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Nano Res
1
Designed synthesis of cobalt-oxide-based
nanomaterials for superior electrochemical energy
storage devices
Hua-Jun Qiu, Li Liu, Yan-Ping Mu, Hui-Juan Zhang, and Yu Wang ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0589-6
http://www.thenanoresearch.com on September 18, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0589-6
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Designed synthesis of cobalt-oxide-based
nanomaterials for superior electrochemical energy
storage devices
Hua-Jun Qiu, Li Liu, Yan-Ping Mu, Hui-Juan Zhang , Yu
Wang*
School of Chemistry and Chemical Engineering,
Chongqing University, 174 Shazheng Street, Shapingba
District, Chongqing City, P.R. China, 400044.
Page Numbers. The font is
ArialMT 16 (automatically
inserted by the publisher)
Recent development on designed synthesis of various
cobalt-oxide-based nanomaterials for Li-ion battery are summarized,
including one-dimensional nanowires, two-dimensional nanosheets,
three-dimensional spheres, and hybrids with carbon and other metal
oxides.
Provide the authors’ website if possible.
Author 1, website 1
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2
Designed synthesis of cobalt-oxide-based nanomaterials for superior electrochemical energy storage devices
Hua-Jun Qiu, Li Liu, Yan-Ping Mu, Hui-Juan Zhang, and Yu Wang ()
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China.
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT Cobalt oxides, such as Co3O4 and CoO, have received increased attention as potential anode materials for
rechargeable lithium-ion batteries (LIBs) owing to their high theoretical capacity. Nanostructure engineering
has been demonstrated as an effective approach to improve the electrochemical performance of electrode
materials for LIBs. In this review, we summarize recent development in the rational design and fabrication of
various cobalt oxide-based nanomaterials and their lithium storage performance, including 1D nanowires/belts,
2D nanosheets, 3D hollow/hierarchical structures, hybrid nanostructures with carbon (amorphous carbon,
carbon nanotubes and graphene) and mixed metal oxides. By focusing on the structure effects on their
electrochemical performance, effective strategies for the fabrication of cobalt oxide/carbon hybrid
nanostructures are highlighted. This review shows that by rational design, these cobalt-oxide-based
nanomaterials are very promising as next generation LIBs’ anodes.
KEYWORDS Anodes, Lithium ion battery, hybrid, peapod structure, graphene
1 Instruction
With high energy density, long cycle life, and
environmental friendliness, rechargeable Li-ion
batteries (LIBs) have attracted wide research interest.
However, there still have been some great challenges
in LIBs since the related technology was realized
[1-3]. It is known that LIBs usually suffer from
pulverization of the electrode material and
Li-alloying agglomeration and amorphization during
cycling, resulting in deterioration of the battery
performance [4-6]. Thus, one of the main emphases is
how to enhance the specific capacity and capacity
retention over long term charge/discharge cycles. It
is clear that effective architectures have to be
designed to maintain the integrity of the electrode
Nano Res DOI (automatically inserted by the publisher)
Review Article/Research Article Please choose one
————————————
Address correspondence to [email protected]
3
material and prevent its disintegration as much as
possible in order to enhance the cycle stability and
rate capability of the battery. For this aim, it is
reasonable to believe that nanotechnology is the key
to solve the tough problem. This belief arises from
the fact that nanostructured materials prepared
under precise control are very desirable for the
improvement in surface areas, organization of
electrode components, and electrolyte penetration [7].
Meanwhile, it is worth mentioning that when
particles’ size is down to nanoscale, they are highly
active for catalyzing electrochemical reactions and
electrolyte reversible degradation, which greatly
contribute to the cycleable capacities. However, the
conventional nanotechnology is not qualified to fully
exhibit the advantages of nanomaterials over their
bulk counterparts. For example, when
charge/discharge cycling is imposed on pure
nanostructured electrode, their structural integrity
will be severely damaged, resulting in limited
lifespans, no matter these simple nanostructures are
free-standing, randomly dispersed, or uniformly
organized [8-10]. To improve the performance,
integrating carbonaceous matrix into the active
nanomaterials (such as metal oxides) to form hybrid
nanostructures has been proved to be very effective
because it can increase the electrical conductivity and
better buffer the volume change due to the elastic
feature of carbon supports [11,12].
Today’s secondary LIBs usually use graphitic
carbon as the anode material, owing in part to its low
cost, good electrical conductivity and stable
solid-electrolyte interphase. Graphitic carbon anodes,
however, suffer from relatively low specific capacity
(372 mA·h·g-1) and low rate capability. With the
increasing demand from portable electronics and
electric/hybrid electric vehicles, alternative materials
with enhanced Li+ storage properties such as
increased specific capacity, rate capability and
lifespan must be developed, together with a
consideration of production cost and environmental
impact. A number of potential alternatives to carbon
have emerged in recent years and attracted much
research effort. Among these candidates, cobalt
oxides (CoO and Co3O4) have drawn particular
attention owing to their high theoretical capacity and
relatively rich earth reserve [13-15]. With a high
theoretical capacity of 892 mA·h·g-1, Co3O4 is perhaps
the most suitable cobalt oxide for LIBs through a
complete conversion reaction (Co3O4 + 8Li+
+8e- 3Co + 4Li2O). The recovered species on
charging of Co3O4 has been identified as CoO,
resulting in a reversible 6e- conversion reaction with
a capacity near 670 mA·h·g-1 [16]. Besides Co3O4,
cobalt monoxide (CoO) has also been considered as a
promising alternative anode material, which has a
relatively high theoretical capacity of 716 mA·h·g-1
[17]. Meanwhile, unlike other metal oxides such as
SnO2 and ZnO, CoO can provide a completely
reversible electrochemical reaction (CoO + 2Li+
+2e- Co + Li2O) during charge/discharge cycles.
