1. Ultrafast pyro-synthesis of NiFe2O4 nanoparticles
within a full carbon network as a high-rate and
cycle-stable anode material for lithium ion
batteries
Preetham P, Subhalaxmi Mohapatra, Shantikumar V. Nair,
Dhamodaran Santhanagopalan and Alok Kumar Rai*
NiFe2O4 nanoparticles fully anchored within a carbon network were prepared via a facile pyro-synthesis
method without using any conventional carbon sources. The surface morphology was investigated using
field-emission scanning electron microscopy, which confirmed the full anchoring of NiFe2O4
nanoparticles within a carbon network. The primary particle size of NiFe2O4 is in the range of 50–100
nm. The influence of the carbon network on the electrochemical performance of the NiFe2O4/C
nanocomposite was investigated. The electrochemical results showed that the NiFe2O4/C anode
delivered a reversible capacity of 381.8 mA h gÀ1
after 100 cycles at a constant current rate of 1.0C, and
when the current rate is increased to a high current rate of 5.0C, a reversible capacity of 263.7 mA h gÀ1
is retained. The obtained charge capacity at high current rates is better than the reported values for
NiFe2O4 nanoparticles. The enhanced electrochemical performance can be mainly ascribed to the high
electrical conductivity of the electrode, the short diffusion path for Li+
ion transportation in the active
material and synergistic effects between the NiFe2O4 nanoparticles and carbon network, which buffers
the volume changes and prevents aggregation of NiFe2O4 nanoparticles during cycling.
1. Introduction
The nanostructured transition metal oxides which incorporate
Li+
ions through a conversion reaction and exhibit large
reversible capacities compared to insertion-type electrodes such
as graphite or Li4Ti5O12, are considered to be efficient anode
materials for high power lithium ion batteries.1,2
Among several
transition metal oxides, iron based spinel oxides, such as Fe3O4,
as anode materials are signicantly researched due to their high
theoretical capacity, rich resources, and environmental benig-
nity.3
Spinel type metal oxides have received much attention due
to their stable structure and high theoretical capacities. More
importantly, partial doping of transition metals into binary iron
oxides and modication to ternary ferrites (AFe2O4) also showed
a promising and feasible approach to obtain an outstanding
electrochemical performance of iron based oxides.4–7
It is
believed that these ternary mixed metal oxides could offer better
electrochemical performance due to the enhancement in
stability of the active phases, the existence of multiple valences
of the cations, good electrical properties and the synergetic
effect of all these characteristics.8
More importantly, it is also
expected that the volume expansion/contraction during charge/
discharge in both phases will happen in sequence and reduce
the strain of the material. Among various ternary ferrites,
NiFe2O4 is quite attractive because both nickel and iron are
abundant elements on Earth, are relatively nontoxic and inex-
pensive and have high thermal stabilities. NiFe2O4 can store 8
moles of Li through a conversion reaction, which corresponds
to a theoretical capacity of 915 mA h gÀ1
(NiFe2O4 + 8Li+
+ 8eÀ
4 Ni + 2Fe + 4Li2O).9
However, it has been reported that
NiFe2O4 exhibits drastic capacity fading and poor cycling
stability due to the limited diffusion rate of Li+
ions in the
material, poor electrical conductivity and the large volume
changes during the Li+
ion insertion/deinsertion processes,
leading to pulverization of the electrode and break-down of the
electrical contact pathways between adjacent particles.7,9,10
Hence, in order to avoid the above problems, a new strategy
should be developed for NiFe2O4 to achieve both a high
reversible capacity and long cycling life. It is well-known that
nanostructured electrodes offer a high surface area; a large
contact area between electrode and electrolyte, which enhances
the diffusion kinetics by reducing the diffusion pathway for
electronic and ionic transport and ensures a relatively greater
number of guest-ions during insertion/deinsertion. Thus, a few
attempts such as different synthesis methods to control the
