Nanoscale

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aState Key Laboratory of Silicon Materials, K

Applications for Batteries of Zhejiang Provi

and Engineering, Zhejiang University, Han

edu.cn; tujplab@zju.edu.cn; Fax: +86 571 8bDivision of Physics and Applied Physics,

Sciences, Nanyang Technological University

Cite this: Nanoscale, 2013, 5, 7906

Received 3rd May 2013Accepted 20th June 2013

DOI: 10.1039/c3nr02258g

www.rsc.org/nanoscale

7906 | Nanoscale, 2013, 5, 7906–791

A three-dimensional hierarchical Fe2O3@NiO core/shellnanorod array on carbon cloth: a new class of anode forhigh-performance lithium-ion batteries

Qin-qin Xiong,a Jiang-ping Tu,*a Xin-hui Xia,b Xu-yang Zhao,a Chang-dong Gua

and Xiu-li Wanga

AFe2O3@NiOcore/shell nanorodarrayoncarbonclothwaspreparedwith theaidofhydrothermal synthesis

combined with subsequent chemical bath deposition. The resultant array structure is composed of Fe2O3

nanorods as the core and interconnected ultrathin NiO nanoflakes as the shell. As an anode material for

lithium-ion batteries, the heterostructured array electrode delivers a high discharge capacity of

1047.2 mA h g�1 after 50 cycles at 200 mA g�1, and 783.3 mA h g�1 at a high current density of 2000 mA

g�1. The excellent electrochemical performance is attributed to the unique 3D core/shell nanorod array

architecture and a rational combination of two electrochemical active materials. Our growth approach

offers a simple and effective technique for the design and synthesis of a transition metal oxide

hierarchical array that is promising for high-performance electrochemical energy storage.

1 Introduction

Due to the rapidly growing global energy consumption andworsened environmental pollution, the development of greenpower sources has become an urgent and increasing demand invarious elds such as electric vehicles, hybrid electric vehicles,and other power-supply devices.1,2 In this context, lithium-ionbatteries (LIBs) have attracted attention in the past decade dueto their relatively high energy density, long cycle life, andenvironmental friendliness.3–12 However, the search for high-performance LIBs and their miniaturization never stops andthere are continuous demands for the development of LIBs withhigher power and energy densities. Recently, one majorapproach pursued is based on the fabrication of 3D electrodestructures at the nanoscale. Compared with compact 2D elec-trodes, the 3D architectures possess higher surface/body ratios,larger surface areas and more surface active sites within a smallfootprint area.13–15

Currently, transition metal oxides (MxOy) have long beenfocused on as anode materials for LIBs due to their high elec-trochemical capacities compared to carbonaceous mate-rials.4,16–24 However, despite their high capacity, the practicalapplication of these materials as anodes for LIBs is stillhampered by large initial irreversible loss and poor capacity

ey Laboratory of Advanced Materials and

nce and Department of Materials Science

gzhou 310027, China. E-mail: tujp@zju.

7952856; Tel: +86 571 87952573

School of Physical and Mathematical

, Singapore 637371, Singapore

2

retention over extended cycling. One promising route is scru-pulous design of nanostructured electrodes and smart hybrid-ization of materials. By combining unique properties ofindividual constituents, improved electrochemical performancehas been demonstrated in such an electrode.25–28 A key chal-lenge in this approach is to build up an integrated smartarchitecture, in which structural features and electroactivities ofeach component are fully manifested, and the interface/chem-ical distributions are homogeneous at the nanoscale.

