Journal of Alloys and Compounds 561 (2013) 54–58

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Journal of Alloys and Compounds

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High rate capability of TiO2/nitrogen-doped graphene nanocompositeas an anode material for lithium–ion batteries

Dandan Cai a, Dongdong Li a, Suqing Wang a, Xuefeng Zhu b, Weishen Yang b, Shanqing Zhang c,Haihui Wang a,⇑a School of Chemistry & Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou, Chinab State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Chinac Centre for Clean Environment and Energy, Environmental Futures Centre and Griffith School of Environment, Gold Coast Campus, Griffith University, QLD 4222, Australia

a r t i c l e i n f o

Article history:Received 20 November 2012Received in revised form 10 January 2013Accepted 11 January 2013Available online 13 February 2013

Keywords:TiO2

Nitrogen-doped grapheneAnode materialLithium–ion batteries

0925-8388/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2013.01.068

⇑ Corresponding author. Tel./fax: +86 20 87110131E-mail address: hhwang@scut.edu.cn (H. Wang).

a b s t r a c t

TiO2/nitrogen-doped graphene nanocomposite was synthesized by a facile gas/liquid interface reaction.The structure and morphology of the sample were analyzed by X-ray diffraction analysis, X-ray photo-electron spectroscopy, scanning electron microscopy and transmission electron microscopy. The resultsindicate that nitrogen atoms were successfully doped into graphene sheets. The TiO2 nanoparticles (8–13 nm in size) were homogenously anchored on the nitrogen-doped graphene sheets through gas/liquidinterface reaction. The as-prepared TiO2/nitrogen-doped graphene nanocomposite shows a better elec-trochemical performance than the TiO2/graphene nanocomposite and the bare TiO2 nanoparticles.TiO2/nitrogen-doped graphene nanocomposite exhibits excellent cycling stability and shows high capac-ity of 136 mAh g�1 (at a current density of 1000 mA g�1) after 80 cycles. More importantly, a high revers-ible capacity of 109 mAh g�1 can still be obtained even at a super high current density of 5000 mA g�1.The superior electrochemical performance is attributed to the good electronic conductivity introducedby the nitrogen-doped graphene sheets and the positive synergistic effect between nitrogen-dopedgraphene sheets and TiO2 nanoparticles.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Lithium–ion batteries have been widely used in portable elec-tronic devices because of their high energy density and long cy-cling lifetime. Recently, they have also been proposed for use inelectric vehicles and hybrid electric vehicles [1,2], which havemade it crucial to improve the safety and rate performance for lith-ium–ion batteries. As a typical commercial anode material for lith-ium–ion batteries, graphite still has safety concerns due to theproduction of lithium dendrites during the continuous charge/dis-charge process [3]. Thus, a safe anode material has become an ur-gent requirement for high performance lithium–ion batteries.

Among the anode materials, TiO2 has been regarded as a prom-ising anode material for lithium–ion batteries because of its lowcost and environmental friendliness. More importantly, TiO2 showsa higher lithium intercalation potential (1.5–1.8 V versus Li+/Li)compared with graphite, which could avoid the formation of lith-ium dendrites [4–6]. However, the rate performance of the bareTiO2 is poor due to its low lithium–ion diffusivity and electronicconductivity [7,8]. To solve the problem, nanotechnology has been

ll rights reserved.

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introduced to improve electrochemical performance by shorteningthe transport distance of electrons and lithium–ion and increasingcontact area between electrode and electrolyte [9–13]. In addition,the other effective approach to improve the rate performance is thecombination of TiO2 and carbon-based materials (polymer [14],carbon [15] and graphene [16–22]). It is known that carbon-basedmaterial could not only improve electronic conductivity of TiO2 butalso suppress its severe agglomeration.