Amrita Centre for Nanosciences and Molecular Medicine, Amrita Vishwa
Vidyapeetham, Amrita University, Kochi 682041, Kerala, India. E-mail:
alokkumarrai1@gmail.com; alokkumar21223@aims.amrita.edu; Fax: +91-4842
802020; Tel: +91-4842 858750
Cite this: RSC Adv., 2016, 6, 38064
Received 9th February 2016
Accepted 4th April 2016
DOI: 10.1039/c6ra03670h
www.rsc.org/advances
38064 | RSC Adv., 2016, 6, 38064–38070 This journal is © The Royal Society of Chemistry 2016
RSC Advances
PAPER

2. morphology and particle sizes have been reported.9–12
In addi-
tion, the partial substitution of host-structures with different
metal cations to enhance the long-term Li cycling of NiFe2O4
has also been achieved.13
Recently, it is also believed that the
fabrication of nanocomposites with graphene nanosheets or
carbon materials could be an effective approach to buffer the
volume changes due to graphene’s unique properties such as
high exibility, excellent electrical conductivities, large surface
area and high mechanical strength.14–17
However, the reaction
between oxygen containing functional groups (such as OH,
COOH etc.) on the graphene nanosheets and Li+
ions during the
insertion/deinsertion processes was believed to be the primary
reason for the loss of irreversible capacity.14
On the other hand,
a carbon coating is also preferential to improve the interparticle
connectivity by developing a conducting network throughout
the electrode and suppress excessive particle growth, which can
result in improvement of the cycle life.8
However, it is difficult to
achieve a uniform dispersion of nanoparticles on a carbon
matrix. Hence, a facile synthesis of NiFe2O4 nanoparticles
incorporated within a full carbon network in a one step process
would be an excellent strategy to improve the electrochemical
performance of the NiFe2O4 electrode.
Therefore, in the present work, for the rst time we have
adopted a polyol-assisted pyro-synthesis method to synthesize
NiFe2O4 nanoparticles within a full carbon network without
using any conventional carbon sources. The polyol solvent acts
as a primary fuel and instantly provides ultrahigh exothermic
energy to the surroundings during combustion, which leads to
thermochemical decomposition of the precursors (endo-
thermic) and nucleation under an oxygen-limited atmosphere
by the useful consumption of exothermic energy.18
The feasi-
bility of forming a carbon network under very short reaction
times can suppress the particles’ growth and improve the
interparticle connectivity. This method has not been investi-
gated with NiFe2O4 nanoparticles to see whether such nano-
particles could be efficiently distributed within a full carbon
network, and what would be the electrochemical performance
of such a unique nanostructured electrode. The objective is to
investigate whether such an electrode would give rise to both
high capacity and high stability. The obtained architecture of
NiFe2O4 nanoparticles within a full carbon network exhibits
a high reversible capacity (768.0 mA h gÀ1
at 1.0C), a better cycle
life (381.8 mA h gÀ1
at 1.0C aer 100 cycles) and an improved
rate capability (263.7 mA h gÀ1
at 5.0C).
2. Experimental
2.1. Materials synthesis
NiFe2O4 nanoparticles within a full carbon network were
synthesized by a polyol-assisted pyro-synthesis method and the
preparation procedure can be seen in Scheme 1. In a typical
synthesis, stoichiometric amounts of nickel(II) acetate tetrahy-
drate [Ni(OCOCH3)2$4H2O, 98% Sigma Aldrich] and iron(II)
acetate [Fe(OCOCH3)2, 95% Sigma Aldrich] were dissolved in 80
ml of diethylene glycol [C4H10O3, 99% Spectrochem] by stirring
for 24 h at room temperature. The use of diethylene glycol here
acts not only as a solvent and reducing agent but also a carbon
source. Subsequently, 50 ml of liquid thinner (Sheenlac, India,
benzene free D13X NC thinner) was added to the homogenous
solution which was again stirred for 30 min. Then, the resultant
solution was transferred onto a hot-plate preheated to 250
C.
The ammable solution was ignited with an electric torch to
induce a self-extinguishable combustion process. A black
colored powder was directly obtained aer the combustion
process. Finally, to crystallize the as-prepared powder, it was
annealed at 500
C for 5 h under an argon atmosphere.