In this present work, we develop a simple strategy to designand fabricate 3D heterostructured Fe2O3@NiO core/shellnanorod arrays on a carbon cloth substrate. The designedelectrode has the following advantages. First, both the core andshell materials are good active materials, hence contributing tothe electrochemical energy storage. Second, the Fe2O3 nanorodarray of high crystalline quality directly grown on conductivecarbon cloth serves as the backbone to ensure quick electrontransport. Third, the 3D Fe2O3@NiO nanorod array is well incontact with and strongly supported on the carbon clothsubstrate, avoiding the use of polymer binder/conductiveadditives, and thus the inactive interface is signicantlyreduced. Fourth, the ultrathin NiO nanoakes are well wrappedon the Fe2O3 nanorod surfaces, which can provide a short Li+

diffusion path and maintain the structural integrity of the coreduring the charge–discharge process. Fih, by using the carboncloth as the current collector to replace traditional copper foil,highly exible LIBs with excellent mechanical performance arefabricated. Therefore, in this electrode design, not only are allthe desired functions of each constituent effectively utilized,but also the strong synergistic effect of both the active materialscan be realized.

This journal is ª The Royal Society of Chemistry 2013

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2 Experimental2.1 Preparation of core/shell nanorod array

Before the fabrication of Fe2O3@NiO array, the carbon cloth wascleaned by sonication sequentially in acetone, deionized (DI)water, and ethanol for 30 min. First, a self-supported Fe2O3

nanorod array was synthesized by a typical hydrothermalmethod. A 70 mL aqueous solution containing 0.8 g FeCl3$6H2Oand 0.479 g Na2SO4 was stirred for 10 min, and then transferredto a Teon-lined stainless-steel autoclave. Aerwards, the driedcarbon cloth was immersed into the resultant solution andhydrothermally treated at 120 �C for 6 h. Finally, the Fe2O3

nanorod array on the carbon cloth was obtained aer annealingin Ar gas at 450 �C for 3 h. The load weight of the Fe2O3 nanorodarray was about 1.3 mg cm�2.

The self-supported Fe2O3 nanorod array on carbon clothwas used as the scaffold for NiO nanoake growth in a simplechemical bath. The Fe2O3 nanorod array was placed verticallyin a 250 mL Pyrex beaker. The solution for chemical bathdeposition (CBD) was prepared by adding 20 mL of aqueousammonia (25�28%) to the mixture of 100 mL of 1 MNiSO4$6H2O and 80 mL of 0.25 M K2S2O8. Aer immersing inthe CBD solution at 20 �C for 10 min, the samples wererinsed with distilled water, and then annealed at 350 �C in Arfor 1.5 h. The average loading of NiO nanoakes was about0.85 mg cm�2.

2.2 Characterization

The morphology and microstructure of the products werecharacterized by an X-ray diffractometer (XRD, Rigaku D/max2550 PC, Cu Ka), scanning electron microscope (SEM, HitachiS-4700) with an X-ray energy dispersive spectroscope (EDS,BRUKER AXS), and transmission electron microscope (TEM,JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV).

Fig. 1 XRD pattern of Fe2O3 nanorod, NiO nanoflake and Fe2O3@NiO core/shellnanorod array.

2.3 Electrochemical investigation

The Fe2O3@NiO hierarchical core/shell nanorod array oncarbon cloth was directly used as the working electrode in coin-type half cells (CR 2025) which were assembled in an argon-lled glove box with a metallic lithium foil as the counterelectrode, 1 M LiPF6 in ethylene carbonate (EC)–dimethylcarbonate (DME) (1 : 1 in volume) as the electrolyte, and apolypropylene (PP) micro-porous lm (Cellgard 2300) as theseparator. The galvanostatic charge–discharge tests were con-ducted on a LAND battery program-control test system atvarious current densities between 0.01 and 3.0 V at roomtemperature (25 � 1 �C). Cyclic voltammetry (CV) was per-formed on a CHI660C electrochemical workstation in thepotential range of 0 to 3.0 V (vs. Li+/Li) at a scanning rate of0.1 mV s�1. In the electrochemical impedance spectroscopy(EIS) measurement, the excitation voltage applied to the cellswas 5 mV and the frequency range was from 100 kHz to 10 mHz.In this experiment, the mass ratio of Fe2O3 and NiO is around3 : 2. The gravimetric capacity of the Fe2O3@NiO nanorod arrayis based on the mass of both the active materials.