Recently, nitrogen-doped graphene has been explored anddelivers better electrochemical performance than the pristinegraphene [23–27]. Besides, nitrogen-doped graphene have beenused as electronic conducting framework to improve the lithiumstorage properties. In this regard, nitrogen-doped graphene/SnO2

[28], nitrogen-doped graphene/MnO [29] and nitrogen-dopedgraphene/Fe2O3 [30] hybrids or composite are proved to be ableto improve rate capacity and enhance cycle life of lithium–ion bat-teries. It is believed that the nitrogen-doped graphene providesmore active sites to control the growth of metal oxide nanoparti-cles. Moreover, the doped nitrogen atoms can provide extra lonepair electrons and form complexes with metal ions to help thecombination of metal oxide and nitrogen-doped graphene. How-ever, to the best of our knowledge, very few studies have been

20 30 40 50 60

10 15 20 25 30 35 40 45 50 55 60

Nitreogen-doped graphene

Graphene

(002

)

cb

(204

)

(211

)

(200

)

(004

)

(105

)

2θ (degree)

2θ (degree)(101

)

a

Fig. 1. XRD patterns of (a) the bare TiO2 nanoparticles, (b) the TiO2/graphenenanocomposite and (c) the TiO2/nitrogen-doped graphene nanocomposite. Theinset is XRD patterns of nitrogen-doped graphene and graphene.

D. Cai et al. / Journal of Alloys and Compounds 561 (2013) 54–58 55

done on TiO2/nitrogen-doped graphene nanocomposite as an an-ode material for lithium–ion batteries.

Herein, we utilized gas/liquid interface reaction method to syn-thesize TiO2/nitrogen-doped graphene nanocomposite as an anodematerial for lithium–ion batteries. The simple and low-cost syn-thesis approach presents a promising route for a large-scale pro-duction of TiO2/nitrogen-doped graphene nanocomposite. Thenanocomposite shows a superior electrochemical performancewith excellent rate capability and outstanding cycling stability,which exhibits a great potential for an anode material for lith-ium–ion batteries.

2. Experimental

2.1. Materials preparation

The graphene sheets were prepared by rapid thermal expansion method, as pre-viously reported by our group [31]. Then, the graphene sheets were further an-nealed at 800 �C for 6 h under an ammonia atmosphere to obtain nitrogen-dopedgraphene sheets.

The TiO2/nitrogen-doped graphene nanocomposite was synthesized by a facilegas/liquid interface reaction [32–35]. Briefly, 0.7589 g of TiCl4 (Tianjin KermelChemical Reagent Co., Ltd., China) and 0.0564 g of nitrogen-doped graphene sheetswas added in 20 mL of ethylene glycol (EG) (Beijing Chemical Reagent Co., Ltd., Chi-na), and then sonicated for 10 h to yield a homogeneous suspension. Then the bea-ker with the as-prepared suspension was placed into a 100 mL Teflon-linedautoclave with 14 mL of ammonia solution (Guangzhou Chemical Reagent Factory,China). Then the autoclave was sealed and heated to 180 �C and held at this temper-ature for 12 h. After cooling and centrifugation, the product was washed with ethylalcohol (Beijing Chemical Reagent Co., Ltd., China) for several times. Finally, theblack solid product was dried at 80 �C in a vacuum to obtain the TiO2/nitrogen-doped graphene nanocomposite. For a comparison, the TiO2/graphene nanocom-posite and the bare TiO2 nanoparticles were also synthesized by similar method.

2.2. Characterization of materials

The structures and morphologies were characterized by X-ray diffraction (XRD)(Bruker D8 Advance), scanning electron microscopy (SEM) (Quanta 200F) and trans-mission electron microscopy (TEM) (FEI, Tecnai G2 F30 S-Twin). X-ray photoelec-tron spectroscopy (XPS) analysis was carried out with a Kratos Axis Ultra DLDusing the mono Al Ka radiation (1486.6 eV) under a pressure of 5 � 10�9 torr.

2.3. Electrochemical measurements

The electrochemical properties of the TiO2/nitrogen-doped graphene nanocom-posite, the TiO2/graphene nanocomposite and the bare TiO2 nanoparticles wereinvestigated using coin cells (CR2025). In order to make working electrodes, a slurrycontaining 75 wt.% active materials (the TiO2/nitrogen-doped graphene nanocom-posite, the TiO2/graphene nanocomposite or the bare TiO2 nanoparticles), 15 wt.%Super P carbon black, and 10 wt.% polyvinylidene (PVDF) was dispersed in an N-methyle-2-pyrrolidone (NMP). Then the slurry was coated on copper foil. Lithiumfoil was used as the counter electrode while celgard 2325 membrane was used asa separator. The electrolyte consisted of 1 mol L�1 LiPF6 in ethylene carbonate(EC)/diethylcarbonate (DEC) (1:1 by volume) (Beijing Institute of Chemical Re-agents, China). The coin cells were assembled in an argon-filled glove box (Mikro-una, super 1220) where the oxygen and moisture contents were less than 1 ppm.

