Synthesis and characterization of graphene-nickel oxide nanostructures for fast charge-discharge application

Article abstract of DOI:10.1016/j.electacta.2011.03.143

Graphene-metal oxide composites as anode materials for Li-ion batteries have been investigated extensively, but these attempts are mostly limited to moderate rate charge-discharge applications. Here, graphene-nickel oxide nanostructures have been synthesised using a controlled hydrothermal method, which enabled in situ formation of NiO with a coralloid nanostructure on graphene. Graphene/NiO (20%), graphene/NiO (50%) and pure NiO show stable discharge capacities of 185 mAh/g at 20 C (1 C = 300 mA/g), 450 mAh/g at 1 C, and 400 mAh/g at 1 C, respectively. High rate capability and good stability in prolonged charge-discharge cycling permit the application of the material in fast charging batteries for upcoming electric vehicles. To the best of our knowledge such fast rate performance of graphene/metal oxide composite as anode and such stability for pure NiO as anode have not been reported previously.

Full text of DOI:10.1016/j.electacta.2011.03.143

Electrochimica Acta 56 (2011) 5815–5822
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and characterization of graphene–nickel oxide nanostructures for fast
charge–discharge application
Iresha R.M. Kottegodaa,b,∗, Nurul Hayati Idrisa,c, Lin Lua, Jia-Zhao Wanga,∗∗, Hua-Kun Liua
a Institute for Superconducting and Electronic Materials, ARC Centre of Excellence for Electromaterials Science, University of Wollongong, NSW 2519, Australia
b Materials Technology Section of Industrial Technology Institute, Sri Lanka
c Department of Physical Sciences, Faculty of Science, University Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Graphene–metal oxide composites as anode materials for Li-ion batteries have been investigated exten-
sively, but these attempts are mostly limited to moderate rate charge–discharge applications. Here,
graphene–nickel oxide nanostructures have been synthesised using a controlled hydrothermal method,
which enabled in situ formation of NiO with a coralloid nanostructure on graphene. Graphene/NiO (20%),
graphene/NiO (50%) and pure NiO show stable discharge capacities of 185 mAh/g at 20 C (1 C = 300 mA/g),
450 mAh/g at 1 C, and 400 mAh/g at 1 C, respectively. High rate capability and good stability in prolonged
charge–discharge cycling permit the application of the material in fast charging batteries for upcom-
ing electric vehicles. To the best of our knowledge such fast rate performance of graphene/metal oxide
composite as anode and such stability for pure NiO as anode have not been reported previously.
© 2011 Elsevier Ltd. All rights reserved.
Received 24 January 2011
Received in revised form 23 March 2011
Accepted 23 March 2011
Available online 28 April 2011
Keywords:
NiO nanoparticles
Graphene
Nickel oxide–graphene composites
Lithium batteries
1. Introduction
and attracted wide attention in materials science and condensed
matter physics [5–9]. The most interesting feature of graphene
Lithium-ion batteries for hybrid and plug-in hybrid electric
vehicles are currently under intensive investigation. The storage
and release of electrical energy depend mainly on how fast the
electrons and ions can move through the electrodes. In practice,
it is hard to meet all the battery performance requirements, such
as high specific energy, high rate capability, long cycle life, low
cost, perfect safety, and minimal environmental impact, at the
same time. The Li-ion battery is attractive in terms of specific
energy and power, and also has the potential to be “the battery of
choice” for electric vehicles [1–3]. Graphite and LiCoO2 are com-
monly used as anode and cathode, respectively, in commercial
Li-ion battery for portable consumer electronics [4]. The intrinsic
properties of the electrodes limit the lithium insertion rate, the
charge–discharge current, and the energy density. To circumvent
the rate issues, several alternative approaches have been pursued,
in which either the existing electrodes were replaced by better
materials or modified to nanostructured form. For the anode, quite
recently, graphene (a single atomic layer of graphite) has emerged
nanostructures is the possibility that they can be synthesized from
natural graphite, which is perceived as “green”. Graphene has
shown faster charge–discharge rates than graphite as an anode in
Li-ion battery, fulfilling major performance requirements [10–14].
Being a single atomic sheet, graphene can accommodate Li-ions
on both sides of the sheet, providing high lithium storage capacity
above 600 mAh/g at the 1 C rate, which is higher than the theo-
retical capacity of graphite (372 mAh/g). Recently, graphene has
been utilized to improve the electrochemical properties of metal
alloys [15–17] and metal oxides [18–26], the theoretical capacities
of which are much higher (700–4000 mAh/g) than that of graphene.
