Reduced graphene oxide and nanosheet-based nickel oxide microsphere composite as an anode material for lithium ion battery

Article abstract of DOI:10.1016/j.electacta.2011.12.040

Reduced graphene oxide and nickel oxide composite was prepared by homogeneous coprecipitation and subsequent annealing. Characterizations show that NiO particles have a nanosheet-based microsphere structure and anchor uniformly on the surface of reduced graphene oxide platelets. As an anode for Li ion batteries, the composite has 1641 mAh g^-1 and 1097 mAh g ^-1 specific capacities for the first discharge and charge, respectively, higher than the theoretical capacity of NiO. It shows an excellent cycling performance with a discharge capacity of 1041 mAh g^-1 after 50 cycles at a current of 100 mA g^-1, and has a good rate capacity with 727 mAh g^-1 at a current of 1600 mA g^-1. The results show a synergistic effect of nanosheet-based NiO microspheres and reduced graphene oxide platelets in the composite for improved electrochemical performance.

Full text of DOI:10.1016/j.electacta.2011.12.040

Electrochimica Acta 64 (2012) 23–28
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Reduced graphene oxide and nanosheet-based nickel oxide microsphere
composite as an anode material for lithium ion battery
Xian-Jun Zhu∗, Jing Hu, Huai-Li Dai, Lei Ding, Li Jiang
College of Chemistry, Central China Normal University, 152 Luoyu Rd, Wuhan, Hubei 430079, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 October 2011
Received in revised form 8 December 2011
Accepted 10 December 2011
Available online 20 December 2011
Reduced graphene oxide and nickel oxide composite was prepared by homogeneous coprecipitation and
subsequent annealing. Characterizations show that NiO particles have a nanosheet-based microsphere
structure and anchor uniformly on the surface of reduced graphene oxide platelets. As an anode for Li ion
batteries, the composite has 1641 mAh g−1 and 1097 mAh g−1 specific capacities for the first discharge and
charge, respectively, higher than the theoretical capacity of NiO. It shows an excellent cycling performance
with a discharge capacity of 1041 mAh g−1 after 50 cycles at a current of 100 mA g−1, and has a good
rate capacity with 727 mAh g−1 at a current of 1600 mA g−1. The results show a synergistic effect of
nanosheet-based NiO microspheres and reduced graphene oxide platelets in the composite for improved
electrochemical performance.
Keywords:
Reduced graphene oxide
Nickel oxide
Lithium ion batteries
Anode
© 2011 Elsevier Ltd. All rights reserved.
Homogeneous coprecipitation
1. Introduction
fast charge–discharge application [26]. However, a method to
prepare the hybrid materials in which metal oxide particles are
Transition metal oxides have been widely studied as anode
materials for Li ion batteries since the pioneering studies of Taras-
con’s group [1,2]. These oxides are potential substitutes for graphite
as anode materials for Li ion batteries due to their high spe-
cific capacities [3–7]. Among them, with a theoretical capacity of
718 mAh g−1, NiO is one of the most promising anode materials for
Li ion batteries. Nevertheless, its practical application is constrained
by its poor cycling performance. This is because NiO has a large
volume change with aggregation of active particles during charge
and discharge, leading to pulverization and electrical disconnection
from the current collector [8,9].
One approach to avoid this limitation is to hybridize NiO with
carbonaceous materials for better accommodation of the strain
during volume change [10–12]. Graphene can be used as a con-
ducting additive for hybrid nanostructured materials because of
its excellent electronic conductivity, high theoretical surface area
of 2630 m2 g−1, and good mechanical properties [13–17]. Recently,
high surface area chemically modified graphene has been used to
form hybrid materials with metal oxides and Ni(OH)2 nanoparticles
with the aim of improving the energy storage performance [18–25].
Kottegada et al. reported graphene–nickel oxide nanostructure
composite prepared by a controlled hydrothermal method for
uniformly distributed in the ‘graphene’ matrix still remains a
challenge.