The cells were galvanostatically discharged and charged in the voltage range of1.0–3.0 V using a Battery Testing System (Neware Electronic Co., China). Electro-chemical impedance spectra (EIS) of the TiO2/nitrogen-doped graphene nanocom-posite, the TiO2/graphene nanocomposite and the bare TiO2 nanoparticles after80 cycles were measured at a discharged potential of 1.81 V versus Li/Li+ at the elec-trochemical workstation (Zahner IM6ex). The frequency range was set from 10 mHzto 1 MHz and the potential amplitude was 5 mV.

3. Results and discussion

The XRD patterns of the TiO2/nitrogen-doped graphene nano-composite, the TiO2/graphene nanocomposite and the bare TiO2

nanoparticles are shown in Fig. 1. All the strong diffraction peakscan be perfectly indexed to the typical anatase phase structure(JCPDS No. 21-1272). The broad diffraction peaks of the synthe-sized samples suggest that the size of the TiO2 particles is verysmall. There is a weak diffraction peak (002) in the XRD patternof the nitrogen-doped graphene sheets and the graphene sheets,

respectively (see inset of Fig. 1). The result implies that the nitro-gen-doped graphene sheets and the graphene sheets could stackinto multilayers in the solution system in the absence of TiO2 nano-particles. However, no obvious diffraction peak attributed tographite is observed, which indicates that the TiO2 crystallinephase was well maintained after the introduction of grapheneand nitrogen-doped graphene [16,20,21]. The absence of thegraphene and nitrogen-doped graphene peaks demonstrates thatthe graphene sheets were randomly distributed between TiO2

nanoparticles, which can be confirmed by the following SEMobservation.

The morphologies and microstructures of the as-prepared bareTiO2 nanoparticles, the TiO2/graphene nanocomposite and theTiO2/nitrogen-doped graphene nanocomposite were investigatedby SEM and TEM. As shown in Fig. 2a and d, most of the bareTiO2 nanoparticles have obvious agglomeration and aggregatedinto larger particles size (ca. 25 nm). The large particles could leadto the poor rate capabilities of TiO2 as anode materials for lithium–ion batteries because of the long diffusion path for lithium–ion andelectron during the lithium–ion insertion/extraction process [16–22]. After being modified by the graphene, the TiO2 nanoparticleshave a wide size distribution of 10–15 nm (Fig. 2b and e). The phe-nomenon suggests that graphene can prevent the aggregation ofthe TiO2 nanoparticles to a certain extent. The partial aggregatedTiO2 nanoparticles could be attributed to the weak adhere strengthbetween the TiO2 nanoparticles and the graphene. In contrast, uni-form TiO2 nanoparticles (8–13 nm in size) are better dispersed onthe nitrogen-doped graphene than pristine graphene as shown inFig. 2c and f. The result suggests that the nitrogen-doped graphenewith more topological defects could provide more active sites thanpristine graphene to limit the aggregation of TiO2 nanoparticles[30]. Thus, the presence of both graphene and nitrogen-dopedgraphene can decrease the particle size of TiO2 nanoparticles.

In order to analyze the chemical binding state of the as-pre-pared samples, the XPS was carried out. The binding energies inthe XPS analysis were corrected with reference to the C 1s peak(284.6 eV). As shown in Fig. 3a, the peaks of C 1s, O 1s, Ti 2p, andN 1s can be clearly detected in the survey spectrum of the TiO2/nitrogen-doped graphene nanocomposite. However, there is noobvious N 1s peak in the XPS of the TiO2/graphene nanocomposite.The XPS analysis indicates that nitrogen was successfully dopedinto graphene sheets [36]. In Fig. 3b, the main peak the N 1s peak

Fig. 2. SEM micrographs of (a) the bare TiO2 nanoparticles, (b) the TiO2/graphene nanocomposite and (c) the TiO2/nitrogen-doped graphene nanocomposite. TEM images of(d) the bare TiO2 nanoparticles, (e) the TiO2/graphene nanocomposite and (f) the TiO2/nitrogen-doped graphene nanocomposite.