2.2. Materials characterization
The crystal structure of the annealed sample was studied using
X-ray diffraction on a Shimadzu X-ray diffractometer with Cu Ka
radiation (l ¼ 1.5406 ˚A). The particle size and surface
morphology of the annealed sample was further studied using
eld-emission scanning electron microscopy (FE-SEM; S-4800
Hitachi). Field-emission transmission electron microscope
(FE-TEM) images were also recorded using a FEI 20 FEG elec-
tron microscopy instrument at 200 kV. The samples were rst
soaked in ethanol and dispersed by ultrasonic vibration before
drop casting onto copper grids for FE-TEM examination. The
carbon content in the annealed sample was determined by CHN
elemental analysis on an Elementar Vario EL III. In order to
investigate the nature of the carbon, Raman spectroscopy
(Witec Alpha 300 RA) was used with a diode laser as the exci-
tation source.
2.3. Electrochemical testing
The electrochemistry of the electrode materials was studied
using CR2032 coin type cells. To measure the electrochemical
performance, the electrodes were constructed by mixing the
annealed NiFe2O4 sample, carbon black and a polyvinylidene
uoride binder in a weight ratio of 70 : 20 : 10, respectively in N-
methyl-2-pyrrolidone to form a homogenous slurry. Then it was
cast on a Cu foil current collector through doctor blading and
dried at 80
C for 12 h in a vacuum oven. Subsequently the slurry
was pressed between stainless steel twin rollers to improve the
adhesion between the Cu foil and active materials. Lithium foil
was used as the counter electrode. 1 M LiPF6 dissolved in an
mixture of ethylene carbonate and dimethyl carbonate (molar
ratio 1 : 1, in volume) was used as the non-aqueous electrolyte.
A glass ber was also used as a separator. Cyclic voltammetry
was carried out using an Autolab potentiostat (PGSTAT 302 N)
with a scan rate of 0.1 mV sÀ1
within the range of 0–3.0 V. The
discharge/charge measurements were performed at room
temperature using an Arbin battery tester (BT 2000) in the
voltage range of 0.005 V to 3.0 V.
3. Results and discussion
3.1. Structural and morphological analysis
Fig. 1 shows the X-ray diffraction pattern of the annealed
NiFe2O4 sample. The XRD pattern displays the majority of the
peaks of cubic spinel NiFe2O4 (JCPDS card no. 010742081) with
a small fraction of crystalline Fe2O3 (JCPDS card no. 330664)
and Ni (JCPDS card no. 011260). It is highly probable that the as-
This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 38064–38070 | 38065
Paper RSC Advances

3. prepared NiFe2O4 sample was slightly reduced to Fe2O3 and Ni
metallic phases during annealing under a reduced atmosphere.
As per the stoichiometry, 1 mol nickel needs 2 mol iron to
produce NiFe2O4. It is believed that a small fraction of NiO is
reduced to Ni during annealing and consequently Fe2O3 did not
have access to a full amount of NiO to form the stoichiometric
NiFe2O4, which leads to the segregation of Fe2O3 as an extra
phase within the sample.19
Fig. 2(a) represents a typical FE-SEM image of the annealed
NiFe2O4/C nanocomposite. From the image, it is obvious that
the NiFe2O4 nanoparticles are highly agglomerated. Further-
more, these NiFe2O4 agglomerates appear to be embedded in
a carbon matrix, which can be explained by the interconnection
of the NiFe2O4 nanoparticles by carbon. The primary particle
size is ranging from 50–100 nm. In order to clearly know the
decoration of carbon in the sample and the distribution of
nanoparticles within the carbon network, FE-TEM analysis was
further investigated. Fig. 2(b)–(d) shows the FE-TEM images of
the annealed NiFe2O4/C sample obtained at different magni-
cations. The FE-TEM images clearly show two distinct regions in
the sample. The color contrast between the light gray edge and
the dark centre is easily observed. The region with the dark
centre is the NiFe2O4 nanoparticles, whereas the light gray
region is the carbon network in the sample.18
The low magni-
cation FE-TEM images in Fig. 2(b) and (c) show that the
NiFe2O4 nanoparticles are almost uniformly decorated within
the carbon network without severe aggregation. More
importantly, the inset images of Fig. 2(b) and (c) clearly reveal
that no freestanding NiFe2O4 nanoparticles are found in this
sample. Furthermore, the high magnication FE-TEM image in
Fig. 2(d) markedly exhibits the carbon network between the
nanoparticles and the small sized NiFe2O4 nanoparticles are
closely anchored within the carbon network. The small sizes
and uniformity of these nanoparticles are maintained even aer
the sonication process during the preparation of the TEM
specimen, which apparently shows the strong interaction
between the nanoparticles and the carbon sheet. The under-
lying conductive carbon network would benet fast electron
transfer and maintain structural integrity during the volume
change.