However, one concern about these cobalt oxides is
their toxicity. Therefore, these cobalt oxide
containing energy storage devices should be
carefully recycled to avoid their effect on
environment.
Since comprehensive description and discussion of
various electrode materials for LIBs have been
provided in recent years [5,11,16,18,19], this review
mainly focuses on the recent development in the
structural design and fabrication methodology of
nanostructured cobalt oxides, cobalt oxide/carbon
hybrid, and mixed cobalt-based oxides and their
performance in LIBs. Specifically, we focus on the
reasonable design of peapod-like carbon
nanotube-encapsulated and sandwich-like
graphene-encapsulated Co3O4 nanocomposites and
their enhanced performance in LIBs. Finally, future
trends and outlook in the development of advanced
cobalt-oxide-based anodes are discussed.
2 Nanostructured cobalt oxides
Fabricating nanostructured materials has been
considered as a promising approach for enhancing
the performance of LIBs in terms of energy density,
power density, and cycling stability [6,16,20]. In
general, nanostructured active materials such as
metal oxides can significantly boost the LIB
performance due to the large electrochemical active
area and greatly reduced ion and electron transport
distance. Recent research has found that some of the
structural features, such as low-dimensional
structures (1D nanowires [10] and 2D nanosheets
[21]) and nanoporous/hollow structures [22], are able
to better withstand the large volume change during
4
the charge/discharge process, thus leading to
enhanced cycling performance. Moreover,
nanoporosity has been found to play a key role in
enhancing the electrochemical performance of
solid-state battery electrode. Thus, the combination
of nanoporosity with low-dimensional nanostructure
or hollow nanostructure is expected to further
improve their cycling performance in LIBs.
2.1 One-dimensional nanowires and nanobelts
(a) (b)
(c) (d)
Figure 1 SEM image of the mesoporous Co3O4 nanowire
array (a) and schematic illustration of the structure change of
the nanowires during the discharge process (b). Inset in (a) the
zoom-in top view image of the nanowires. Specific capacity of
the Co3O4 nanowire arrays on Ti foil (red), non-self-supported
nanowires (magenta), and commercial powders (blue) as a
function of the cycle number. For each set of data, the upper
curve is for discharge and the lower curve is for charge (c).
Charge/discharge curves of the nanowire arrays from 1 to 50 C
(d). Reproduced with permission from Ref. [10]. Copyright
American Chemical Society, 2008.
In an early work in 2007, it was reported that solid
Co3O4 nanowires with a diameter of 7-8 nm by a
template method exhibited a drastic capacity
decrease after 15 cycles [23]. The poor capacity
retention of the Co3O4 nanowires was suggested to
be a result of inter-particle disconnection following
complete conversion. The formed Co nanograins
were electrically isolated from each other by the
formed insulating polymeric layers [23]. Instead of
the solid nanowires, Li et al., then fabricated
mesoporous Co3O4 nanowires with a mean
diameter of ~500 nm and a pore size of ~3.3 nm, by
a template-free ammonia-evaporation-induced
method (Fig. 1a and b), which could be extended to
grow the nanowires on different substrates (such as
Si, Ti, Cu, glass, etc.) [10]. The mesoporous Co3O4
nanowire array displayed a capacity as high as 700
mA·h·g-1 after 20 cycles at 1 C rate (~78% of the
theoretical value), and showed significantly
enhanced performance compared with
non-supported nanowires and commercial Co3O4
powder (Fig. 1c). In this case, the Co nanograins
formed during discharge were suggested to remain
interconnected throughout the insulating Li2O
matrix, forming a conductive network where
electron transfer may propagate through the
nanowire (Fig. 1b). Such interconnected structure
could facilitate the electron transfer and result in a
high rate capability (discharge capacities of 450 and
240 mA·h·g-1 were obtained when cycled at rates of
20 C and 50 C, respectively, Fig. 1d) [10]. Porous
Co3O4 nanowires prepared by a hydrothermal
synthesis using nitrilotriacetic acid as a
structure-directing ligand, also displayed a high
capacity (up to 1366 mA·h·g-1 in the first discharge
cycle) and appreciable cycling stability (810
mA·h·g-1 after 30 cycles at 300 mA g-1) [24]. The
significantly improved performance of the
mesoporous Co3O4 nanowires confirmed the
importance of porosity in maintaining structural
integrity during electrochemical cycling. The
porosity can obviously accommodate much of the
volume strain associated with the early
intercalation of Li+ (LixCo2O3), and the phase
changes brought about during the electrochemical
conversion. These mesoporous nanowires can also
assemble into a 3D hierarchical pompon-like
structure, which showed a higher specific capacity
and better cycle performance than Co3O4
nanoparticles and separated nanowires [25].
5
Recently, mesoporous Co3O4 nanobelts [26] and
nanoneedles [27], which were prepared by the
calcination of α-Co(OH)2 nanobelts, delivered high
capacities and stable early cycling abilities. The
mesopore size could be somewhat tailored by
altering the calcination temperature, which
ultimately impacted the electrochemical performance.