Considerable fast charge–discharge capability has been shown by
graphene/TiO2 [18] and graphene/Co3O4 [20] composites.
Reports on graphene composites with Si [16], TiO2 [18], Co3O4
[20], Fe3O4 [22], SnO2 [23–25], Cu2O [26], and CuO [27] claim
that a uniform distribution of metal oxide on graphene sheets
can eliminate restacking of the sheets during the synthesis and
stabilizes the volume changes in the metal/metal oxide during
charge–discharge cycling. Studies based on alternatively assem-
bled graphene and nanocrystalline metal oxides such as SnO2 or
NiO reveal similar explanations for their improved electrochemical
performances [24]. However, the preparation method in this report
was not an in situ method, and the performance at high current den-
sities such as 300–6000 mA/g was not investigated. Quite recently,
graphene/NiO composite with 45% NiO has been synthesised using
an in situ method [21]. However, this particular study was limited
∗
Corresponding author at: Institute for Superconducting and Electronic Materi-
als, ARC Centre of Excellence for Electromaterials Science, University of Wollongong,
NSW 2519, Australia. Tel.: +94 7 1443 3343; fax: +94 1 1237 9816.
∗∗
Corresponding author. Tel.: +61 2 4298 1478; fax: +61 2 4221 5731.
E-mail addresses: iresha@iti.lk (I.R.M. Kottegoda), jiazhao@uow.edu.au
(J.-Z. Wang).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.03.143

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to the preparation and characterization of the composite. We have
focused on synthesizing graphene/NiO composite with lower NiO
content (20 wt%), enabling a battery based on graphene/NiO anode
to be charged within a few minutes. NiO synthesised by the same
method also gave good stability in prolonged charge–discharge
cycling at moderate current densities. To the best of our knowl-
edge, the effectiveness of using lower percentage of metal oxide for
higher charge/discharge current application has not been reported.
2. Experimental
2.1. Preparation of materials
Graphite oxide was synthesised by the Hummers method [28].
1 g of natural graphite was added to 20 cm3 of C. H2SO4 at 0 ◦C. 3 g
of KMnO4 was added gradually to the mixture while stirring. The
mixture was then stirred at room temperature for 30 min. 50 cm3 of
water was added slowly and heated at 90 ◦C for 15 min. 150 cm3 of
distilled water was added, followed by 10 cm3 of 30% H2O2 solution.
The solids were separated by centrifuging and washed repeatedly
with 5% HCl until sulphate could not be detected with BaCl2. The
product was washed 3–4 times with acetone and dried in an oven
at 65 ◦C for 12 h. Graphite oxide synthesized as above was used to
synthesize graphene and graphene/NiO composites. Hydrothermal
method was used to reduce graphite oxide to graphene as described
below [26,29,30]. Graphite oxide is dispersed in ethylene glycol
by ultrasonic agitation for 30 min before autoclave the mixture.
To synthesize the composite, an appropriate ratio of nickel acetate
tetrahydrate and graphite oxide was dispersed in ethylene glycol.
The mixtures were then transferred to stainless steel autoclaves
filled with Ar and kept at 160 ◦C for 16 h. The resulting products
(graphene and graphene composites) were washed with acetone
followed by water and then dried at 65 ◦C in air. The samples
were finally thermally annealed at 500 ◦C in Ar. For comparison,
nickel acetate tetrahydrate was treated under similar conditions
(hydrothermal reaction in ethylene glycol at 160 ◦C and pyrollysis
at 500 ◦C) to synthesize NiO. Graphite oxide converts to graphene
and nickel acetate turns to an intermediate product at the end of
the hydrothermal reaction. Therefore, heat treatment is necessary
to convert the intermediate product to NiO.
Fig. 1. XRD spectra of graphite oxide, graphene, and graphene/NiO composites with
different quantities of NiO.
electrode and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate
(EC/DMC, 1:1 by volume) provided by MERCK as the electrolyte.
The cells were galvanostatically charge–discharge cycled between
2.5 V and 0.01 V vs Li/Li+ on a LAND battery tester at 25 ◦C. Differ-
ent current rates ranging from 300 mA/g to 6000 mA/g were used
to cycle the cells and for simplicity 300 mA/g rate was used as 1 C
rate for all cells irrespective to their real theoretical values.