CHN analysis is performed to determine the accurate
percentage of carbon in the annealed sample. CHN analysis of
the annealed NiFe2O4/C nanocomposite indicates that the
amount of carbon is 4.7%, which would account for a carbon
network in the sample, as observed in FE-TEM images of Fig. 2.
Raman spectroscopy is further used to determine the degree
of graphitization of the carbon in the annealed sample of
NiFe2O4/C. Fig. 3 shows a Raman spectrum of the annealed
NiFe2O4/C nanocomposite between the wavenumbers of 500
and 2500 cmÀ1
. As shown in Fig. 3, two bands at wavenumbers
of 1358.7 cmÀ1
and 1596.2 cmÀ1
are observed, which can be
assigned to the D-band and G-band of carbon, respectively.
Generally, the D-band is associated with disordered sp3
carbon
and defects such as topological defects, dangling bonds, and
vacancies, whereas the G-band corresponds to ordered sp2
hybridized carbon.5
The ratio between the D and G band (ID/IG)
is used to evaluate the disordered and ordered crystal structures
of carbon. Here, the ID/IG ratio for the NiFe2O4/C sample is 0.85,
which clearly indicates the good graphitization behaviour of the
carbon in the sample.20,21
3.2. Electrochemical performance
Fig. 4(a) shows the cyclic voltammograms of the NiFe2O4/C
electrode at a scan rate of 0.1 mV sÀ1
for the rst six cycles in the
voltage range of 0–3.0 V (vs. Li/Li+
). The rst cycle is substan-
tially different from the second one. In the rst cathodic scan,
a weak cathodic peak at 1.48 V is observed, which could be
attributed to the insertion of Li+
ions into the NiFe2O4 elec-
trode,10
whereas the broad intense peak at 0.57 V corresponds to
the initial reduction of Ni2+
and Fe3+
to metallic Ni0
and Fe0
nanoparticles along with the formation of a solid electrolyte
interface (SEI) layer and amorphous Li2O.10,11,17
However, in the
Scheme 1 Schematic illustration of the preparation of NiFe2O4 nanoparticles within a carbon network.
Fig. 1 X-ray powder diffraction pattern of annealed NiFe2O4
nanoparticles.
38066 | RSC Adv., 2016, 6, 38064–38070 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper

4. subsequent cycles, the cathodic peak shis to a higher potential
and clearly separates into two peaks at 0.98 V and 0.84 V,
indicative of different electrochemical reactions governing the
two processes. The large decrease in the peak intensity and
integrated area between the rst cycle and the following cycles
is consistent with a low initial coulombic efficiency, indicating
a capacity loss due to the electrolyte decomposition and SEI lm
formation. Meanwhile, a broad anodic peak around $1.64 V
could be attributed to the oxidation of metallic Ni0
and Fe0
into
Ni2+
and Fe3+
, and this peak slightly shis positively to 1.68 V in
the subsequent cycles.10,11,17
Remarkably, all the characteristic
peaks and their integrated areas and shapes remained the same
aer the rst cycle, suggesting reversible reduction and oxida-
tion of the NiFe2O4/C electrode.
Fig. 4(b) shows the galvanostatic charge/discharge proles
for the 1st
, 2nd
and 5th
cycles at a constant current rate of 1.0C
[1C ¼ 915 mA gÀ1
] in the voltage range of 0.005 to 3.0 V. The 1st
discharge curve consists of two slope regions and a plateau.