The Co3O4 nanobelts delivered a capacity of up to
~1900 mA·h·g-1 in the first cycle, and a reversible
capacity of ~1400 mA·h·g-1 after 20 cycles at a current
density of 40 mA·g-1 [26]. Although the unique
structure enabled high capacities at relatively low
rates, the nanobelts exhibited a relatively poor rate
performance and stable capacities could only be
obtained at low current density. To enhance the
conductivity and rate performance, we designed and
synthesized mesoporous Co3O4 nanobelt arrays on a
conducting substrate through a two-step method
(Fig. 2a) [28]. First, Co(CO3)0.5(OH)0.11H2O nanobelts
were grown on a Ti substrate by a hydrothermal
reaction, and then the nanobelt array was annealed
to form the desired mesoporous Co3O4 nanobelt
array (Fig. 2b). When evaluated in LIBs, the
mesoporous Co3O4 nanobelt array was capable of
retaining the specific capacity of 770 mA·h·g-1 over 25
cycles at high charge-discharge current density of
177 mA g-1 (Fig. 2c). Moreover, even though the
charge-discharge rates were increased to 1670 and
3350 mA g-1, it could still display a stable retention of
the specific capacity of 510 and 330 mA·h·g-1,
respectively, after 30 cycles, indicating an excellent
rate capability. A similar two-step approach has also
been used to fabricate mesoporous CoO array on
conductive Ti support [29]. After consecutive 20
cycles at 1 C, a relatively high capacity of ~670
mA·h·g-1 can still retain.
2.2 Two-dimensional nanosheets
The 2D nanostructures possess a finite lateral size
and enhanced open edges, which facilitate Li+ and
electron diffusion through the structure and also
better accommodate the volume change during the
charge/discharge process [20]. Zhan et al.,
synthesized a 2D Co(OH)2 hexagonal nanosheet
structure by a hydrothermal method, which was
then transformed to mesoporous Co3O4 nanosheets
by a proper annealing treatment [30]. Due to the
Figure 2 Schematic illustration of the fabrication of the Co3O4
nanobelt array on Ti substrate (a). The corresponding SEM
image (b) and cycling performance (c) of the array. Reproduced
with permission from Ref. [28]. Copyright American Chemical
Society, 2010.
mesoporosity (pore diameter: ~3.3 nm), the resulting
Co3O4 nanosheets had a high
Brunauer-Emmett-Teller (BET) surface area of 69 m2
g-1. Consequently, it delivered a considerable high
capacity of 1450 mA·h·g-1 after 27 cycles at 50 mA g-1.
The high surface area combined with the unique 2D
structure enabled the Co3O4 nanosheets to achieve a
capacity far in excess of theoretical value (892
mA·h·g-1). Porous nanostructured Co3O4 thin sheets
have also been prepared by electro-deposition in an
aqueous solution of Co(NO3)2 and followed by
calcination at 300 oC for 3 h [31]. The porous thin
sheets exhibited a specific surface area of 68.64 m2 g-1
and a capacity of 513 mA·h·g-1 after 50 cycles at a
current density of 100 mA g-1 [31]. Recently, to further
enhance the performance, we designed and
synthesized an ultrathin mesoporous Co3O4
nanosheet (named nanomesh) with a thickness of
only ~10 nm and size of tens or even hundreds of
square micrometers by thermal treatment of
single-crystal (NH4)2Co8(CO3)6(OH)6·4H2O
nanosheets [21]. Thanks to the ultrathin property and
mesoporous structure (pore size: ~3.27 nm), the
nanomesh possesses the highest surface area (382 m2
g-1) as compared to other Co3O4 nanostructures.
When evaluated in LIBs, the nanomesh exhibited a
(a)
(b) (c)
6
high specific capacity (1800 mA·h·g-1) in initial tests,
good rate capability and stability (above 380 mA·h·g-1
at a current density of 1000 mA g-1 over 50 cycles)
[21].
3. Hollow structures
Hollow structured metal oxides with sufficient void
space and nanoscale building blocks have been seen
as an appealing family of nanomaterials for LIBs.
The enhanced electrochemical performance of the 3D
hollow nanostructure can be mainly attributed to the
permeable thin shells with nanoporosity which
provide a shorten path for Li+ diffusion and the
hollow interior which provides sufficient space for
accommodating the large volume change during the
repeated Li+ insertion/extraction processes [12,19]. So
far, various strategies have been developed to
fabricate hollow nanostructures. Here we
highlighted some recently developed approaches to
fabricate cobalt-oxide-based hollow structures for
LIB applications.
Hollow spheres with tunable shells assembled by
Co3O4 nanosheets have been synthesized through a
hydrothermal method (Fig. 3a-c) [32]. It was found
that both the poly(vinylpyrrolidone) (PVP) soft
templates and the formation of cobalt glycolate
played key roles in the formation of these
multi-shelled hollow structures. When tested as
anode materials in LIBs, the double-shell sphere
displayed a capacity of 866 mA·h·g-1 after 50 cycles at
178 mA g-1 and its Coulombic efficiency could reach
about 100% after the first cycle. The enhancement
was suggested to result from the synergetic effect of
small diffusion lengths in the nanosheet building
blocks and sufficient void space to buffer the volume
expansion. It was also found that the size of the void
space played a key role for the cycling stability. Due
to the efficient void space, the single-shell and
double-shell samples clearly exhibited a better
cycling stability compared with the multi-shell
sample (Fig. 3d and e). Du et al., fabricated porous
Co3O4 nanotubes by using carbon nanotubes as
templates [33]. The carbon nanotube template was
removed by high temperature (500 oC) treatment in
air. The Co3O4 nanotubes with a diameter of ~30 nm
and interconnected nanoparticle-assembled shell
(particle size: 5-10 nm) exhibited a high capacity of
1200 mA·h·g-1 after 20 cycles at a current density of
50 mA g-1. Porous Co3O4 hollow nanorods have also
been prepared by using a bacteria bio-template [34].
First, nanostructured cobalt oxides with a size of ~2-5
nm were formed on the bacteria surface at room
temperature through an electrostatic interaction
between the functional surface of the bacteria and
cobalt ions. Porous Co3O4 hollow rods were then
obtained through a subsequent heat treatment at 300 oC to remove the bio-template. The Co3O4 hollow
rods exhibited a reversible capacity of 903 mA·h·g-1
after 20 cycles at a rate of 240 mA g-1 [34]. Needle-like
Co3O4 nanotubes have also been prepared by a
self-supported topotactic transformation approach
[35]. When cycled at a current density of 50 mA g-1,
the self-supported nanotube electrode showed a
capacity of 918 mA·h·g-1 with negligible capacity loss
Figure 3 TEM images of spheres with single shell (S-Co, a),
double shells (D-Co, b) and triple shells (T-Co, c). Their cycling
performance (d) and schematic illustration of the structure
change during cycling (e). Reproduced with permission from Ref.