We report a simple method for obtaining a composite con-
sisting of nanosheet-based NiO microspheres decorating reduced
graphene oxide (RG-O) platelets, which we hereafter refer to as
the “RG-O/NiO”. The process involves homogeneous coprecipita-
tion of NiCl2 in a suspension of graphene oxide platelets using urea
and subsequent reduction of the graphene oxide with hydrazine by
refluxing to yield RG-O platelets decorated with Ni(OH)2 nanopar-
ticles, and finally annealing at 400 ◦C for 3 h under a nitrogen
atmosphere to obtain NiO particles. As an anode for Li-ion bat-
teries, the RG-O/NiO composite exhibits 1641 and 1097 mAh g−1
of initial discharge and charge capacities, respectively, normalized
to the mass of NiO in the composite (∼1313 and 878 mAh g−1
,
respectively, based on the total mass of the composite). Our
synthesis method presents a promising route for large-scale pro-
duction of metal oxide/RG-O platelet composites for lithium ion
batteries.
2. Experimental
2.1. Synthesis of graphite oxide
Graphite oxide was synthesized from natural graphite powders
(500 mesh, Sinopharm Chemical Reagent Co. Ltd.) by a modi-
fied Hummers method [27]. Typically, graphite powders (2 g) and
∗
Corresponding author. Tel.: +86 27 67868388.
E-mail address: xianjunzhu@yahoo.com.cn (X.-J. Zhu).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.12.040

24
X.-J. Zhu et al. / Electrochimica Acta 64 (2012) 23–28
NaNO3 (1 g; >99 wt%) were mixed, then put into concentrated
H2SO4 (96 ml; 98 wt%) in an ice bath. Under vigorous stirring,
KMnO4 (6 g; 99.5 wt%) was gradually added and the temperature
of the mixture was kept below 20 ◦C. After removing the ice bath,
the mixture was stirred at 35 ◦C in a water bath for 4 h. As the reac-
tion progressed, the mixture became pasty with a brownish color.
150 ml H2O was then slowly added to the pasty mixture. Because
large amounts of heat were generated by addition of water into the
concentrated H2SO4 medium, water should be added slowly and at
the same time keeping the mixture in an ice bath to maintain the
temperature below 50 ◦C. After dilution with 240 ml H2O, 5 ml of
30 wt% H2O2 (Sinopharm Chemical Reagent Co. Ltd.) was added to
the mixture, and the color of this diluted solution became a bril-
liant yellow. After continuously stirring for 2 h, the mixture was
filtered and washed with 10 wt% HCl (250 ml), then DI water and
then anhydrous ethanol to remove other ions. Finally, the resulting
solid was dried under vacuum.
was Celgard 2300. The cells were discharged and charged galvano-
statically in a potential window from 0.005 to 3.0 V using a Land
battery tester (China) at room temperature.
3. Results and discussion
As shown in Scheme 1, graphite oxide prepared by a modified
Hummers method [27,28] was sonicated in water to form a suspen-
sion of graphene oxide platelets. For the synthesis of the RG-O/NiO
composite, NiCl2 was hydrolyzed in the graphene oxide suspen-
sion in the presence of urea at 90 ◦C for 1.5 h in an oil bath. The
molar ratio of NiCl2 to urea was 1:30. This step yielded a uniform
Ni(OH)2 coating on the surface of the graphene oxide platelets. Dur-
ing hydrolysis, urea releases hydroxyl ions slowly and uniformly in
the suspension, resulting in the formation of Ni(OH)2 as suggested
by the following reactions:
CO(NH2)2 + 3H2O → 2NH4+ + CO2 + 2OH−
Ni2+ + 2OH− → Ni(OH)2
(1)
(2)
2.2. Preparation of RG-O/NiO composite
The RG-O/NiO composite was prepared by homogeneous copre-
cipitation and then subsequent reduction with hydrazine by
refluxing. In a typical experiment, 5 mmol NiCl2 (0.65 g; >98 wt%)
was dissolved in 50 ml water, 150 mmol urea (9.0 g; 99 wt%) was
dissolved in another 50 ml water, then urea and NiCl2 solutions
were slowly and sequentially added to 50 ml of 2 mg ml−1 graphite
oxide suspension under stirring. After exposure to ultrasound
from an ultrasonic bath for 30 min, the mixture was heated at
90 ◦C for 1.5 h. When cooled to room temperature, 0.5 ml N2H4
(85 wt%, Sinopharm Chemical Reagent Co. Ltd.) was added to the
mixture while it was stirred. Then the mixture was refluxed at
100 ◦C for 24 h in an oil bath, during which the mixture color
changed from black-brown to black. Then, the black mixture was
collected by filtration. After washing with DI water in an attempt to
remove any excess hydrazine as well as other ions, the as-prepared
product was annealed at 400 ◦C for 3 h under an atmosphere of
nitrogen in order to obtain a RG-O/NiO composite. For compar-
ison, NiO without RG-O was also synthesized using the same
procedure.