The rst slope between 3.0 V–0.7 V can be attributed to Li+
ion
insertion into the electrode. Furthermore, the long at voltage
plateau between 0.7 V–0.45 V, followed by a gradual slope to the
cut-off voltage of 0.005 V can be ascribed to the reduction of
NiFe2O4 and the formation of Li2O as well as the formation of
SEI layers. More precisely, it can also be observed that the large
discharge plateau disappears in the later cycles, indicating that
irreversible reactions such as electrolyte decomposition or SEI
layer formation occurred. The 1st discharge and charge
capacities were found to be 1301.7 mA h gÀ1
and 768.0 mA h
gÀ1
, respectively with an initial coulombic efficiency of 59%.
The large irreversible capacity loss in the rst cycle can be
attributed to the polymer gel like SEI layer formation, irre-
versible decomposition of the electrolyte and some of the
undecomposed Li2O phase.10
In the subsequent 2nd
and 5th
cycles, the discharge and charge capacities are reduced to 808.9
mA h gÀ1
and 734.7 mA h gÀ1
, and 713.9 mA h gÀ1
and 670.1
mA h gÀ1
, respectively with a high coulombic efficiency of 91%
and 94%. The coulombic efficiency rapidly increases and then
remains almost the same in the subsequent cycles. It is
believed that the carbon network favours better interparticle-
connectivity and hence improves the electronic conductivity
of the electrode.
Fig. 2 (a) FE-SEM and (b–d) FE-TEM images of NiFe2O4/C nanocomposite.
Fig. 3 Raman spectrum of the NiFe2O4/C nanocomposite.
This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 38064–38070 | 38067
Paper RSC Advances

5. Fig. 4(c) shows the cycling behaviour of the NiFe2O4/C elec-
trode up to 100 cycles at a constant current rate of 1.0C. From
the prole, it can be seen that the NiFe2O4/C electrode shows
a stable and high reversible capacity except for the initial few
cycles. The reversible charge capacity of the NiFe2O4/C electrode
is 381.8 mA h gÀ1
aer 100 cycles, which is still higher than the
theoretical capacity of the graphite anode (372 mA h gÀ1
). It is
highly probable that the good cycling capacity of the electrode is
due to the small size of the nanoparticles, which shorten the Li+
ion transport distance and increase the kinetics of the conver-
sion reactions. In addition, the existence of a carbon network
imparts excellent electronic conductivity to the electrode
material and buffers the volume changes during cycling to
decrease pulverization and deterioration of the NiFe2O4 elec-
trode during cycling. In order to clearly evaluate the perfor-
mance of the pyro-synthesized NiFe2O4/C electrode, a detailed
comparison between the current study and previous NiFe2O4
studies based on different synthesis methods, various micro-
structures and nanocomposites with graphene nanosheets and
carbon materials is shown in Table 1.7,10,15,17,22–24
It can be seen
that the obtained capacity of the current NiFe2O4/C electrode is
better than those reported in the literature. More importantly,
the synthesis strategy of the present study is also cost-effective
and simple compared to the previous reports.
Fig. 4(d) demonstrates the remarkable rate capability of the
NiFe2O4/C electrode at various current rates between 0.5C and
5.0C. With the increase of current rate, the capacity decreases.
It maintains a reversible charge capacity of 791.2 mA h gÀ1
,
615.4 mA h gÀ1
, 461.5 mA h gÀ1
, 307.7 mA h gÀ1
and 263.7 mA h
gÀ1
at a current rate of 0.5C, 1.0C, 2.0C, 4.0C and 5.0C,
respectively. It can also be seen that a charge capacity of 648.4
mA h gÀ1
could be recovered when the current density returns
to 0.5C. In addition, the NiFe2O4/C electrode exhibits stable
capacities at all the high current rates of 1.0C, 2.0C, 4.0C and
5.0C. It is possible that the carbon existing in the system is
responsible for the lower charge transfer resistance of the
hybrid material.
In order to further conrm that the NiFe2O4 nanoparticles
are protected by the carbon network during cycling, an ex situ
TEM study is performed on the cycled electrode aer 60 cycles.