[32]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 2010.
after 30 cycles. Single-crystalline Co3O4 hollow
octahedral nanocages prepared by the
carbon-assisted carbothermal method [36] and CoO
octahedral nanocages prepared by using NH3 as
coordination etching agent [37] also showed higher
cycling stabilities compared with commercial Co3O4
particles and their solid counterparts. Quite recently,
porous Co3O4 nanospheres [22] and hollow Co3O4
dodecahedrons [38] with controllable interiors have
also been synthesized by thermal treatment of metal
7
organic frameworks. Enhanced performances were
then observed when compared with their solid
counterpart. Particularly, the ball-in-dodecahedron
Co3O4 nanostructure demonstrated an extremely
high reversible capacity of 1550 mA·h·g-1 and
excellent cycling stability (1335 mA·h·g-1 after 100
cycles) at a current density of 100 mA g-1 [38].
Although the hollow structure seems very promising,
their void space has to be optimized since a too large
hollow space would dramatically decrease the
volumetric capacitance.
4 Hybrid nanostructures based on cobalt
oxide and carbon
Carbonaceous materials have been widely used to
enhance the performance of electrode materials in
electrochemical energy-related applications such as
fuel cells [39], supercapacitors [40,41], Li-air batteries
[42,43] and LIBs [44,45]. In order to achieve high rate
capability and cycling stability, fabrication of
desirable nanocomposites, in which the
nanostructured metal oxides are assembled onto or
embedded into the carbon support, has attracted
great interest. Typically, the carbon
component in the composites was expected to
play dual roles: as a conducting support to
promote the electron transport of the poorly
conductive metal oxides and as an elastic
buffer layer to effectively relieve the strain
induced by the volume change during
charge/discharge process [46]. By combining
nanostructured cobalt oxide and carbon in
different ways, a wide variety of cobalt
oxide/carbon nanocomposites have been
fabricated and evaluated as anode materials
for LIBs.
Coating carbon on metal oxide surface is
one of the simplest and most popular
approaches to improve the battery
performance of metal oxides [47-50]. Despite
their great potentials, the integrity
maintenance of these materials during
continuous cycling is still unsatisfactory due
to very limited elasticity of carbon on intact
metal oxide surface. This brings a critical
problem because crack and fracture of the carbon
coating layer would appear as a result of the
volumetric expansion of the core. Therefore,
conventional core/shell structures are usually
ineffective in keeping the structure integrity for
long-term cycling, resulting in a limited cycle life of
less than 100 cycles and low Coulombic efficiency. To
provide more void space for the inside active
materials, we have designed and synthesized
peapod-like Co3O4/carbon core/shell nanostructures
[51,52]. The fabrication process is shown in Fig. 4a.
First, glucose as a green carbon source was
polymerized on the precursor
(Co(CO3)0.5(OH)0.11H2O nanobelts) surface by a
hydrothermal reaction, and then the calcination at
different conditions was carried out to obtain
peapod-like Co3O4/carbon and/or Co/carbon
nanocomposites due to the carbonization of the
polymerized glucose and decomposition of the
encapsulated precursor. We can observe that the
Co3O4 nanoparticles (20-30 nm) are uniformly
distributed inside the carbon nanotube (Fig. 4b).
Importantly, the void space inside the carbon
nanotube would accommodate the volume
expansion of the cobalt oxide core during the
Figure 4 Schematic illustration of the fabrication of the peapod-like carbon
tube encapsulated Co3O4 nanoparticles (a). The corresponding TEM image
(b), charge-discharge curves (c) and cycling performance (d) of the
composite. Reproduced with permission from Ref. [51]. Copyright American
Chemical Society, 2010.
8
electrochemical cycles. As a result, the peapod-like
Co3O4/carbon nanocomposite demonstrated a very
high specific capacity (~1000 mA·h·g-1 at the rate of 1
C) and excellent cyclability (at least 80% capacity was
retained when cycled back from a high
charge/discharge rate of 10 C), indicating it’s
potential as a promising candidate for LIBs’ anode
(Fig. 4c and d) [51]. To further enhance the rate
capability of the peapod-like Co3O4/carbon
nanocomposite, the peapod arrays were then directly
grown on a conducting Ti substrate [52]. As a result,
a very stable capacity of 1150 mA·h·g-1 was delivered
over 200 cycles at a current density of 100 mA g-1.
Moreover, with the current densities gradually
increased to 0.5, 1, 2, and 10 A g-1 (each for 20 cycles),
the correspondingly delivered capacities were 800,
760, 730 and 450 mA·h·g-1, respectively, affirming the
wonderful rate capability and enhanced performance
[52]. Significantly, the developed strategy is also
capable of fabricating other peapod-like functional
nanomaterials such as carbon tube coated Ni12P5 [53]
and MnO [54] nanocomposites which also exhibited
excellent LIB performance when used as anodes. In
another method, carbon-coated CoO porous
nanowires have been prepared by chemical vapor
deposition (CVD) of carbon on the mesoporous
Co3O4 nanowire surface [55]. The carbon-coated CoO
porous nanowires showed a capacity of ~800
mA·h·g-1 after 70 cycles at a current density of 100
mA g-1, which was more stable compared with bare
mesoporous Co3O4 nanowires [55]. Carbon-coated
porous CoO nanocubes have also been synthesized
by first polymerization of pyrrole monomers on
Co3O4 nanocube surface and then heat treatment of
the polypyrrole-coated nanocubes at 500 °C for 3 h
under N2 atmosphere [56]. After 50 cycles, the
carbon-coated porous CoO nanocubes can still
deliver a discharge capacity of 598.3 mA·h·g-1, which
is much higher than that of the bare Co3O4
nanocubes (203.7 mA·h·g-1).