This journal is ª The Royal Society of Chemistry 2013

3 Results and discussion

XRD measurement was carried out to substantiate the structureand phase composition of the products. Fig. 1 shows typicalXRD patterns of the resultant Fe2O3@NiO core/shell nanorods,Fe2O3 nanorods and NiO nanoakes. The formation of rhom-bohedral crystalline NiO (JCPDF card no. 44-1159) in the hybridis revealed by the diffraction peaks centered at 37.2, 43.3 and62.9� corresponding to (101), (012) and (110) crystal planes,respectively. The diffraction peaks at 33.2, 35.6, 40.9, 49.5 and54.1�, which correspond to (104), (110), (113), (024) and (116)crystal planes respectively, are ascribed to Fe2O3 (JCPDF cardno. 33-0664). Besides, a peak at around 26� is also observed,coming from the carbon cloth template.29 Furthermore, thediffraction peaks of NiO and Fe2O3 in the hybrid are sharp andintense, implying the successful synthesis of a high crystallinecore/shell nanorod array.

The morphologies of the Fe2O3 nanorods, NiO nanoakesand nal Fe2O3@NiO core/shell nanorod arrays on a carboncloth substrate are shown in Fig. 2. Obviously, the Fe2O3

nanorods are homogeneously aligned and separated apartadequately, forming an array with a highly open structure on alarge scale (Fig. 2a and b). The Fe2O3 nanorods have an averagediameter of 25�30 nm and length up to around 300�400 nm.Fig. 2c and d show the NiO nanoake lm, which is uniform inappearance and exhibits cellular-like morphology. The lmthickness obtained from the cross-sectional SEM image (insetin Fig. 2c) is around 350 nm. Fig. 2e and f clearly exhibit thetypical morphology of the Fe2O3@NiO core/shell nanorod array.As shown in Fig. 2e, each Fe2O3@NiO core/shell nanorod array/carbon composite ber has a uniform diameter of approxi-mately 10 mm. The composite ber is composed of numeroushighly ordered Fe2O3@NiO core/shell nanorod arrays withrelatively high density grown on an individual carbon micro-ber. Aer chemical bath deposition, the Fe2O3 nanorods aredecorated with NiO nanoakes (Fig. 2f). The NiO nanoakeshells with a thickness of �100 nm are interconnected and fully

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Fig. 2 SEM images of (a and b) Fe2O3 nanorod array, (c and d) NiO nanoflakefilm and (e and f) hierarchical Fe2O3@NiO core/shell nanorod array.

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cover the entire Fe2O3 core, forming a 3D network structure. Ofimportance, even with two components integrated, the uniformarray structure is still well retained.

The detailed structural features of the products are alsoexamined by TEM. Fig. 3a displays the TEM image of a singleFe2O3 nanorod. The inset is the selected area electron diffrac-tion (SAED) pattern, revealing that the Fe2O3 nanorod is a singlecrystal. Fig. 3b shows the image of a NiO nanoake which isseparated from the lm by ultrasonic treatment and it can beseen that it is very smooth. In addition, all the diffraction ringsindicate that the NiO lm is polycrystalline in nature. Fig. 3cshows a typical TEM image of the Fe2O3@NiO hybrid structurein which ultrathin NiO nanoakes uniformly cover the surfaceof the Fe2O3 nanorods. A close examination of the exposedprole reveals that the thickness of the outer symmetric NiOlayer is about 100 nm, which agrees with the results of SEMobservation. The high-resolution TEM (HRTEM) image ofcurling nanoakes shown in Fig. 3d reveals an interplanarspacing of 0.24 and 0.21 nm, corresponding to the (101) and(012) planes of NiO, respectively. This core/shell heterostructureis also supported by the EDS elemental mapping analysis of thehybrid nanorods (Fig. 3e). The mapping result is unambigu-ously consistent with the EDS spectrum shown in Fig. 3f, inwhich the Cu signal comes from Cu grid.

