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Electrochimica Acta 74 (2012) 65– 72

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

igh specific capacity of TiO2-graphene nanocomposite as an anode material forithium-ion batteries in an enlarged potential window

andan Caia, Peichao Liana, Xuefeng Zhub, Shuzhao Lianga, Weishen Yangb, Haihui Wanga,∗

School of Chemistry & Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou 510640, ChinaState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

r t i c l e i n f o

rticle history:eceived 6 January 2012eceived in revised form 26 March 2012ccepted 31 March 2012vailable online 25 April 2012

eywords:

a b s t r a c t

TiO2-graphene nanocomposite was first synthesized by a facile gas/liquid interface reaction. The structureand morphology were characterized by X-ray diffraction, scanning electron microscopy, transmis-sion electron microscopy, and Brunauer–Emmett–Teller measurements. The results indicate that TiO2

nanoparticles (ca. 10 nm in mean grain size) were successfully deposited onto the graphene sheets dur-ing the gas/liquid interfacial reaction process. The electrochemical performance was evaluated by usingcoin-type cells versus metallic lithium in an enlarged potential window of 0.01–3.0 V. A high specific

−1 −1

rapheneiO2

anocompositenode materialithium-ion batteries

charge capacity of 499 mAh g was obtained at a current density of 100 mA g . More strikingly, the TiO2-graphene nanocomposite exhibits excellent rate capability, even at a high current density of 3000 mA g−1,the specific charge capacity was still as high as 150 mAh g−1. The high specific charge capacities can beattributed to the facts that graphene possesses high electronic conductivity, and the lithium storage per-formance of graphene is delivered during discharge/charge processes of TiO2-graphene nanocompositebetween 0.01 and 3.0 V.

© 2012 Elsevier Ltd. All rights reserved.

. Introduction

Lithium-ion batteries are the most attractive secondary batter-es because of their high energy density, long cycling lifetime andxcellent safety. The performance of lithium-ion batteries mainlyepends on the physical and chemical properties of the cathodend anode materials. TiO2 has been regarded as a promising anodeaterial for lithium-ion batteries due to its structural characteris-

ics, low cost, safety and environmental benignity [1–3]. However,ts practical capacity and high-rate capability are limited due to theow Li-ion diffusivity and electronic conductivity during reversiblei-ion insertion/extraction process [4–6]. In order to improve thelectrochemical performance of TiO2 materials, nanotechnologyas been explored to provide increased reaction active sites andhort diffusion lengths for electron and Li-ion transport [7–13].n addition, a variety of approaches have also been developed toncrease the electronic conductivity of the TiO2, such as addingonductive agents (e.g. W [14], RuO2 [15]) and using conductiveoating (e.g. carbon [16–21], Sn [22], Ag [23]).

Graphene, a monolayer of carbon atoms with tight packing ofhe honeycomb lattice, possesses unique physicochemical proper-ies including large surface area, good flexibility, superior electronic

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

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.03.170

conductivity and high chemical stability [24,25]. In the past twoyears, graphene sheets as an anode material for lithium-ion bat-teries have been investigated and exhibit a large reversible specificcapacity (540–1264 mAh g−1) between 0.01 and 3.0 V [26–30]. Inaddition, graphene is also regarded as an ideal carbon nanostruc-ture to improve the rate capability of TiO2 owing to its superiorelectronic conductivity and large surface area. Recently, severalTiO2-graphene composites have been reported in lithium-ionbatteries and other fields [31–38]. For instance, Li et al. [31] synthe-sized mesoporous anatase TiO2 nanospheres/graphene compositesand the composites were applied in lithium-ion batteries andphotocatalysis. Yang et al. [32] fabricated sandwich-like, graphene-based titania nanosheets as an anode for lithium-ion batteries bya nanocasting method. However, all the reported TiO2-graphenenanocomposites were studied only in a narrow potential window of1.0–3.0 V, which lead to low reversible capacity because graphenein these composites only acted as a conductive agent instead of alithium storage material. In the past, in order to increase reversiblecapacity of TiO2-based nanomaterials, the electrochemical perfor-mance in an enlarged potential window was proposed. Marinaroet al. [39] investigated electrochemical behavior of rutile TiO2 usingtwo different potential windows. They found that a high reversible

capacity can be achieved by using an enlarged potential window.Yang et al. [19] prepared nanosized anatase TiO2 loaded porouscarbon nanofibers (TiO2/PCNFs), and the TiO2/PCNFs presented asuper high charge capacity of 687.2 mAh g−1 at a current density