Carbon nanotubes with high conductivity and
flexibility have also been used as an excellent
conductive support for the growth of cobalt oxide
nanostructure and fabricating cobalt-oxide-based
nanocomposites for LIBs [57,58]. For example, Co3O4
nanoparticles (10-25 nm) with a mass loading over 70
wt% were homogeneously grown on a super-aligned
carbon nanotube array by a pyrolysis method [58].
Benefiting from the flexible and highly conductive
carbon nanotube scaffold, the Co3O4/carbon
nanotube composite electrode exhibited an
exceptional cycling stability (910 mA·h·g-1 after 50
cycles at 0.1 C) as well as excellent rate performance
(820 mA·h·g-1 at 1 C). Similarly, octahedral Co3O4
particles have been anchored on ultra-long carbon
nanotube arrays by a hydrothermal process and
subsequent calcination [59]. The composite exhibited
a reversible capacity of 725 mA·h·g-1 at a current
density of 100 mA g-1 and no capacity degradation
over 100 cycles at a current density of 500 mA g-1 [59].
Mesoporous Co3O4 nanoparticles have also been
attached to carbon nanotubes by chemical deposition
method, which delivered a capacity of 873 mA·h·g-1
after 50 cycles at a current density of 100 mA g-1 [60].
When the current density increased to 250, 350 and
500 mA g-1, it still maintained a capacity of 895, 834
and 757 mA·h·g-1, respectively.
(a)
(b) (c)
Figure 5 Schematic illustration of the fabrication process of
Co3O4/graphene composite. TEM image (b) and cycling
performance (c) of the composite. Inset in (b) is the selective
area electron diffraction (SAED) pattern of Co3O4 nanoparticles
with [110] plane. Reproduced with permission from Ref. [67].
Copyright American Chemical Society, 2010.
In the last decade, graphene has become the most
popular carbon nanomaterial due to its unique
physicochemical properties, such as high electrical
conductivity, large specific surface area, high
mechanical strength, and flexibility [61]. These
fascinating properties have driven researchers to
design and fabricate graphene-based hybrid
nanostructures for energy-related applications
9
[62-66]. When combined with metal oxide for LIBs,
graphene not only facilitates the electron transport
but also effectively buffers the volume change due to
the “flexible” property. Thus, the cycle life of the
hybrid electrode is significantly prolonged. So far,
graphene has been widely studied as a highly
conducting support for nanostructured cobalt oxides
[14,67-75]. For example, Co3O4 nanoparticles (10-30
nm) has been immobilized on graphene sheets by
calcination of directly grown Co(OH)2 on graphene
(Fig. 5a and b) [67]. Since graphene remains
essentially intact during cycling, it should act as a
structural scaffold, supporting the volume and
phase changes during the electrochemical
conversion of Co3O4. The Co3O4/graphene composite
displayed a slowly increased capacity to 935
mA·h·g-1 after 30 cycles at a current density of 50 mA
g-1 with a Coulombic efficiency of ~98%, and an
enhanced rate performance compared with bare
Co3O4 sample (Fig. 5c). These results suggest
important roles of
(a)
(b) (c)
Figure 6 Schematic illustration of the fabrication of the
graphene coated Co3O4 spheres (a). SEM image (b) and cycling
performance (c) of the composite. G-1 and G-2 in (c) indicates
different graphene content. Reproduced with permission from
Ref. [78]. Copyright American Chemical Society, 2014.
structural buffers and increased electronic
conductivity on the performance of Co3O4 anodes. By
using an electrostatic induced spread growth
method, Huang et al., prepared a fully coated
Co(OH)2 sheets on graphene [76]. The
Co(OH)2-coated graphene was then deposited on Cu
foil by a layer-by-layer stacking method. Finally,
annealing was applied to obtain a binder-free and
mechanically robust CoO/graphene electrode. When
used as anode for LIBs, it exhibited a capacity as
high as 640 mA·h·g-1 after 150 cycles at 100 mA g-1,
which was 89.6% of the theoretical capacity of CoO
(714 mA·h·g-1) and good rate capability of 172
mA·h·g-1 at a high current density of 20 A g-1, More
importantly, the electrode can be stably cycled for
5000 times at 1 A g-1 [76]. Octahedral CoO
nanocrystals with a high density have been anchored
onto graphene nanosheets by thermal decomposition
of Co(acac)3 in an oil-phase solution in the present of
CVD-grown hydrophobic graphene [77]. After 60
discharge/charge cycles at 100 mA g-1, the
CoO/graphene nanocomposite with a mass ratio of
9:1 still exhibited a reversible capacities of 1401
mA·h·g-1 [77]. Recently, graphene-coated hollow
Co3O4 spheres have also been designed and prepared
by first modifying the Co3O4 surface with amine end
groups, and then the negatively charged graphene
oxide sheets were assembled on the positively
charged amine-modified Co3O4 surface through the
electrostatic interaction (Fig. 6a and b) [68,78]. The
resulted graphene/Co3O4 nanocomposite electrode
displayed an exceptional cycling stability with a high
reversible capacity (1000 mA·h·g-1 after 130 cycles at
a current density of 74 mA g-1 for the solid Co3O4
electrode [68] and over 600 mA·h·g-1 after 500 cycles
at a high current density of 1.0 A g-1 for the hollow
Co3O4 electrode (Fig. 6c) [78]).