To evaluate the electrochemical lithium storage, the Fe2O3

nanorod, NiO nanoake and nal Fe2O3@NiO core/shellnanorod array on carbon cloth were directly applied as anodesfor LIBs. Fig. 4 shows the rst three CV curves of the Fe2O3

nanorod, NiO nanoake and Fe2O3@NiO core/shell nanorodarray electrodes in the voltage range of 0–3 V at a scan rate of

7908 | Nanoscale, 2013, 5, 7906–7912

0.1 mV s�1. The cathodic peak at 0.001 V and the anodic peak at0.24 V are ascribed to the lithium insertion and extraction fromthe carbon cloth for all the electrodes. Fig. 4c shows the CVcurves of the Fe2O3@NiO core/shell nanorod array electrode. Inthe rst cathodic scan, an intensied peak (Peak I) at 0.60 V anda weak peak (Peak II) at 0.49 V respectively correspond to thecomplete reduction of Fe2O3 to Fe and the initial reduction ofNiO to Ni, as well as the formation of amorphous Li2O and solidelectrolyte interphase (SEI).30–32 In comparison with Fig. 4a andb, peaks I and II deviate due to the inuence of the hybridsystem on the processes of electrochemical reactions. The twobroad peaks at about 1.5 and 2.25 V in the rst anodic scanmainly correspond to the NiO formation.22,33–35 The anodic peakat about 1.85 V corresponds to the reversible oxidation of Fe0 toFe3+, which agrees well with early studies.36 The difference of thesubsequent cycles from the rst one is mainly due to the irre-versible phase transformation during lithium insertion andextraction in the initial cycle. The present results, to a certainextent, sustain the electrochemical reactions for Fe2O3 and NiOanodes as follows:

Fe2O3 + 6Li+ + 6e� 4 Li2O + Fe (1)

NiO + 2Li+ + 2e� 4 Li2O + Ni (2)

It is interesting that the area integrated within the current–potential curves greatly increases for the 3D Fe2O3@NiO core/shell nanorod array electrode, which efficiently increases theaccessible surface area of active materials to electrolyte,leading to a much higher capacity. Fig. 4d shows the repre-sentative discharge–charge curves of the 3D Fe2O3@NiO core/shell nanorod array electrode at current densities from 100 to1000 mA g�1 in the voltage range of 0.01–3 V. There are anobvious plateau (Plateau I) at around 0.6 V and a short plateau(Plateau II) at around 0.45 V followed by a sloping curve duringthe rst discharge cycle at a current density of 100 mA g�1,which is in agreement with the results shown in Fig. 4c(respectively corresponding to Peaks I and II). To estimate thecapacity contribution of the carbon cloth in the composite, thedischarge–charge curves at the same current densities are alsoshown in the inset. The capacity contribution of the carboncloth is ascribed to lithium insertion and extraction below0.4 V. In the following test, we have eliminated the contribu-tion of the carbon cloth. As anticipated, the increase of currentdensity results in higher overpotential of the Fe2O3@NiO core/shell nanorod array electrode, in terms of lower dischargeplateau and higher charge potential. However, the dischargeand charge curves at different current densities still kept asimilar shape and delivered high capacity, suggesting the goodrate performance of the Fe2O3@NiO core/shell nanorod arrayelectrode.

The EIS test was carried out to further understand theadvantage of the 3D Fe2O3@NiO core/shell nanorod arrayelectrode. Fig. 5 shows the Nyquist plots of the Fe2O3 nanorod,NiO nanoake and nal Fe2O3@NiO core/shell nanorod arrayelectrodes at a current density of 200 mA g�1 at 2.0 V in the 51th

This journal is ª The Royal Society of Chemistry 2013

Fig. 3 TEM images of (a) Fe2O3 nanorods (inset is the SAED pattern), (b) NiO nanoflakes (inset is the SAED pattern), (c) hierarchical Fe2O3@NiO core/shell nanorods, (d)HRTEM image of NiO nanoflakes of hybrid nanorods, (e) EDS mapping results of Fe2O3@NiO core/shell nanorods, and (f) EDS spectrum of the Fe2O3@NiO core/shellnanorods in (d).