The Ni(OH)2 particles produced likely anchor onto the surface of
the graphene oxide platelets through oxygen-containing functional
groups, such as hydroxyl, epoxyl, and carboxyl, but further work is
needed to elucidate the detailed chemical bonding, if any, at the
surface.
After the suspension was cooled to room temperature, a trace
amount of hydrazine was added to the suspension under con-
tinuous stirring and the suspension was refluxed at 100 ◦C for
24 h in an oil bath, during when graphene oxide gets converted
to RG-O. After filtration, the as-obtained sample was annealed
at 400 ◦C for 3 h under a nitrogen atmosphere; Ni(OH)2 decom-
poses to yield NiO particles following Ni(OH)2 → NiO + H2O. Fig. 1
shows the XRD of the as-prepared composite, it has the charac-
teristic peaks of NiO (JCPDS: 65-2901) and RG-O, indicating that
the as-prepared product is composed of NiO and RG-O. The Raman
spectrum in Fig. 2 also shows characteristic peaks of NiO as well
as the D and G characteristic peaks of RG-O. The composite con-
tains 20 wt% RG-O as measured by thermal gravimetric analysis
(see Fig. S1).
The morphology of the RG-O/NiO composite was studied by SEM
as shown in Fig. 3. Fig. 3a and b shows that RG-O/NiO composite
consists of thin, crumpled RG-O platelets closely connecting with
each other to form a 3D network structure. NiO particles were dis-
tributed on the curved RG-O platelets. It can be seen that these NiO
particles are nanosheet-based sphere-like structures from Fig. 3b.
The morphology of NiO particles can also be seen from TEM images
of RG-O/NiO shown in Fig. 3c. For an isolated NiO nanosheet-
based sphere-like particle (a ‘microsphere’), the edge portion of
2.3. Characterization
The structure of the as-prepared RG-O/NiO composite was
characterized by X-ray diffraction (XRD, Cu K␣ radiation;
ꢀ = 0.15414 nm) at the scan rate of 2◦ min−1 in the 2Â range of
5 and 70◦. Scanning electron microscopy (SEM) was performed
using JSM-6700F (field emission gun; specimen chamber pres-
sure of about 10−5 Pa; accelerating voltage 5 kV; working distance
8 mm). Transmission electron microscopy (TEM, JEM-2010FEF;
200 keV) was used to study the morphology and microstruc-
ture of the composites. Raman spectrum measurements were
carried out using INVIA (RENISHAW, England) system with a
514.5 nm wavelength incident laser light. Thermal gravimetric
analysis (TGA) was measured with a SDT600 apparatus using a
heating rate of 5 ◦C min−1 under 20 ml min−1 of flow air from 25 to
850 ◦C.
2.4. Electrochemical evaluation
Electrochemical experiments were performed using 2032 coin-
type cells. The working electrode consisted of 95 wt% as-prepared
active material and 5 wt% polytetrafluoroethylene binder. The mass
of active material in the electrode is 5.6 mg. The electrolyte was a
solution of 1 M LiPF6 in EC/DEC (1:1 by volume) (purchased from
Zhangjiagang Guotai-Huarong New Chemical Materials Co. Ltd.).
Pure Li foil was used as the counter electrode and the separator
Fig. 1. XRD patterns of (a) RG-O/NiO, (b) NiO (JCPDS: 65-2901), and (c) RG-O.

X.-J. Zhu et al. / Electrochimica Acta 64 (2012) 23–28
25
Scheme 1. Scheme for making RG-O/NiO composite.
ingly, the first charge capacity of 1097 mAh g−1 is equivalent to
3.05 Li per NiO. The second discharge and charge capacities are
1147 and 1089 mAh g−1, respectively. From the second cycle on,
the discharge and charge curves are similar.
the sphere-like structure is thinner than that of the center, and is
comprised of sheet-like ‘2D’ structures. From the HRTEM image of
Fig. 3d, a single nanosheet in such microspheres shows a single-
crystalline structure. The lattice spacing of 0.21 nm corresponds to
the d spacing between adjacent (2 0 0) crystallographic planes of
NiO crystal [29].