10
It is noted that to date, basically all the fabrication
strategies have strictly followed the procedure that
graphene or graphene oxide was synthesized first
and then the other functional materials (such as
metal oxides) were immobilized onto their surfaces.
Therefore, these strategies inevitably generated a
large variety of graphene-supported simple mixtures
[78]. Since only one side of active materials is
attached to graphene, the cycling stability of the
whole electrode may still be limited due to the
pulverization. Therefore, we developed a completely
different approach to fabricate a sandwich-like
graphene-coated metal oxide nanocomposite [79,80].
The synthesis process is shown in Fig. 7a. First,
glucose molecules as a green carbon source were
polymerized on the thin (NH4)2Co8(CO3)(OH)6·4H2O
nanosheet surface by a hydrothermal reaction and
then the samples were annealed at high
temperatures and inert atmosphere to make the thin
carbon coat crystalize. At the same time, the coated
precursor was decomposed into cobalt nanoparticles
imbedded inside the two graphene layers. Finally,
the sandwich-like graphene/cobalt was annealed at
ambient atmosphere to make the sandwich-like
graphene/Co3O4 nanocomposite (Fig. 7b and c). The
encapsulating graphene sheets can not only enhance
the charge transfer but also prevent the encapsulated
Co3O4 nanoparticles from aggregation and
pulverization during battery cycling process.
Moreover, the sandwich-like structure with void
space is also favorable for electrolyte diffusion so
that the exchange rate of Li+ can be greatly
accelerated. As a result, an outstanding rate
capability and excellent cycling stability of the
composite were obtained. After 50 cycles at 100 mA
g-1, a capacity of 1000 mA·h·g-1 was still maintained
from another 10 cycles at the same current density
(Fig. 7d) [79].
5 Cobalt-based mixed metal oxides and
composites
To further enhance the physicochemical properties of
Co3O4 and lower the cost, effects have been made to
partially replace the Co in Co3O4 by a different
element. For example, MxCo3-xO4 (M=Ni, Mn, Fe, Zn,
etc.) has been synthesized and can reversibly react
with lithium through the conversion reaction and
alloying/dealloying reaction (for Zn) [81-85].
Considering the possibly enhanced properties (such
as enhanced electron conductivity from NiCo2O4 [86])
and even lower cost, these cobalt-based mixed oxides
are very promising for LIBs.
Figure 7 Schematic illustration of the fabrication of the sandwich-like graphene encapsulated Co3O4 nanoparticles (a). The
corresponding TEM images (b: plane view; c: section view) and cycling performance of the composite (d). Reproduced with permission
from Ref. [80]. Copyright The Royal Society of Chemistry, 2013
11
(b) (c)
(a)
(d) (e)
200μm 20μm
50nm
Figure 8 Schematic illustration of the fabrication of ZnCo2O4
nanowire arrays/carbon cloth composite (a). SEM images of the
composite (b, c). TEM image of a single ZnCo2O4 nanowire (d)
and the cycling performance (e). Reproduced with permission
from Ref. [84]. Copyright American Chemical Society, 2012.
For example, uniform porous MnCo2O4 and
CoMn2O4 microspheres have been synthesized by
topotactic chemical transformation (heat treatment)
from solvothermally prepared Mn0.33Co0.67CO3 and
Co0.33Mn0.67CO3 microspheres [87]. When evaluated
as anode materials for LIBs, the porous MnCo2O4
and CoMn2O4 microspheres showed reversible
capacities of 755 and 706 mA·h·g-1, respectively, after
25 cycles at a current density of 200 mA g-1.
Particularly, the MnCo2O4 sample delivered a
capacity as high as 610 mA·h·g-1 even at a high
current density of 400 mA g-1 after 100 cycles,
indicating its potential application in LIBs.
MnCo2O4 submicrospheres with various hollow
structures such as mesoporous spheres, hollow
spheres, yolk-shell spheres, shell-in-shell spheres,
and yolk-in-double-shell spheres have been
synthesized by manipulating the ramping rates
during the heating of MnCo-glycolate
submicrospheres precursor [88]. Interestingly, when
tested as LIB anode, the yolk-shell-like structure
showed the best performance, which might be due
to the high contact area and stable structure
provided by this structure [89]. A well-ordered
mesoporous CuCo2O4 has been synthesized by a
nanocasting strategy using the KIT-6 mesoporous
silica as the template and delivered an initial
discharge capacity of 1564 mA·h·g-1 and a reversible
capacity of 900 mA·h·g-1 after 6 cycles at 60 mA g-1
[90]. Recently, Liu et al., developed a mild and
cost-effective solution route to directly grow
Figure 9 Schematic illustration of the fabrication of the carbon-coated CoSnO3 nanobox (a). The corresponding SEM (b), TEM (c)
images and cycling performance of the nanobox (d). Reproduced with permission from Ref. [92]. Copyright The Royal Society of
Chemistry, 2013.
(a)
(b) (c) (d)
12
(a)
(b) (c)
Figure 10 Schematic illustration of the general fabrication of
multi-shelled sphere (a). SEM (b) and cycling performance (c)
of the triple shelled CoMn2O4 spheres. Scale bar in (b) is 500
nm. Reproduced with permission from Ref. [94]. Copyright
Wiley-VCH Verlag GmbH & Co. KGaA, 2014.
NiCo2O4 nanorod arrays on a Cu substrate [91].
With well-aligned 1D structure and high electrical
conductivity, the binder-free electrode exhibited a
high capacity of ~830 mA·h·g-1 after 30 cycles at 0.5
C and a high rate capability (787 and 127 mA·h·g-1
at 1 C and 110 C, respectively). A hierarchical 3D
ZnCo2O4 nanowire array on carbon cloth has also
been fabricated by using a hydrothermal route (Fig.