3. Results and discussion
Fig. 1 shows X-ray diffraction (XRD) patterns of graphite oxide,
graphene, NiO, and the graphene/NiO composites. Fairly high inter-
planar spacing (d-spacing) of about 0.85 nm was observed in
graphite oxide in contrast to the typical value of about 0.70 nm,
probably due to the high ratio of graphite oxide to H2O [31,32].
Complete conversion of graphite oxide into graphene was observed
from the disappearance of the characteristic peak of graphite oxide
at 2ꢀ ≈ 11◦. Both graphene and the composites show the typical
2ꢀ peak for graphene at about 26◦, corresponding to d-spacing of
about 0.36 nm. Although the d-spacing is higher than that of natu-
ral graphite (∼0.33 nm) the values are similar for pristine graphene
and the composites, suggesting that the NiO nanocrystallites have
not greatly affected the orientation of the graphene layers. This
evidence was confirmed by Raman spectroscopy, which can be
used to investigate the nature of disorder in graphene as shown
in Fig. 2. Raman spectroscopy is a powerful non-destructive tech-
nique to study carbonaceous materials such as graphene and also
for examining the ordered and disordered crystal structures and
distinguishing the single and multilayer characteristics of graphene
layers [13,33,34]. In perfect graphite, the G band is highly intense
(in-plane bond stretching motion of C sp2 pairs), whereas the D
band is generally active in the presence of high disorder in the
graphene. The D/G ratio of the band intensities for the composite
has not increased compared to the pristine graphene, suggesting
that no significant increase in disorder has occurred due to the pres-
ence of NiO crystallites in graphene. Furthermore, G-band observed
at about 1582 cm−1 during the present study. It had been shown
that the peak frequency of the G band of the single layer graphene
at 1585 cm−1 shifts about 6 cm−1 into lower frequencies after stack-
ing several graphene layers. For 2–6 layers of graphene the G band
have even shifted to 1579 cm−1. Therefore the G-band at 1582 cm−1
is attributable to mostly single graphene [13,33,34]. The above con-
cept was supported by FE-SEM as shown later. Crystal sizes of the
2.2. Material characterization
The morphology of the samples was investigated using a JEOL
JSM 6460A scanning electron microscope with an energy disper-
sive X-ray spectroscopy detector with X-ray mapping capability.
X-ray diffraction analysis was carried out using a GBC MMA
generator and X-ray diffractometer with Cu K␣ radiation and a
graphite monochromator. Raman spectroscopy was carried out
to monitor the nature of the disorder in the graphene using a
JOBIN YVON HR800 Confocal Raman system with 632.8 nm diode
laser excitation on a 300 lines/mm grating at room tempera-
ture. Thermogravimetric analysis (TGA) was performed in air via
a Mettler–Toledo TGA/DSC 1 Stare System to determine the actual
weight ratio of graphene and NiO. Fourier transform infrared (FTIR)
spectra were collected using a SHIMADZU-84005.
2.3. Electrochemical measurements
The electrodes were prepared on Cu foil using slurry of 80 wt%
active material and 10 wt% (each) carbon black and polyvinylidene
fluoride dissolved in 1-methyl-2-pyrrolidinone (NMP). The coated
electrodes were dried in a vacuum oven at 100 ◦C for 24 h, pressed,
and cut into 1 × 1 cm2 sizes. The resulting electrodes were assem-
bled into CR 2032 coin-type cells (half cells) in an Ar-filled glove
box (Mbraun, Unilab, Germany), using lithium metal as the counter

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necessary to convert the intermediate product of the composite
to graphene/NiO. Solvothermal reduction of colloidal dispersion
of graphite oxide by various solvents was investigated in liter-
ature and has been used to synthesize graphene and graphene
composites from graphite [26,29,30]. Hydrothermal reaction of
graphite oxide dispersed in ethylene glycol at lower temperature
(120–200 ◦C) has lead to reduction of graphite oxide to gaphene.