To measure the performance of the RG-O/NiO composite as an
anode for Li ion batteries, the composite was mixed with polyte-
trafluoroethylene (PTFE) in a weight ratio of 95:5 for preparing the
working electrode, which is equivalent to a NiO:RG-O:PTFE ratio of
76:19:5. Carbon black or other carbonaceous materials increases
the conductivity of the electrode when preparing the electrode
for electrochemical measurement. However, these additives also
lower the weight specific capacity of the electrode. Carbon black or
other additives were not added to the electrode in this work as com-
pared with the graphene/metal oxides in the literatures previously
reported [19,20,30].
Fig. 4a shows the initial two discharge and charge curves
of the RG-O/NiO composite at a current density of 100 mA g−1
between 3.0 and 0.005 V vs. Li+/Li. The RG-O/NiO composite deliv-
ers 1641 mAh g−1 and 1097 mAh g−1 specific capacities for the first
discharge and charge, respectively, based on the mass of NiO in
the composite (based on the total mass of the RG-O/NiO compos-
ite, the values are 1313 and 878 mAh g−1, respectively). During
the first discharge, the potential decreases steeply from 3.05 to
1.25 V. A plateau region appears at 0.60 V up to a capacity of
1200 mAh g−1 which is equivalent to 3.34 Li per NiO. Another slope
is observed below 0.60 V, yielding a total first discharge capacity
of 1641 mAh g−1, corresponding to 4.57 Li per NiO. Correspond-
For the complete reduction of 2Li+ + NiO + 4e− ꢀ Li2O + Ni, one
would expect a maximum uptake of 2 Li/NiO (718 mAh g−1). The
excess capacity appears to originate from electrolyte decomposi-
tion in the low-potential region, and thus perhaps the subsequent
formation of an organic layer (or polymer gel-like film) [31,32]
on the surface of the particles as well as possible Li inser-
tion/extraction (or simple decoration on open surfaces) of the
RG-O platelets. In the present study, it is suggested that Li inser-
tion/extraction/decoration in RG-O may play a major role in the
overall electrochemical process and could be the primary reason
for the excess capacity of the RG-O/NiO composite electrode. By
comparison of the first cycles among RG-O, NiO and RG-O/NiO com-
posite as shown in Fig. 5, the first charge capacity of 1097 mAh g−1
for RG-O/NiO is higher than that of 851 mAh g−1 for NiO mixed
physically with carbon black, or that of 532 mAh g−1 for RG-O
itself. The results indicate that the RG-O/NiO composite has more
lithium insertion/extraction/decoration sites than NiO or RG-O.
This is perhaps because the NiO microspheres composed of NiO
nanosheets that are anchored on the surface of the RG-O platelets
can act as spacers between the RG-O platelets during discharging
and charging, which would lead to higher discharge and charge
capacities.
Fig. 4b shows the cycle performance of the RG-O/NiO compos-
ite at a current density of 100 mA g−1 between 3.0 and 0.005 V.
The discharge capacity of RG-O/NiO drops from 1641 mAh g−1
to 1147 mAh g−1 in the second cycle. However, the discharge
capacity is gradually restored from the second to 26th cycle.
After the 26th cycle, the specific capacity slowly decreases
again. The discharge capacities in the first, 2nd, 10th, 20th,
26th, 40th and 50th cycles are 1641, 1147, 1190, 1228, 1238,
1168, and 1041 mAh g−1, respectively. The phenomenon of an
initial drop in specific capacity as
a Li ion battery anode
followed by a gradual increase is well documented in the lit-
erature [31–33], and is attributed to some irreversible Li2O
generated in the discharge step. Nevertheless, in the present
study, Li2O comes not only from NiO by 2Li + NiO ꢀ Li2O + Ni
and electrolyte degradation, but also partly from RG-O due
to the electrochemical reduction of oxygen-containing func-
tional groups on the surface of RG-O during the discharge step.
We suggest that the electroreduction reactions could be as
follows:
C
C
C
C
O
+
Li
Li2
O +
(3)
Fig. 2. Raman spectra of (a) RG-O/NiO, (b) NiO, and (c) RG-O.

26
X.-J. Zhu et al. / Electrochimica Acta 64 (2012) 23–28
Fig. 3. SEM images of RG-O/NiO (a and b), and TEM images of RG-O/NiO (c and d).
for NiO 50 wt% or ∼350 mAh g−1 for NiO 20 wt%, after 20 cycles
Li + HO
C
Li O+ C
2
+ H
2
(4)
(5)
at 300 mA g−1) [26].