8) and when used directly as a flexible anode for
LIBs, it displayed a capacity of ~1200 mA·h·g-1 even
after 160 cycles at a current density of 200
mA g-1 [83,84]. The appealing
electrochemical performance of such a
flexible electrode can be attributed to the
synergetic effects of the ZnCo2O4 nanowire
arrays and conductive carbon cloth substrate.
Besides the isostructured metal oxides with
Co3O4, in another case, Wang et al., prepared
carbon-coated amorphous CoSnO3
nanoboxes by thermal annealing of
CoSn(OH)6 nanoboxes which were
synthesized by fast stoichiometric
co-precipitation of Sn4+, Co2+ in the presence
of OH- and followed by alkaline etching (Fig.
9) [92,93]. Thanks to the unique structure,
the carbon-coated nanoboxes exhibited an
excellent cycling stability over 400 cycles with a
capacity of over 450 mA·h·g-1 at a current density of
200 mA g-1 [92]. Quite recently, a general
“penetration-solidification-annealing” strategy has
been developed to fabricate mixed metal oxide
spheres (such as MnCo2O4, Mn2CoO4, NiCo2O4, etc.)
with multiple shells (Fig. 10a and b) [94]. First,
hollow carbon sphere as templates were dispersed
into an ethylene glycol solution of metal acetate
precursors for the “penetration” of metal ions.
Second, a “solidification” process at a relatively high
temperature was carried out to form metal glycolate
on both the interior and outer surface of the carbon
spheres. Finally, annealing in air was carried out to
form the triple shelled structure. To demonstrate
their potential application in LIB, after coated with
carbon, the CoMn2O4 triple-shelled hollow spheres
exhibited a specific capacity of 726.7 mA·h·g-1 after
200 cycles at a current density of 200 mA g-1 (Fig. 10c)
[94].
Another strategy for enhancing the performance
was to design and integrate two or more metal
oxides in one system. The interaction of different
metal oxides may result in synergetic effect and thus
enhanced performance. For example, Fe2O3/Co3O4
double-shelled hierarchical microcubes were
synthesized by annealing the double-shelled
(a) (b) (c)
(e)(d)
Figure 11 Schematic illustration of the two-step hydrothermal growth of
the Co3O4/α-Fe2O3 branched nanowires (a). The corresponding SEM (b),
TEM (c) images, charge-discharge curves and cycling performance at 100
mA g-1(d) of the composite. Reproduced with permission from Ref. [97].
Copyright Springer, 2013.
13
Fe4[Fe(CN)6]3/Co(OH)2 microcubes [95]. The robust
Fe2O3 hollow microcube at the inner layer not only
displayed a good electronic conductivity but also
acted as a stable support for the Co3O4 outside shell
consisted of nanosized particles. The Fe2O3/Co3O4
nanocomposites delivered a specific capacity of 500
mA·h·g-1 after 50 cycles at a current density of 100
mA g-1, 3 times higher than that of pure Co3O4
nanoparticles. Co3O4 coated SnO2 hollow
nano-spheres have been prepared by chemical
deposition of Co3O4 on SnO2 hollow sphere surface.
Owing to the cobalt-enhanced reversibility of the
Li2O reduction reaction, the SnO2/Co3O4 hollow
nano-sphere electrode exhibited an enhanced
reversible capacity (962 mA·h·g-1 after 100 cycles at
100 mA g-1) [96]. Wu et al., synthesized a
Co3O4/α-Fe2O3 core/shell structured nanowire array
by a two-step hydrothermal method (Fig. 11) [97].
First, single-crystalline Co3O4 nanowire arrays were
grown on Ti substrates for efficient electrical and
ionic transport. Then α-Fe2O3 branches were grown
on the nanowire arrays, resulting in enhanced
surface area and high theoretical Li+ storage capacity.
The Fe2O3 branches also served as volume spacers
between neighboring Co3O4 nanowires to maintain
electrolyte penetration as well as reduce the
aggregation during Li+ intercalation. When examined
in LIBs, the Co3O4/α-Fe2O3 core/shell structured
nanowire anode exhibited an almost constant
capacity from the second cycle and retained a value
of 980 mA·h·g-1 after 60 cycles at 100 mA g-1 [97]. By a
quite similar approach, Co3O4/MnO2 core/shell
nanowire arrays have also been fabricated, which
exhibited a capacity of ~924 mA·h·g-1 after 100 cycles
at a current density of 120 mA g-1 [98]. CoO-coated
NiSix core/shell structured nanowire arrays have
been fabricated by radio-frequency sputtering CoO
on CVD-prepared NiSix nanowire surface [99]. The
core/shell structured nanowire arrays displayed a
gradually increased capacity and a stabilized
capacity of ~900 mA·h·g-1 after 200 cycles at a current
density of 358 mA g-1. Similarly, NiSix/Co3O4 [100]
and CuO/Co3O4 [101] core/shell nanowire arrays
have been fabricated by chemical bath deposition of
Co3O4 nanosheets on NiSix and CuO nanowire arrays,
respectively. The core/shell nanowire arrays can
maintain a reversible capacity of 1279 mA·h·g-1 after
100 cycles at a current density of 400 mA g-1 (for
NiSix/Co3O4 [100]) and a reversible capacity of 1191
mA·h·g-1 after 200 cycles at a current density of 200
mA g-1 (for CuO/Co3O4 [101]).
6 Conclusions and outlook
This article highlights recent developments on
rational design and fabrication of nanostructured
cobalt oxides, cobalt oxide/carbon hybrid and cobalt
oxide-based mixed metal oxides as anodes in LIBs.