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charging. They have the same shapes of Nyquist plots,composed of a high-frequency semicircle and a medium-frequency semicircle and a long low-frequency subsequent 45�

line. In this experiment, the impedance data are analyzed bytting the equivalent electrical circuit shown in the inset, whereR0, Rf and Rc indicate electrolyte resistance, surface lm resis-tance and charge transfer resistance, respectively; Cf and Cd

stand for the corresponding capacitances of Rf and Rc; and Wrepresents the diffusion-controlled Warburg impedance. InFig. 5, the symbols represent the experimental data whereas thecontinuous lines represent the tted spectra. For the Fe2O3@NiO core/shell nanorod array electrode, the diameter of themedium-frequency semicircle, reecting the Rc, is obviouslysmaller than the Fe2O3 nanorod and NiO nanoake. As expec-ted, such a result indicates that the core/shell nanorod arrayelectrode possesses a rapid charge transfer reaction during

This journal is ª The Royal Society of Chemistry 2013

lithium insertion and extraction. The curve tting furtherconrms this assumption. It reveals that the value of Rc is 98.2Ufor the pure Fe2O3 and 127.4 U for the pure NiO, whereas it is80.8 U for the Fe2O3@NiO core/shell nanorod array electrode.

Fig. 6 shows the cyclic performance and coulombic efficiencyof the Fe2O3@NiO core/shell nanorod array electrode measuredat a current density of 200 mA g�1. The cyclic performances ofthe Fe2O3 nanorod and NiO nanoake electrodes are also shownfor comparison. It is observed that the Fe2O3@NiO core/shellnanorod array electrode delivers a reversible capacity up to1047.2 mA h g�1 aer 50 cycles and its coulombic efficiency isabove 98% from the 5th cycle. Although the pure Fe2O3 showsthe same cycle trend as the Fe2O3@NiO core/shell nanorodarray, its capacity is much lower. Aer 50 cycles, the dischargecapacity of the pure Fe2O3 and NiO electrodes reduces to719.5 mA h g�1 and 482.7 mA h g�1, respectively. The capacity

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Fig. 4 CV curves of (a) Fe2O3 nanorod, (b) NiO nanoflake film, (c) Fe2O3@NiOcore/shell nanorod array for the first three cycles at a scan rate of 0.1 mV s�1 in thepotential range of 0–3.0 V (versus Li/Li+) and (d) charge–discharge profiles of theFe2O3@NiO core/shell nanorod array between 0.01 and 3.0 V at current densitiesfrom 100 mA g�1 to 1000 mA g�1.

Fig. 5 Nyquist plots of Fe2O3 nanorod, NiO nanoflake and Fe2O3@NiO core/shell nanorod array electrodes at 2.0 V in the 51th charging in the frequency rangefrom 100 kHz to 10 mHz. The inset is the equivalent circuit.

Fig. 6 Cycling performance of Fe2O3 nanorod, NiO nanoflake and Fe2O3@NiOcore/shell nanorod array electrodes at a current density of 200 mA g�1.

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difference between the Fe2O3@NiO core/shell nanorod andFe2O3 nanorod arrays exhibits that the synergistic effect of thecombined electrochemical contributions from the Fe2O3 coreand the ultrathin NiO shell is apparent. Based on the theoreticalcapacities of pure Fe2O3 and NiO, the calculated theoreticalcapacity for the Fe2O3/NiO composite (with a mass ratio of 3 : 2)is 888.2 mA h g�1, which is about 159 mA h g�1 smaller than themeasured value of 1047.2 mA h g�1. Moreover, compared to thepreviously reported results for Fe2O3 and NiO, the reversiblecapacity of the Fe2O3@NiO core/shell nanorod array electrode ishigh and very stable, which is mainly attributed to the specialcore/shell nanostructures, the synergistic effects of both theFe2O3 and NiO active materials and the reversible growth of apolymeric gel-like lm resulting from kinetically activatedelectrolyte degradation.29,37–40