Ethylene glycol performs as the solvent and the reducing agent
for the reduction of graphite oxide. Ultrasonic agitating of graphite
oxide in ethylene glycol resulted in highly dispersed brown–yellow
colour graphite oxide in ethylene glycol. The presence of oxygen
containing functional groups in graphite oxide facilitates anchor-
ing of various polar molecules such as ethylene glycol and water
[26,30]. The reduction of graphite oxide anchored to ethylene gly-
col takes place at low temperature with the development of high
pressure in the autoclave. The complete reduction has observed
even at 120 ◦C with ethylene glycol [29] although the present study
used 160 ◦C for complete reduction. Thermal annealing at 500 ◦C
was employed after the hydrothermal reaction to convert the inter-
mediate nickel hydroxide to NiO and also to remove any residues
such as chemically and physically bonded ethylene glycol from all
materials. It is interesting to understand the intermediate products
resulting from the hydrothermal reaction before production of the
graphene/NiO composite and the pure NiO. Fig. 4 shows FE-SEM
micrographs of the intermediate products. Similar morphology has
been observed for the microwave-assisted hydrothermal synthesis
of nickel hydroxide [35]. The morphology of the intermediate is
similar to those of NiO, suggesting that the nucleation and growth
of the nanostructure have begun before conversion of the inter-
mediate product to NiO. FE-SEM micrographs further reveal that
NiO intermediate in the composite has a similar morphology to
the intermediate of pure NiO, implying that the synthesis of both
pure NiO and the composite involves the same intermediate. How-
ever, after the heat treatment of the composite, the morphology
of NiO is completely different from the intermediate. We suggest
that, the pyrollysis of the composite at 500 ◦C in an Argon flow lead
to cleavage of agglomeration of NiO and attachment to new cites
or functional groups on the surface of graphene which presum-
ably created from the heat treatment. Therefore, dispersion of NiO
throughout the graphene sheets is visible after the heat treatment
as revealed in Fig. 3c and d instead of the growth and agglomeration
observed in the intermediate stage (Fig. 4b). Detailed investiga-
tion is underway for further clarification of the morphology and
the mechanism of formation of the composite which would be
published separately. The typical XRD peaks corresponding to the
(0 0 3), (0 0 6), (1 0 1), and (1 1 0) planes (Fig. 5) confirmed the for-
mation of ␣-Ni(OH)2·xH2O from nickel acetate precursor [35–38].
Therefore, the intermediate product of both graphene/NiO and
pristine NiO is Ni(OH)2·xH2O. The interlayer spacing, which was
derived from the (0 0 3) plane, is about 0.89 nm, suggesting that the
acetate and ethylene glycol have intercalated into the layered nickel
hydroxide. Generally, ␣-nickel hydroxide stacks with intercalated
ions such as the above to restore its charge neutrality [35–38]. Such
stacking obviously stabilizes nickel hydroxide in ethylene glycol.
We propose that the intermediate of the composite is nickel
hydroxide bonded to the graphene surface. From Fig. 6, Fourier
transform infrared spectroscopy (FTIR) showed vibrational bands
Fig. 2. Raman spectra of graphene and of the composite with 20% NiO.
NiO were also compared from the XRD peaks in Fig. 1. The pris-
tine NiO and the composites show (1 1 1), (2 0 0), and (2 2 0) crystal
planes of strongly crystalline NiO with space group R−3m. The crys-
tal sizes of NiO, which were calculated using the Scherrer equation
applied to the (2 0 0) plane, followed the order 32, 43, and 120 nm
for samples containing 20%, 50%, and 100% NiO, respectively.
Graphene, when imaged via field emission scanning electron
microscopy (FE-SEM), shows disassembly of graphite stacks into
nanosheets with a flower petal-like appearance (Fig. 3a andb). Gen-
erally, attempts made to synthesize nanometer sized crystallites
fail as their lateral size increases. The thermal vibrations rapidly
build up, which diverts the nanoparticles to the macroscopic scale
and changes the two-dimensional (2D) crystallites into a variety of
stable 3D structures [8]. Accordingly, graphene layers are naturally
crumpled and flexible, in contrast to graphite. The graphene layers
are substantially thin and disordered according to the Fig. 3a which
is consistent with the results of Raman. The composites consisting
of 20% and 50% NiO show NiO nanocrystallites a few nanome-
ters in size attached firmly to certain sites on graphene sheets
(Fig. 3c and d). This phenomenon was confirmed by energy dis-
persive spectroscopy, where NiO particles at high concentrations
were observed at certain sites (not shown here). The growth pat-
tern of NiO nano-structures as observed in Fig. 3e, and f is not visible
in the composite and agglomerations of NiO nanoparticles are not
much prominent either. Presumably the growth of nano-structures
could exist in limited manner which is not visible through scan-
ning electron microscope. We believe that interconnections of the
graphene layers are not entirely interspaced by NiO nanocrystal-
lites at 20% NiO in graphene. If all such contacts incorporated NiO,
the electrical resistance would be too extreme to deliver such stable
capacity as we observed for the composite, as discussed later, and
a high charge-transfer rate would not be attainable either. Obvi-
ously electrical contacts can be lost due to the insulating nature
of NiO in high doses. A completely different growth pattern of
NiO was observed in the pristine NiO, the particle size of which
is much bigger (100–300 nm) than in the composites (Fig. 3e and
f). Coralloid-type nanostructures of NiO are formed. Higher magni-
fication revealed that a flower petal-like morphology on the order
of 1 nm thick grew radially from the core. Formation of stable 3D
structures is common to all the nanoparticles. Such striking mor-
phology may presumably lead to high Li-ion storage capacity, as
shown later.
