delivered a specific capacity retention of 84%
(445 mAh g−1
) after 2000 cycles at 2000 mA g−1
(4C) [304]. Note, however, that the amount of
carbon was important, i.e. 43 wt%.
8.6.2. Iron Oxide-Carbon Nanosheets
In a different geometry, uniform-sized ferrite nanocrystals/carbon hybrid nanosheets as an
anode maintained 600 mAh g−1
after 50 cycles where the cell was discharged with a constant current
of 100 mA g−1
to 0.05 V (vs. Li0
/Li+
) and constant voltage at 0.05 V vs. Li0
/Li+
to 50 mA g−1
and
charged with a constant current of 100 mA g−1
to 3.0 V , and 81.5% of original capacity was retained
after 5 cycles, where the lithiation and delithiation current density were fixed at 100 mA g−1
and
5 A g−1
, respectively [305]. A major improvement was achieved with a facile and scalable in situ
synthesis of carbon-encapsulated Fe3O4 nanoparticles (<18.2 nm) homogeneously embedded in
two-dimensional (2D) porous graphitic carbon nanosheets with a thickness of less than 30 nm. This
composite as an anode delivered 858, 587, and 311 mAh g−1
at 5, 10, and 20C, respectively
(1C = 1 A g−1
) and suffered only 3.47% capacity loss after 350 cycles at a high rate of 10C [306].
8.6.3. Iron Oxides-Graphene Composites
Two Fe2O3/graphene composites have been investigated [307,308]. The results are comparable.
The capacity was 1027 mAh g−1
. Recently, Fe2O3
nanoparticles encapsulated in g-C3N4/graphene (CN-G) sandwich-type nanosheets were synthesized.
This Fe2O3/CN-G composite delivered 980 mAh g−1
after 50 cycles at a current density of 50 m g−1
,
preserving 76.4% of the initial capacity. 532 mAh g−1
and 436 mAh g−1

1086
densities of 1 and 2 A g−1
, respectively [309].
Fe3O4/graphene composites were also synthesized. Considering only the better results obtained
in the recent years, we note free standing (i.e. binder less) hollow Fe3O4/graphene (39.6 wt%
graphene) with hollow and porous Fe3O4 that delivered 1400 and 660 mAh g−1
after 50 cycles at
current densities100 and 500 mA g−1
, respectively [310]. Another composite with 25 wt% graphene
after 100 cycles at current density of 1 A g−1
. The capacity was
still 460 mAh g−1
at 5 A g−1
[311]. The improved performance was attributed to a new method to
obtain well-dispersed Fe3O4 particles uniformly with high loading.
8.6.4. Ternary Compounds
Ternary composites have been recently proposed. To compensate the lower capacity of TiO2, the
hierarchical co-assembly of TiO2 nanorods and Fe3O4 nanoparticles on pristine graphene nanosheets
has been proposed to address simultaneously the lower capacity of TiO2 by coupling it with Fe3O4,
and their low electrical conductivity by coupling them with graphene [312]. The TiO2
nanorods/Fe3O4 nanoparticles/pristine graphene ternary heterostructures (40:20:40) delivered a
reversible capacity up to 703 mAh g−1
at the end of 200 charging-discharging cycles at a current
, corresponding to 90% cycle retention, and 524 mAh g−1
after 200 cycles at
1 A g−1
. This performance is remarkable as it allies high capacity, especially at high rates, and high
cycleability. The capacity is higher than the TiO2/reduced-GO anode reported previously, e.g.
190 mAh g−1
at 320 mA g−1
[313], 161 mAh g−1
at 170 mA g−1
[314], 200 mAh g−1
at
100 mA g−1
[315] or 175 mAh g−1
at 100 mA g−1
[316]. This is attributed by the authors to the
high-capacity Fe3O4 as an auxiliary active material, and also the choice of pristine graphene rather
than reduced graphene oxide (r-GO), because the electronic properties of r-GO are damaged during
the oxidation-reduction process needed to prepare it. The good cycleability is due to the fact that the
fraction (20 wt%) of Fe3O4 is small, to take full advantage that the change of volume of TiO2 during
cycling is small, at contrast with the other active anode elements except Li4Ti5O12, so that the
resulting aging of the anode is reduced. For comparison, a TiO2/Fe2O3 anode where the Fe2O3
fraction is up to 42.9 wt%, had reduced performance, namely 430.2 mAh g−1
at 200 mA g−1
after 100
cycles [317].
8.7. Co3O4-based Composites
According to the conversion reaction, the theoretical capacity of Co3O4 is 890 mAh g−1
.
However, its electrical conductivity is very small. One possibility to overcome this difficulty is to
dope this compound, and indeed, C-N co-doped Co3O4 hollow nanofibers gave promising
performance as an anode [318]. Otherwise, the solution was to build nano-Co3O4/carbon composites.
Recently, a Co3O4/porous carbon nanofibers composite delivered 869.5 mAh g−1
at 0.1C 94.9%
capacity retention at 50 cycles at 0.1C-rate, and 403.6 mAh g−1
at 2C at 25 cycles [319]. The best
results, however, were obtained with graphene.
Co3O4 nanoparticles in a size range of 2–3 nm confined in a few-layered porous graphene
nanomesh framework (70% Co3O4) delivered 1543 mAh g−1
at 150 mA g−1
and 1075 mAh g−1
at