Briey, for the ex situ TEM studies, the electrode was separated
from the coin cell inside the glove box and subsequently washed
thoroughly with solvent (dimethyl carbonate) to remove the
electrolyte. Fig. 5(a) and (b) show the ex situ FE-TEM images of
the NiFe2O4/C electrode aer 60 cycles at different magnica-
tions. It can be clearly seen that the carbon network and elec-
trode integrity is still maintained aer long term cycling. The
NiFe2O4 nanoparticles are still completely anchored on the
carbon network, ensuring good electrical contact between the
NiFe2O4 nanoparticles and carbon. Finally, it is also reasonable
to suggest that the small NiFe2O4 nanoparticles anchored on
the carbon network could alleviate the pulverization problem
and enhance the electrical conductivity, cycling performance
and rate capabilities of the NiFe2O4/C electrode.
Fig. 4 Electrochemical performance of the NiFe2O4/C nanocomposite electrode: (a) cyclic voltammogram at a scan rate of 0.1 mV sÀ1
; (b)
discharge/charge voltage profiles at the constant current rate of 1.0C; (c) cycling performance plot at the constant current rate of 1.0C; (d) C-rate
capability at various current rates between 0.5C and 5.0C.
38068 | RSC Adv., 2016, 6, 38064–38070 This journal is © The Royal Society of Chemistry 2016
RSC Advances Paper

6. 4. Conclusions
In summary, an easy, fast and low cost pyro-synthesis approach
was chosen to fabricate NiFe2O4 nanoparticles decorated onto
a conductive carbon network in order to overcome the issues of
NiFe2O4 such as large volume changes and poor electrical
conductivity. The material was systematically characterized
using X-ray diffraction, eld-emission scanning electron
microscopy, eld-emission transmission electron microscopy,
Raman spectroscopy and electrochemical measurements.
Morphology observation clearly shows that the NiFe2O4 nano-
particles are well anchored within the carbon network with
particle sizes of 50–100 nm. As an anode, the NiFe2O4/C elec-
trode exhibits a high reversible capacity of 381.8 mA h gÀ1
aer
100 cycles at a constant current rate of 1.0C and a better rate
capability of 263.7 mA h gÀ1
at 5.0C. It is believed that the
carbon network in this architecture is not only providing the
electrical network to facilitate Li+
ion and electron transport,
but also offers buffering during electrochemical cycling to
maintain the structural integrity of the electrode. In addition, it
may also be probable that the presence of the extra phase of
Fe2O3 in the nanocomposite sample may also contribute to
improving the electrochemical performance of the parent
NiFe2O4/C electrode by the storage of six Li+
ions per formula
unit. The current strategy can be extended to produce other
hybrid metal oxides with novel architectures for high-
performance energy storage applications.
Acknowledgements
A. K. Rai is grateful for the nancial support by the Science and
Engineering Research Board (SERB), Government of India, vide
grant no. YSS/2015/000489. We are also thankful to the Amrita
Centre for Nanosciences for providing the infrastructure.
References
1 T. V. Thi, A. K. Rai, J. Gim and J. Kim, J. Power Sources, 2015,
292, 23.
2 L. Huang, G. H. Waller, Y. Ding, D. Chen, D. Ding, P. Xi,
Z. L. Wang and M. Liu, Nano Energy, 2015, 11, 64.
3 Y. N. NuLi and Q. Z. Qin, J. Power Sources, 2005, 142, 292.
4 L. Yao, X. Hou, S. Hu, J. Wang, M. Li, C. Su, M. O. Tade,
Z. Shao and X. Liu, J. Power Sources, 2014, 258, 305.
5 A. K. Rai, S. Kim, J. Gim, M. H. Alfaruqi, V. Mathew and
J. Kim, RSC Adv., 2014, 4, 47087.
6 L. Lu, J. Z. Wang, X. B. Zhu, X. W. Gao and H. K. Liu, J. Power
Sources, 2011, 196, 7025.
7 L. Liu, L. Sun, J. Liu, X. Xiao, Z. Hu, X. Cao, B. Wang and
X. Liu, Int. J. Hydrogen Energy, 2014, 39, 11258.
8 M. H. Alfaruqi, A. K. Rai, V. Mathew, J. Jo and J. Kim,
Electrochim. Acta, 2015, 151, 558.