at about 3644, 3405, 1623, 1358, 1280, 990, 645, and 484 cm−1
,
which is typical for ␣-type hydroxides [35,36]. Although the broad-
ness of the peaks for the composite is not the same as for the nickel
hydroxide, the characteristic peaks are common to both samples.
Vibration bands for free hydroxyl groups at 3644 cm−1 and for
H bonded to water at 3405 cm−1 seem to be well separated and
small in the composite compared to the broad band of the pure
NiO intermediate sample, although there is no apparent shift of
the band position. This result suggests that the free OH vibration
The preparation process for the graphene, the NiO, and the com-
posite involved two steps. The hydrothermal method was used to
reduce graphite oxide (present in the composite and in the pris-
tine graphite oxide) to graphene, and thermal annealing was then

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Fig. 3. FE-SEM images of (a) and (b) graphene, (c) 20% NiO composite, (d) 50% NiO composite, and (e) and (f) pure NiO.
is low in the composite. Fig. 7 shows a schematic representation
of the formation of NiO and NiO/graphene composite. The inter-
mediate product for both the composite and the pristine NiO is
nickel hydroxide, the thermal annealing of which converts it to
NiO:(1)Ni(OH)2·xH2O → NiO·xH2O + H2O
sites in graphene. Graphene has a high surface to volume ratio, and
various defects appear after exfoliation of graphite. Therefore, we
suppose that the higher quantity of NiO (50%) can firmly attach to
the reactive sites of graphene.
In the present investigation, the graphene and NiO precursor
ratio was chosen so that free nickel hydroxide was not formed
among the graphene sheets, which would affect adversely the
conductivity of the graphene. As explained previously, high reac-
tive sites such as edge surface and defect sites of graphene attach
metal oxide crystallites as much as possible. Free NiO is obviously
formed after attachment of all such reactive sites on graphene sur-
face. At higher concentrations of NiO precursor (above 60%), free
nickel hydroxide is observed, even by the naked eye, due to the
unique green colour of the intermediate. The composite with 80%
In the composite, graphene edge surfaces and defect sites can act
as nucleation sites for nickel hydroxide. Obviously, nickel hydrox-
ide selectively attaches to the edge surfaces and defect sites, as
shown in the schematic diagram. This is because the edge surfaces
and any other defects present on the surface of the graphene are
more reactive towards various chemical reactions [39]. A recent
study on atomic layer deposition of Al2O3 reveals that Al2O3 is pref-
erentially grown at the edge surface of graphene [40]. The authors
suggest that the method can be used to detect defects and edge

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5819
Fig. 4. FE-SEM images of the intermediates for (a) NiO and (b) NiO/graphene composite.
(not shown here).
Fig.