1087
1000 mA g−1
[320]. Even at the high current density of 1000 mA g−1
, the capacity was still
1072 mAh g−1
, while that of previously reported Co3O4/graphene composites was in the range
500–600 mAh g−1
[321,322]. At low current density, the capacity is comparable to that of the
CoO/graphene composite [249], but the decrease with the number of cycles is smaller in the
Co3O4/graphene composite. Mesoporous Co3O4 nanosheets-3D graphene networks (specific surface
area and pore size 34.5 m2
g−1
and <3.8 nm, respectively) delivered 630 mAh g−1
after 50 cycles at
0.2C, and 130 mAh g−1
at 5C [323]. A well-dispersed Co3O4 nanoparticles with high density
anchored on nitrogen-doped graphene (CNGs) delivered a reversible capacity of 867.9 mAh g−1
after
350 cycles at current density 50 mAh g−1
and exhibited a high rate capability (155.6 mAh g−1
at
2 A g−1
[324]. Recently, a mesoporous atomic layer-by-layer Co3O4 nanosheets/graphene mesoporous
composite discharge capacities of 2014.7 and 1134.4 mAh g−1
at 0.11 and 2.25C, respectively, and
capacity retention of 92.1% after 2000 cycles at 2.25C [325]. This outperforms all previous results.
However, this synthesis process seems limited to the laboratory scale.
Composite composed of anatase TiO2 nanofibers and secondary porous Co3O4 nanosheets have
been prepared to take advantage of the high capacity of anatase TiO2 (in the same way as in the
TiO2/Fe3O4 case already discussed in the section devoted to the iron oxides). This TiO2/Co3O4
composite delivered 632.5 mAh g−1
and 95.3% capacity retention after 480 cycles, and
449.5 mAh g−1
at current density of 1 A g−1
[326].
9. Conclusion
Titanium oxides have the disadvantage of a lower energy density than that of graphite, typically
160 mAh g−1
for Li4Ti5O12, 250 mAh g−1
for TiO2 vs. 372 mAh g−1
. Also, the voltage at which Li
cycling occurs in the titanium oxides is high, namely 1.3–1.6 V, which reduces the operating voltage
of the Li-ion battery, and thus its energy density. On another hand, this potential has the advantage of
avoiding the large irreversible capacity lost due to the formation of the SEI on anode particles
operating below 1 V. The titanium oxides have other remarkable advantages: low cost, environmental
safety, very good stability both in the discharged and charged state, very good cycleability and very
high power density, very good abuse tolerance. That is why Li4Ti5O12 is expected to be accepted for
hybrid and different applications, in particular when high power density is needed, in particular
under the form of carbon-coated Li4Ti5O12. This composite anode has a remarkable rate capability. In
addition, the nano-size of the structures that have been built during last years, increases the surface
over volume ratio, and introduces additional active sites at the surface, so that the experimental
reversible capacities can be larger than the theoretical one, solving the problem of the capacity
retention.
Alloying materials such as Si, Ge, SiO, SnO2 can provide much larger capacities, and energy
densities than the titanium oxides and the other elements of the previous group, but they suffer from
the important capacity loss upon cycling due to the large variation of volume upon Li insertion and
de-insertion. Si and SnO2 are the most promising elements of this family, as germanium is expensive
and not abundant in nature. The same problem is met with oxide anodes based on conversion
reactions. In addition, these anodes exhibit large potential hysteresis between the charge and
discharge reactions. The voltage hysteresis, however, is expected to be solved by using some

1088
catalysts and surface coating. Important improvements in the cycleability and rate capability of these
anodes have been obtained in the last few years.
Both the alloying anode materials and the oxide anodes based on conversion reactions have
benefited from the progress in the preparation of porous nanostructures by different processes
involving thermal decomposition of hydroxide, carbonate, oxalate, etc., which are much more
scalable than prior methods. Another progress at the laboratory scale has been the synthesis of
composites, in particular with graphene. Graphene has a very good electronic conductivity, a good
mechanical flexibility and high chemical functionality. Therefore, it can serve as an ideal 2D support
for assembling nanoparticles with various structures and very good results have been obtained with
composites graphene-nanoparticles of metal oxides. In addition the graphene is able to prevent
agglomeration of the nanoparticles upon cycling, and the nanoparticles prevent the re-stacking of the
graphene sheets. The development of metal oxides based on alloying or conversion reaction, in
addition of titanium oxides based on the intercalation process we have reviewed can now be prepared
under the form of nanoporous structure supported by graphene sheets, which are able to deliver a
reversible capacity the order of 700 mAh g−1
for 1000 cycles at the laboratory scale.
The handicap for the development of such anodes is now the cost, and a scalable production of
these nanostructured composites. This is a big challenge that hinders the mass production of such
anodes until the problem has been solved. Also the composites carbon nanotubes-nanoparticles of
metal oxides can be presently prepared only at the laboratory scale, as production cost and the low
speed at which they can be synthesized does not permit their application in the battery industry.
Hopefully, recent results suggest that the forcespinning technology is a viable method for the large
scale production of nano/micro fibers for battery electrodes [327]. This technology has the capability
for dual materials feed with an almost 100% yield and solvent-free processing for melt spinning.
The main efforts in the recent years have been to increase the performance of the anode
materials. However, the capacity is limited more by the cathode than the anode element in Li-ion
batteries: typically, the capacity of a cathode element is in the range 140–200 mAh g−1
. The gain in
capacity of the total cell by increasing that of the anode element alone is thus limited, as the total
capacity almost saturates when the anode capacity reaches typically 500 mAh g−1
[2]. Many
composites today can deliver such a capacity and even much more for a thousand of cycles.
Somehow, the run on the performance of anodes is over. Time is to focus on scalable synthesis of
these composites at a reasonable price to supplement the graphite anodes.
Conflict of Interest
The authors declare that there is no conflict of interest regarding the publication of this
manuscript.
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