56 D. Cai et al. / Journal of Alloys and Compounds 561 (2013) 54–58

of the nitrogen-doped graphene sheets can be resolved into threecomponents centered at 398.4, 400.3, and 404.2 eV, representingpyridine-like, pyrrolic, and graphitic nitrogen atoms doped in thegraphene, respectively [23–27]. The atomic percentage of dopednitrogen is about 1.75 wt.%.

Fig. 4 illustrates the cycling performance of the TiO2/nitrogen-doped graphene nanocomposite, the TiO2/graphene nanocompos-ite and the bare TiO2 nanoparticles at a current density of1000 mA g�1. Both the TiO2/nitrogen-doped graphene nanocom-posite and the TiO2/graphene nanocomposite show more excellentcycling stability compared to the bare TiO2 nanoparticles. After80 cycles, the discharge capacity of the TiO2/nitrogen-dopedgraphene nanocomposite is 136 mAh g�1, which is much higherthan those of the TiO2/ graphene nanocomposite (123 mAh g�1)and the bare TiO2 nanoparticles (78 mAh g�1). The improvementin capacity retention can be ascribed to the introduction of the

nitrogen-doped graphene with superior conductivity. On the onehand, nitrogen-dopants in graphene sheets further increase theinterfacial and electronic conductivity of TiO2 nanoparticles [37].On the other hand, the active sites of the nitrogen-doped graphenecan induce the formation of tiny TiO2 nanoparticles in the processof the facile gas/liquid interface reaction.

As shown in Fig. 5, the rate capabilities of the TiO2/nitrogen-doped graphene nanocomposite, the TiO2/graphene nanocompositeand the bare TiO2 nanoparticles were evaluated by charging/dis-charging at various current densities from 100 to 5000 mA g�1.Apparently, the rate performance of the TiO2/nitrogen-doped graph-ene nanocomposite is superior to those of the TiO2/graphene nano-composite and the bare TiO2 nanoparticles. For the TiO2/nitrogen-doped graphene nanocomposite, even at the high current densityof 5000 mA g�1, a reversible capacity of 109 mAh g�1 can still bedelivered. However, the reversible capacities of the TiO2/graphene

TiO2/nitrogen-doped graphene

Ti 3p

Inte

nsity

(a.u

.)

Binding Energy (eV)

O(Auger)Ti 2s

O 1s

Ti 2p

N 1s

C 1s

Ti 3s

TiO2/graphene

Survey

1000 800 600 400 200 0

411 408 405 402 399 396 393

404.2

400.3398.4

TiO2 /graphene

N 1s

Inte

nsity

(a.u

.)

Binding Energy (eV)

TiO2 /nitrogen-doped graphene

(a)

(b)

Fig. 3. (a) XPS survey spectra and (b) XPS N1s spectra of the TiO2/nitrogen-dopedgraphene nanocomposite and the TiO2/graphene nanocomposite.

0 10 20 30 40 50 60 70 800

50

100

150

200

250

300

c

b1000mAg-1

100mAg-1

5000mAg-13000mAg-1

2000mAg-1

500mA-1

100mAg-1

Spec

ific

Cap

acity

(mA

h g-1

)

Cycle Number

Discharge Charge

a

Fig. 5. Rate capability of (a) the TiO2/nitrogen-doped graphene nanocomposite, (b)the TiO2/graphene nanocomposite and (c) the bare TiO2 nanoparticles at variouscurrent densities, from 100 to 5000 mA g�1.

D. Cai et al. / Journal of Alloys and Compounds 561 (2013) 54–58 57

nanocomposite and the bare TiO2 nanoparticles are only 90 and23 mAh g�1, respectively. More importantly, the specific capacityof the TiO2/nitrogen-doped graphene nanocomposite can recoverto the initial value when the current density is reduced back to

0 20 40 60 800

20

40

60

80

100

120

140

160

180

Spec

ific

Cap

acity

(mA

h g-1

)

Cycle Number

Discharge Charge

b

c

a

Fig. 4. Cycle performance of (a) the TiO2/nitrogen-doped graphene nanocomposite,(b) the TiO2/graphene nanocomposite and (c) the bare TiO2 nanoparticles at acurrent density of 1000 mA g�1.

