Cite this: RSC Advances, 2013, 3, 699
Received 30th October 2012,
Accepted 9th November 2012
Mesoporous iron oxide directly anchored on a graphene
matrix for lithium-ion battery anodes with enhanced
strain accommodation3
DOI: 10.1039/c2ra22702a
www.rsc.org/advances
Mingbo Zheng,{ Danfeng Qiu,{ Bin Zhao, Luyao Ma, Xinran Wang, Zixia Lin,
Lijia Pan, Youdou Zheng and Yi Shi*
A continuous mesoporous iron oxide nanofilm was directly
formed on graphene nanosheets through the in situ thermal
decomposition of Fe(NO
3
)
3
?9H
2
O and was anchored tightly on the
graphene surface. The lithiation-induced strain was naturally
accommodated, owing to the constraint effect of graphene and
the mesoporous structure. Hence, the pulverization of the iron
oxide nanofilm was effectively prevented.
Transition metal oxides, such as FeO
x
,Co
3
O
4
and NiO, have long
been considered as anode materials for lithium-ion batteries
(LIBs) due to their high capacities.
1–3
In their practical applica-
tions, the pulverization problem induced by large volume changes,
which leads to loss of electrical contact and subsequent rapid
capacity fading, has to be resolved.
4
As typical transition metal
oxides, iron oxides have been extensively investigated as LIB
anodes and many efforts have been focused on controlling the
pulverization. The proposed methods mainly include the follow-
ing: 1) the preparation of nanostructured iron oxides to accom-
modate the large lithiation-induced strain
5–7
and 2) the use of
carbon as a coating or matrix for iron oxides to ensure electrical
connectivity.
8–15
The emergence of graphene provided a novel
carbon matrix for LIB active materials, as graphene possesses
superior electrical conductivity, excellent mechanical flexibility
and good chemical stability.
16–20
Fe
3
O
4
21–31
and Fe
2
O
3
32–34
nanoparticles combined with graphene matrices as LIB anode
materials have been reported. Here, graphene serves as an
efficient conductive network and prevents the aggregation of iron
oxide nanoparticles. At the same time, mesoporous materials have
attracted significant interest in LIBs, due to their unique proper-
ties.
35–38
Some of these properties are the mesopores’ ability to
allow the smooth transport of electrolyte, the thin walls providing
a short diffusion length for lithium ions and the high porosity that
provides free space to accommodate the lithiation-induced
strain.
35–38
Recently, Yang et al. reported that mesoporous TiO
2
as LIB anode materials could be coated on a graphene surface by a
hard template method and the results showed that the combina-
tion of the mesoporous structure and graphene enhanced the rate
capability of the electrode.
39
For iron oxide anode materials that
undergo large volume changes during cycling, the strong bonding
of mesoporous iron oxide (MIO) nanofilms with graphene is
highly desirable. In a previous work by our group, it was
demonstrated that the alumina matrix has a constraint effect on
the volume shrinkage of polymer nanofibers during carbonization
due to the strong bond between the polymer and alumina at the
interface.
40
Accordingly, for a nanocomposite system, where MIO
is strongly bonded with graphene, the graphene matrix may also
have a constraint effect on the volume change of iron oxide,
thereby uniformly accommodating the lithiation-induced strain of
the loaded continuous MIO nanofilms. Fortunately, the existence
of covalent chemical bonding formed through oxygen-containing
defect sites on the graphene surface provides an opportunity to
tightly anchor iron oxide on graphene.
16,31,41,42
The combination
of the constraint effect of the graphene matrix and the
mesoporous structure of MIO is expected to enhance effectively
the strain- accommodating capability, thereby preventing the
pulverization of iron oxide upon lithium-ion insertion/extraction.
In the present work, the direct anchoring of continuous MIO
nanofilms on graphene nanosheets (GNSs) as LIB anodes via a
new and versatile in situ thermal decomposition method is
reported (Fig. 1, steps 1 and 2) and a new constraint effect of the
GNS on the volume change of MIO at the molecular level is
presented. The electrochemical test results show that the MIO/
GNS nanocomposite has excellent capacity retention (no capacity
fading after 400 cycles at 1000 mA g
21
), high specific capacity
(about 1000 mAh g
21
at 100 mA g
21
) and good rate capability as
an LIB anode material. The excellent cycling performance is
attributed to the superior strain accommodating capability of the
MIO/GNS nanocomposite (Fig. 1, steps 3 and 4). The results
present a promising method for the synthesis of graphene-based
nanocomposites and an interesting nanostructured system for LIB
Nanjing National Laboratory of Microstructures, School of Electronic Science and
Engineering, Nanjing University, Nanjing 210093, China. E-mail: yshi@nju.edu.cn;
Fax: 86-25-83621220; Tel: 86-25-83621220
3
Electronic supplementary information (ESI) available: experimental details, XRD
patterns, additional SEM and TEM images, SEM mapping, N
2
adsorption–
desorption analysis, FT-IR spectrum, XPS spectrum and additional electrochemical
data. See DOI: 10.1039/c2ra22702a
{ These authors contributed equally to this work.
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