8
shows the charge–discharge behaviour of the
graphene/NiO composites, graphene, and NiO when assem-
bled as an anode in a coin type Li-ion cell. The charge–discharge
current density used provides capacity of 300 mAh/g in 1 h, which
is denoted as the 1 C rate for each half cell. All cells show high
irreversible capacity loss during the first charge–discharge cycle
and improvement of stability during the subsequent cycles. The
initial capacity loss is attributed to the surface film formation
[41]. NiO generally shows a voltage plateau at about 0.6 V dur-
ing the first discharge, corresponding to the reaction of Li-ions
and NiO according to 2Li + NiO → Li2O + Ni and formation of the
surface films [42]. The plateau shifts to about 1.2 V in subse-
quent charge–discharge cycles and becomes smaller in prolonged
charge–discharge cycling. The peak at 2.25 V during the charge
cycle corresponds to the reverse reaction of formation of NiO from
Ni and Li2O. However, such voltage hysteresis, as is observed in
NiO based cells, is unsuitable for practical application. Graphene
does not show such hysteresis, which starts appearing only with
increasing content of NiO in the composite. The composite with
20% NiO shows similar charge–discharge behaviour to graphene,
which is useful in practical application. It is one of the advantages
of using lower percentage of metal oxide in a composite. The
charge–discharge behaviour is not much deviate from the pristine
graphene. Generally, composites at high percentage of metal
oxide show voltage plateaus [18,20] which are unfavourable in
practice. Cyclic voltammograms (CV) of the cells also show high
irreversibility during the first cycle (Fig. 9), corresponding to the
Fig. 5. XRD spectra of intermediate products for (a) NiO and (b) NiO/graphene com-
posite.
NiO shows metallic nickel in the composite, the performance of
which is also interesting and different from the present systems, as
will be discussed separately. The actual weight percent of NiO in
the composite was investigated using thermo-gravimetric analysis
Fig. 7. Schematic illustration of the formation of nickel hydroxide and NiO, includ-
ing the formation of Ni(OH)2/graphene composite and the phase transformation to
NiO/graphene.
Fig. 6. FTIR spectra of intermediates for NiO and the composite with 50% NiO.

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Fig. 8. Voltage profiles showing the 1st, 2nd, 40th, and 100th charge–discharge cycle behaviour of (a) graphene, (b) 20% NiO composite, (c) 50% NiO composite, and (d) 100%
NiO half cells at 1 C rate.
formation of the surface film. Graphene edge surface is highly
exposed compared to that of graphite due to the exfoliation of the
graphite stacks; consequently, the surface area is also higher than
graphite. As a result, higher initial capacity loss is observed which
is common to all graphene based anodes [15–26]. NiO also showed
high initial capacity loss presumably due to high surface area
resulting from the growth of nanostructures. Nevertheless high
initial is not suitable for practical application. Extensive work has
been done to reduce the initial capacity loss as well as the capacity
fade on charge–discharge cycling of graphite and other materials
[41]. Similar work is necessary to solve the problems associated
with graphene and composites which needs to address separately.
The present work aimed at investigation long term cycle stability
and high rate capability which is also equally important in practical
application.
Fig. 10 shows the Li-ion storage capacity of the half cells consist-
ing of graphene, NiO, and composites as anode in charge–discharge
cycling at 1 C rate. Continuous irreversibility in varying degree
is evident during the initial charge–discharge cycles. Such irre-
versibility occurs until a stable surface film is formed, which
prevents further capacity loss. In the present study, almost all
constituents were used without further purification. Using clean-
dried constituents or using alternative ones obviously decreases the
irreversibility resulting from those constituents. However, the per-
formance of the materials in the present system can be compared,
as any fault that occurs during the preparation of the electrodes
is common to all cells. The capacity observed for the graphene is
less than for the graphene synthesised by exfoliation of graphite
oxide at 1050 ◦C in Ar [13]. Nevertheless, the present graphene
can be used for comparison with the composite. The compos-
ite with 50% NiO showed capacity as high as 450 mAh/g for 100
cycles. All the cells show good stability in cycling at the 1 C rate
after the cell was stabilized, implying that all of them are suit-
able for charge–discharge at moderate current densities. In other
words, the battery can be charged to its full capacity in 1 h. The
performance of even pristine NiO is good at the 1 C rate, imply-
ing that reversible insertion of lithium is possible at the 1 C rate.
The charge–discharge performance of pristine NiO was further
tested upto 300 charge–discharge cycles and the cell showed good
stability at the 1 C rate (not shown here). NiO generally suffers
many morphological changes during continuous charge–discharge
cycling. Therefore much effort has been made to improve the per-
formance by modification of the NiO [42,44]. However, the pure
NiO obtained in the present study showed better performance
than reported modified NiO [42–45]. The unique nanostructure
observed in the present NiO obviously gave good reversible capac-
ity at moderate current densities.