100 mA g�1, indicating that the nanocomposite shows an excellentcycle performance.

The improved electrochemical performance of the TiO2/nitro-gen-doped graphene nanocomposite may be attributed to the fol-lowing reasons. Firstly, the nitrogen-doped graphene stillmaintained good mechanical properties as the pristine graphene[23–27]. Secondly, the doped nitrogen atoms can provide extra lonepair electrons, which makes the electrical conductivity improvedcompared with the pristine graphene [17]. The conductivity of theas-prepared materials can be obtained by the EIS measurements[25,38]. Fig. 6 shows that the nitrogen-doped graphene electrodeshave much lower electrolyte resistances (RX = 1.515 X) and chargetransfer resistances (Rct = 188.3 X) than those of the graphene elec-trodes (RX = 1.959 X, Rct = 222.7 X). Fitting of impedance spectra tothe proposed equivalent circuit was performed using ZSimpWin soft-ware (see inset of Fig. 6). Based on the equation r = L/(RA) [25,38],where L, A, and R is the thickness, area, and the fitted resistances ofthe electrode pellets, respectively. The calculation results indicatethat the nitrogen-doped graphene electrodes have much higherelectrical conductivity (2.33 � 10�5 S cm�1) than that of the graph-ene electrodes (1.97 � 10�5 S cm�1), which is consistent with the

Fig. 6. Nyquist plots of the pure graphene and nitrogen-doped graphene after3 cycles, the inset shows equivalent circuit for Nyquist plots.

0 50 100 150 200

0

50

100

150

200

b

c

-Zim

(Ohm

s)

Zre (Ohms)

a

Fig. 7. Nyquist plots of (a) the TiO2/nitrogen-doped graphene nanocomposite, (b)the TiO2/graphene nanocomposite and (c) the bare TiO2 nanoparticles after80 cycles.

58 D. Cai et al. / Journal of Alloys and Compounds 561 (2013) 54–58

data previously reported [25,26,39,40]. Thirdly, the nitrogen-dopedgraphene could limit the aggregation of TiO2 nanoparticles [30].

EIS measurements of the TiO2/nitrogen-doped graphene nano-composite, the TiO2/graphene nanocomposite and the bare TiO2

nanoparticles electrode were carried out after 80 cycles. It is usu-ally considered that the semicircle in the high frequency range isrelated to the charge transfer resistance [35,41]. The Nyquist plotsshow that the diameter of the semicircle for the TiO2/nitrogen-doped graphene nanocomposite in the high–medium frequency re-gion is smaller than those of the TiO2/graphene nanocomposite andthe bare TiO2 nanoparticles (Fig. 7). Therefore, the result indicatesthat the charge transfer resistance of the TiO2/nitrogen-dopedgraphene nanocomposite electrode is much lower than those ofthe TiO2/graphene nanocomposite and the bare TiO2 nanoparticleselectrode. The lower charge transfer resistance of the TiO2/nitro-gen-doped graphene nanocomposite can be associated with the in-creased conductivity of the nitrogen-doped graphene andinterfacial interaction of the nitrogen-doped graphene and TiO2

nanoparticles [30].

4. Conclusions

The TiO2/nitrogen-doped graphene nanocomposite has beenfabricated via a facile gas/liquid interface reaction. The TiO2 nano-particles were homogenously anchored on the nitrogen-dopedgraphene sheets. Moreover, the as-prepared nanocomposite exhib-its a high rate capability and good cycling stability. The excellentelectrochemical performance can be attributed to the improvedelectronic conductivity by the introduction of the nitrogen-dopedgraphene and the positive synergistic effect between the nitro-gen-doped graphene and TiO2 nanoparticles. Furthermore, the fac-ile gas/liquid interface approach could be applied to thepreparation of the nitrogen-doped graphene and other transitionmetal oxides.

Acknowledgments

This work was supported by Program for Pearl River Scholar, byEducational Commission of Guangdong Province for Talent Intro-duction, and by the Fundamental Research Funds for the CentralUniversities, SCUT (2009220038).

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