Li + HOOC
Li O +
+ H + CO
2
2
C
C
The high capacity and good cycling stability of the RG-O/NiO
composite as an anode material is attributed to the intimate con-
tact between the nanosheet-based NiO microspheres and the RG-O
platelets. The uniform mixture and interaction between the sphere-
like NiO particles and RG-O platelets can evidently accommodate
the volume change of Ni and NiO active particles during discharg-
ing and charging, and can prevent the aggregation of Ni and NiO
nanoparticles and the restacking of RG-O sheets, which likely also
enhances cycle stability.
2
On the other hand, the RG-O/NiO composite has a moderate BET
surface area (91 m2 g−1). Therefore, the Li2O likely results in the for-
mation and decomposition of a polymeric gel-like film on the large
surface of the NiO particles and RG-O platelets with a high specific
surface area [31–33]. Furthermore, this polymeric film can also pro-
hibit the growth of Ni or NiO active particles during discharge and
charge, leading to gradually increasing discharge and charge capac-
ities due to the good conductivity of RG-O platelets. However, after
long cycling, lithium dendrites were formed and polymeric gel-like
film was partly destroyed, resulting in a slowly decreasing specific
capacity. From Fig. 4a and b, it can also be seen that the Coulom-
bic efficiency of the first cycle for RG-O/NiO composite is 66.8%.
After 4 cycles, the Coulombic efficiency is greater than 97%, indi-
cating that the composite has good capacity retention (see Fig. S2).
The RG-O/NiO composite shows good rate performance as well
(Fig. 4c). The specific capacity is as high as 727 mAh g−1 even at a
current density of 1600 mA g−1. A rough comparison indicates that
the reversible specific capacity (∼1100 mAh g−1 at the current den-
sity of 100–400 mA g−1 during charging–discharging) is higher than
In a comparison experiment, we synthesized NiO by the same
method without any RG-O. Although the NiO particles had a mor-
phology and crystallinity similar to those of the nanosheet-based
NiO microspheres in the RG-O/NiO composite (see Fig. S3), the
electrochemical performance of NiO mixed physically with car-
bon black in the same mass weight ratio as the RG-O/NiO is much
worse than that of RG-O/NiO composite. At a current of 100 mA g−1
,
NiO has 1287 mAh g−1 and 851 mAh g−1 specific capacities for the
first discharge and charge, respectively, but after 50 cycles, capac-
ity decreases rapidly to 91 and 90 mAh g−1, respectively (Fig. 4d).
As comparison, pure RG-O had 1707 mAh g−1 of the first discharge
and 532 mAh g−1 of the first charge capacity, and further decreased
to 227 and 224 mAh g−1 after 50 cycles, respectively. The compar-
ison of the cycling performance among RG-O, NiO and RG-O/NiO
shows that the total specific capacity of RG-O/NiO is higher than
those previously reported, such as NiO-SWNTs (∼523 mAh g−1
,
after 15 cycles at 0.5 C) [10], NiO/MWCNT (∼800 mAh g−1, after
50 cycles at 50 mA g−1) [11], and graphene–NiO (∼500 mAh g−1

X.-J. Zhu et al. / Electrochimica Acta 64 (2012) 23–28
27
Fig. 4. Electrochemical performance of the RG-O/NiO composite. The specific capacities are based on the mass of NiO in the composite. (a) Discharge/charge profiles of
RG-O/NiO for the first and second cycles at the current density of 100 mA g−1. (b) Cycling performance of RG-O/NiO composite at a current density of 100 mA g−1. (c) Rate
capacity of RG-O/NiO composite between 0.005 and 3.0 V with increasing current density. (d) Comparison of cycling performance among RG-O/NiO, RG-O, and NiO physically
mixed with carbon black in the same weight ratio as RG-O/NiO composite at a current density of 100 mA g−1
.
of reduced graphene oxide platelets. The composite exhibits
enhanced electrode performance as an anode for lithium ion
battery. Our simple synthesis method can be readily adapted
to prepare other composites in which reduced graphene oxide
not only acts as a conducting additive to support metal oxide
nanoparticles, but also provides additional electrochemical active
sites to further enhance their electrochemical performance.
Acknowledgements
This work was supported by the Natural Science Foundation
of Hubei Province (No. 2011CDB161) and the Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State Edu-
cation Ministry (SRF for ROCS, SEM).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2011.12.040.
Fig. 5. The comparison of the first cycles among various composites: (a) RG-O/NiO,
(b) NiO, and (c) RG-O.
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