Fig. 7 illustrates the current density dependence of thedischarge capacity of the Fe2O3 nanorod, NiO nanoake andnal Fe2O3@NiO core/shell nanorod array electrodes. Withinthe current density range from 100 to 2000 mA g�1, theFe2O3@NiO core/shell nanorod array always delivers a higher

This journal is ª The Royal Society of Chemistry 2013

Fig. 8 SEM images of (a) Fe2O3 nanorod and (b) NiO nanoflake and (c)

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capacity than the other two electrodes. The discharge capacitydecreases from 1285.7to 1058.7, 949.8, 883.4, and 783.3 mA hg�1 with an increase in the current density from 100 to 200,500, 1000 and 2000 mA g�1, respectively, indicating diffusion-controlled kinetics for the electrode reaction. Upon decreasingthe current density back to 100 mA g�1 again, nearly 99.6% ofthe capacity of the second cycle (1109.3 mA h g�1) can berecovered in the case of the Fe2O3@NiO core/shell nanorodarray, while it is 98.5% for the pure Fe2O3 electrode and 90.6%for the NiO electrode. It is worth noting that the Fe2O3@NiOcore/shell nanorod array exhibits excellent rate capability, andthe capacity is stable at each of the tested current densities.On the basis of the above results, the excellent cyclingstability, high capacity, and outstanding rate performance ofthe 3D hierarchical Fe2O3@NiO core/shell nanorod array/carbon cloth make it a promising candidate for an anodematerial for LIBs and will be favorable for numerous potentialapplications in electrochemical energy storage.

To further understand the excellent electrochemical perfor-mance of the Fe2O3@NiO core/shell nanorod array, the SEMimages of the Fe2O3 nanorod, NiO nanoake and Fe2O3@NiOcore/shell nanorod arrays aer 25 cycles at a current density of200 mA g�1 are shown in Fig. 8. Aer the lithiation/delithiationcycles, the surface of Fe2O3 nanorods becomes rather roughwith an expansive diameter of around 100 nm (Fig. 8a). As forthe NiO nanoake lm, although it maintains the cellular-likemorphology, the nanoakes are damaged severely. Fortunately,the morphology of the Fe2O3@NiO core/shell nanorod arrayremains as it was. The ultrathin NiO nanoakes attached to theFe2O3 nanorods have lightly ruptured, but almost exhibit noagglomeration. From Fig. 8c, it can be seen that the diameter ofthe core/shell nanorod array remains the same and the nano-akes around the neighboring nanorods are connected asbefore, indicating that combining these two components into asingle electrode offers special structural stability through thereinforcement and modication of each other. This 3D hierar-chical core/shell nanorod array structure is benecial to relax

Fe2O3@NiO core/shell nanorod array after 25 cycles at a current density of200 mA g�1.

Fig. 7 Rate performance of Fe2O3 nanorod, NiO nanoflake and Fe2O3@NiOcore/shell nanorod array electrodes at a current density of 100 mA g�1 to2000 mA g�1.

This journal is ª The Royal Society of Chemistry 2013

the volume expansion and alleviate the structure damageduring cycling.

4 Conclusions

In summary, smartly designed 3D Fe2O3@NiO core/shellnanorod array has been successfully synthesized by a hydro-thermal method combined with subsequent chemical bathdeposition. The Fe2O3@NiO core/shell nanorod array electrodeoffers high capacity, excellent rate capability and cyclic stability,compared with its corresponding single counterpart electrode.Such intriguing capacity behavior is attributed to the hierar-chical core/shell nanorod array conguration and the syner-gistic effect of the Fe2O3 core and ultrathin NiO shell. Thepresent work indicates that the designed 3D Fe2O3@NiO core/shell nanorod array possesses great application potential inhigh-performance energy storage devices.

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Acknowledgements

This work is supported by the Key Science and TechnologyInnovation Team of Zhejiang Province (2010R50013). Theassistance of Dr Shao-jun Shi andMr Xin-ting Cong for SEM andTEM analyses is gratefully acknowledged.

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