9 C. T. Cherian, J. Sundaramurthy, M. V. Reddy, P. S. Kumar,
K. Mani, D. Pliszka, C. H. Sow, S. Ramakrishna and
B. V. R. Chowdari, ACS Appl. Mater. Interfaces, 2013, 5, 9957.
10 G. Huang, F. Zhang, X. Du, J. Wang, D. Yin and L. Wang,
Chem.–Eur. J., 2014, 20, 11214.
11 H. Liu, H. Zhu and H. Yang, Mater. Res. Bull., 2013, 48, 1587.
12 P. Lavela, N. A. Kyeremateng and J. L. Tirado, Mater. Chem.
Phys., 2010, 124, 102.
13 C. T. Cherian, M. V. Reddy, G. V. Subba Rao, C. H. Sow and
B. V. R. Chowdari, J. Solid State Electrochem., 2012, 16, 1823.
14 Y. Xiao, J. Zai, X. Li, Y. Gong, B. Li, Q. Han and X. Qian, Nano
Energy, 2014, 6, 51.
Table 1 Comparison of specific capacities of the current NiFe2O4/C electrode with previous NiFe2O4 electrodes reported in the literature
Materials Synthesis method
Current density/
rate
Cycle
number
Specic capacity (mA h
gÀ1
) Ref.
NiFe2O4 octahedron Hydrothermal method 0.1C 50 375 mA h gÀ1
7
NiFe2O4/C composites Polymer pyrolysis method 1/8C 40 780 mA h gÀ1
17
— — 4C 40 200 mA h gÀ1
—
NiFe2O4 mesoporous spheres Template-free hydrothermal
method
0.2C 50 223 mA h gÀ1
22
NiFe2O4/rGO Hydrothermal route 0.2C 50 489 mA h gÀ1
15
Porous core–shell NiFe2O4@TiO2
nanorods
Metal–organic frameworks
template
5.5C 50 202 mA h gÀ1
10
NiFe2O4 nanoparticles Hydrothermal 0.2 mA cmÀ2
3 709 mA h gÀ1
23
NiFe2O4 nanorods Template-engaged reaction 1C 300 520 mA h gÀ1
24
NiFe2O4/C Pyro-synthesis 1C 100 381.8 mA h gÀ1
Current
study
NiFe2O4/C Pyro-synthesis 5C 25 263.7 mA h gÀ1
Current
study
Fig. 5 Ex situ FE-TEM images of the NiFe2O4/C nanocomposite
electrode at different magnifications after 60 cycles of discharging/
charging at a constant current rate of 1.0C.
This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 38064–38070 | 38069
Paper RSC Advances

7. 15 P. Zhu, S. Liu, J. Xie, S. Zhang, G. Cao and X. Zhao, J. Mater.
Sci. Technol., 2014, 30, 1078.
16 A. K. Rai, T. V. Thi, B. J. Paul and J. Kim, Electrochim. Acta,
2014, 146, 577.
17 Y. Ding, Y. Yang and H. Shao, J. Power Sources, 2013, 244,
610.
18 J. Kang, S. Baek, V. Mathew, J. Gim, J. Song, H. Park, E. Chae,
A. K. Rai and J. Kim, J. Mater. Chem., 2012, 22, 20857.
19 A. K. Rai, J. Jim, T. V. Thi, D. Ahn, S. J. Cho and J. Kim, J. Phys.
Chem. C, 2014, 118, 11234.
20 Z. Li, Y. Wang, H. Sun, W. Wu, M. Liu, J. Zhou, G. Wu and
M. Wu, J. Mater. Chem. A, 2015, 3, 16057.
21 J. Yang, J. Wang, X. Li, D. Wang, J. Liu, G. Liang, M. Gauthier,
Y. Li, D. Geng, R. Li and X. Sun, J. Mater. Chem., 2012, 22,
7537.
22 L. Duan, Y. Wang, L. Wang, F. Zhang and L. Wang, Mater.
Res. Bull., 2014, 61, 195.
23 H. Zhao, Z. Zheng, K. W. Wong, S. Wang, B. Huang and D. Li,
Electrochem. Commun., 2007, 9, 2606.
24 N. Wang, H. Xu, L. Chen, X. Gu, J. Yang and Y. Qian, J. Power
Sources, 2014, 247, 163.
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