Fig. 11 shows the charge–discharge performance of the half cells
at different current densities varying from 0.5 C to 20 C. Although
the cell with pristine NiO showed good performance at the 1 C rate,
it shows poor performance at higher rates. The cell with 50% NiO
shows good capacity only at the 1 C and 2 C rates, it shows low
capacity at higher current densities, such as the 10 C and 20 C rates.
It is clear that, NiO contributed to the additional capacity which
has high theoretical capacity. However, it stands solely at lower
current rates. The cells containing 20% NiO show the best perfor-
mance, with Li-storage capacity of 185 mAh/g at the 20 C rate. As
we have taken the current density of 300 mA/g as the 1 C rate,
the 20 C rate corresponds to 6000 mA/g. Such high rate perfor-
mance has been recently shown for LiFePO4 as a cathode material

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5821
Fig. 9. Cyclic voltammograms of (a) graphene, (b) 20% NiO composite, (c) 50% NiO composite, and (d) 100% NiO.
[46]. The present cell can store energy to considerable capacity
within 3 min, suggesting that the present anode is suitable for fast
charge–discharge requirements. Further increase of the rate per-
formance needs increase of the conductor used in the electrode as
indicated in the above Ref. [46]. The fact can be explained as fol-
lows. The rate capability of the electrodes generally increases by
addition of high percentage of the conductor. Especially for high
rate application, higher percentage of conductor is used. The above
publication reveals using of even 30% of the conductor in the elec-
trode at very higher current rates such as 20 C, 30 C, etc. The main
reason is high rate charge transfer which is a stringent requirement
for high rate application. It is clear that a high content of NiO (50%)
leads to attachment of NiO throughout the graphene surface, wher-
ever there are reactive sites such as edges and defects. Presumably,
such morphology leads to higher electrical impedance and low
capacity when charge–discharge takes place at high current den-
sities. Detailed investigation of impedance spectra is underway.
However, the fact that NiO 20% composites deliver higher rate
capability can be explained as follows. Graphene is an atomic or
Fig. 10. Charge–discharge cycling performance of graphene, 20% NiO composite,
Fig. 11. The rate capability of graphene, 20% NiO composite, 50% NiO composite,
50% NiO composite, and 100% NiO half cells at 1 C rate.
and 100% NiO half cells (1 C = 300 mA/g).

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nano layers. Therefore it shows good charge transfer rates at higher
charge–discharge rates than graphite as observed from recent stud-
ies on graphene. Although graphene shows good rate capability, it
does not show high capacity as shown by graphene/NiO 20% com-
posite even at low rates. As a result, graphene shows lower capacity
at higher rates, yet the rate capability of both half cells is simi-
lar. Although, the presence of more NiO leads to high capacity, it
obviously limits the charge transfer rate at fast charge–discharge
currents as NiO is not conductive. Consequently, the rate capability
of both pure NiO and 50% NiO is not satisfactory. The lower percent-
age of NiO in the composite is therefore more suitable for higher
rates. Furthermore, scanning electron micrographs reveal the exis-
tence of NiO nano-crystallites mainly at the edge surface of the
graphene in 20% NiO composite, presumably due to high reactivity
at the edge surface of graphene. Whereas, the 50% NiO composite
shows a distribution of NiO throughout the surface due to higher
amount of NiO which may be adversely affect for the inter particle
conduction of the electrode. Nevertheless, the addition of higher
amount of conductor is necessary for fast rate application of such
materials. The present study used 10% conductor for fabrication of
each electrode. Further investigation is underway to elucidate the
mechanism through impedance spectra analysis of each material at
different C-rates and different charge–discharge cycle. Therefore
higher percentage of NiO is not suitable for application at higher
current densities. The better performance of the 20% NiO compos-
ite at higher charge/discharge rates may be a result of the good
mechanical stability and the electrical conductance at lower level
of NiO. Similar results were obtained for natural graphite surfaces
modified with zirconia (ZrO2) [47,48]. High rate capabilities have
been observed from the presence of zirconia at lower levels. X-
ray absorption near edge surface studies showed that zirconia was
present at the edge surface.
Attard for his invaluable support for field emission scanning elec-
tron microscopy and Raman spectroscopy. The authors would also
like to thank Dr. Tania Silver for critical reading and correction of
this manuscript.
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Acknowledgements
The first author is grateful for fellowship support from the Aus-
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support was also provided by the Australian Research Council (ARC)
through a Discovery Project (DP 0987805). Nurul Hayati Idris is
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