The effects of proper nanostructure engineering on
their electrochemical performance are discussed in
detail. Particularly, we focused on cobalt oxides
nanomaterials with various structures (such as 1D
nanowires and nanobelts, 2D nanosheets, 3D hollow
structures), composites with carbon (including
amorphous carbon, carbon nanotube, and graphene)
and mixed metal oxides. The performances of some
representative cobalt-oxide-based anode materials
for LIBs were summarized in Table 1. In general,
these cobalt-oxide-based nanostructures possess
obvious advantages compared with their bulk
counterparts, in terms of specific capacity, rate
capability, and cycling stability. In particular,
hybridizing the nanostructured cobalt oxides with
carbonaceous materials was proved to be very
effective for enhancing the rate capability and
prolonging the lifespan due to the additionally
provided electrical conductivity and structural
stability. To achieve the greatly enhance performance,
the nature of carbonaceous materials and the
configuration of hybrid structures should be
properly chosen and optimized. For example, the
peapod-like carbon nanotube-coated and
sandwich-like graphene-coated hybrids exhibited
excellent electrochemical performance mainly due to
the inherent interaction between cobalt oxide and
carbon by encapsulating the active material inside
and proper amount of void space provided for the
volume change during cycling. Considering that
these carbonaceous materials have a low theoretical
value, their content should be carefully controlled in
order to obtain optimal performance. Although these
designed cobalt oxide nanomaterials exhibited high
14
specific capacitance and good cyclability, the
practical use of cobalt oxides in LIBs anode remains a
great challenge due to its high charge/discharge
potentials. Integrating or mixing with other
functional materials, such as tin oxide, may partially
solve the problems. For example, designing mixed
metal oxides such as ternary metal oxides or
composites of two simple metal oxides by
compositional engineering, would be a promising
approach. Developing mixed cobalt-oxide-based
nanostructures may tune the inherent
physicochemical properties of cobalt oxides, which
may be hardly achieved by mere structure
engineering. In this regard, the proper compositional
engineering and combination of different metal
oxides will be essential. Also, one has to always keep
in mind that for potential practical applications, the
Table 1 Summary of some representative cobalt-oxide-based anode electrodes for lithium-ion batteries.
Methods Typical samples Performance Ref.
Nanostructuring mesoporous Co3O4 nanowires 700 mA·h·g-1 after 20 cycles at 1 C [10]
mesoporous Co3O4 nanowires 810 mA·h·g-1 after 30 cycles at 300 mA g-1 [24]
mesoporous Co3O4 nanobelts 1400 mA·h·g-1 after 20 cycles at 40 mA·g-1 [26]
mesoporous Co3O4 nanobelts 770 mA·h·g-1 after 25 cycles at 177 mA g-1 [28]
mesoporous Co3O4 nanosheets 1450 mA·h·g-1 after 27 cycles at 50 mA g-1 [30]
Co3O4 nanomesh 800 mA·h·g-1 after 50 cycles at 300 mA g-1 [21]
Hollow structure Double-shell Co3O4 hollow sphere 866 mA·h·g-1 after 50 cycles at 178 mA g-1 [32]
Porous Co3O4 nanotubes 1200 mA·h·g-1 after 20 cycles at 50 mA g-1 [33]
Porous Co3O4 hollow rods 903 mA·h·g-1 after 20 cycles at 240 mA g-1 [34]
Needle-like Co3O4 nanotubes 918 mA·h·g-1 after 30 cycles at 50 mA g-1 [35]
Carbon coating Carbon-coated Co3O4 nanowires 989.0 mA·h·g-1 after 50 cycles at 0.5 C [49]
Carbon-coated peapod-like
Co3O4/C array
1150 mA·h·g-1 after 200 cycles at 100 mA g-1 [52]
Hybrid with carbon
nanotubes
Carbon nanotube supported Co3O4
nanoparticles
910 mA·h·g-1 after 50 cycles at 0.1 C [58]
Carbon nanobutes/Co3O4 hybrid 873 mA·h·g-1 after 50 cycles at 100 mA g-1 [60]
Hybrid with graphene Graphene supported Co3O4
nanoparticles
935 mA·h·g-1 after 30 cycles at 50 mA g-1 [67]
Graphene/CoO hybrid 640 mA·h·g-1 after 150 cycles at 100 mA g-1 [76]
Sandwich-like graphene
encapsulated Co3O4 nanparticles
1000 mA·h·g-1 after 50 cycles at 100 mA g-1 [79]
Mixed metal oxides porous MnCo2O4 microspheres 755 mA·h·g-1 after 25 cycles at 200 mA g-1 [87]
NiCo2O4 nanorod array 830 mA·h·g-1 after 30 cycles at 0.5 C [91]
ZnCo2O4 nanowire array 1200 mA·h·g-1 after 160 cycles at 200 mAg-1 [84]
Co3O4/α-Fe2O3 branched nanowire 980 mA·h·g-1 after 60 cycles at 100 mA g-1 [97]
15
developed synthesis strategy must be simple, low
cost and suitable for large-scale production. On the
other hand, since the energy density of a full cell LIB
depends highly on the development of high
voltage/capacity cathodes, the design and fabrication
of novel cathode nanomaterials is essential. For
example, the fabrication of peapod-like carbon
nanotube-coated and/or sandwich-like
graphene-coated cathode materials such as LiFePO4
is on process in our lab.
With the rapid development of high-performance
nanomaterial-based electrodes, one can confidently
expect that the performance of rechargeable LIBs will
be further enhanced in the near future. We expect
this review would provide some general guidelines
for the design and fabrication of advanced
nanostructured electrodes for next generation LIBs
although it is quite challenging.
Acknowledgements
This work was financially supported by the
Thousand Young Talents Program of the Chinese
Central Government (Grant No.0220002102003),
National Natural Science Foundation of China
(NSFC, Grant No. 21373280, 21403019), Beijing
National Laboratory for Molecular Sciences (BNLMS)
and Hundred Talents Program at Chongqing
University (Grant No. 0903005203205).
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