66 D. Cai et al. / Electrochimica Acta 74 (2012) 65– 72

tion p

ostta0a

wtigoi1

2

2

iif

g0wRaTwFd1wsvnuf

2

rmmw(td

Fig. 1. Schematic diagram for the prepara

f 25 mA g−1 between 0.001 and 3.0 V. All the reported resultshowed that TiO2-based materials possess high reversible capaci-ies in an enlarged potential window. Therefore, it can be expectedhat TiO2-graphene nanocomposite should possess a high capacitynd excellent rate capability in the enlarged potential window of.01–3.0 V due to the graphene not only as a conductive agent butlso as a lithium storage material.

In our previous work, Fe3O4/graphene and SnO2/grapheneere successfully synthesized by a facile gas/liquid interface reac-

ion, which is simple and low cost [40,41]. Herein, the gas/liquidnterface reaction method was first used to synthesize TiO2-raphene nanocomposite. The electrochemical performance of thebtained nanocomposite was investigated as an anode for lithium-on batteries in two different potential windows (0.01–3.0 V and.0–3.0 V).

. Experimental

.1. Materials preparation

Graphene sheets were prepared via a thermal exfoliation routenvolving graphite oxidation, followed by rapid thermal expansionn nitrogen atmosphere. Detailed preparation procedure can beound in our previous paper [26].

TiO2-graphene nanocomposite was synthesized by theas/liquid interface reaction [40–42]. Briefly, in a 20 mL beaker,.7589 g of TiCl4 (Tianjin Kermel Chemical Reagent Co., Ltd., China)as dissolved in 20 mL of ethylene glycol (EG) (Beijing Chemicaleagent Co., Ltd., China), and then 0.0983 g of graphene sheets wasdded and sonicated for 10 h to yield a homogeneous suspension.hen the beaker was placed into a 100 mL Teflon-lined autoclaveith 14 mL of ammonia solution (Guangzhou Chemical Reagent

actory, China). Then the autoclave was sealed and placed in arying oven preheated to 180 ◦C and kept at that temperature for2 h. After cooling and centrifugation, washing was carried outith ethyl alcohol (Beijing Chemical Reagent Co., Ltd., China) for

everal times. Then the black solid product was dried at 100 ◦C inacuum and calcined at 450 ◦C to obtain the desired TiO2-grapheneanocomposite. The bare TiO2 nanoparticles were synthesizednder the same conditions without the addition of graphene sheetsor a comparison.

.2. Characterization of materials

The structure and morphology were characterized by X-ay diffraction (XRD) (Bruker D8 Advance), scanning electronicroscopy (SEM) (Quanta 200F) and transmission electronicroscopy (TEM) (FEI, Tecnai G2 F30 S-Twin). The surface area

as measured using the Brunauer–Emmett–Teller (BET) method

Micromeritics analyzer ASAP 2010 (USA)) at liquid nitrogenemperature. Prior to adsorption experiments, the samples wereegassed at 100 ◦C for 4 h. The graphene content was determined

rocess of TiO2-graphene nanocomposite.

by an element analyzer (Vario EL III Elementar, Germany). Thegraphene in the TiO2-graphene nanocomposite was combustedusing pure oxygen in a quartz tube to form carbon dioxide, whichcan be detected by a thermal conductivity detector (TCD). Thecontent of carbon (i.e. graphene sheets) in the TiO2-graphenenanocomposite was calculated according to the mass of carbondioxide. The content of graphene in the TiO2-graphene nanocom-posite was determined to be 19.49 wt.%.

2.3. Electrochemical measurements

The electrochemical performances of the TiO2-graphenenanocomposite and the bare TiO2 nanoparticles were investigatedusing coin cells (CR2032) in two different potential windows.The working electrodes were prepared from a mixture of 75 wt.%active materials (the TiO2-graphene nanocomposite and the bareTiO2 nanoparticles), 15 wt.% Super P carbon black, and 10 wt.%polyvinylidene (PVDF) binder in an N-methyle-2-pyrrolidone(NMP) solvent. The slurries were coated on copper foil. The cel-gard 2325 microporous membrane was used as the separator. Theelectrolyte consisted of solution of 1 mol L−1 LiPF6 in ethylenecarbonate (EC)/diethylcarbonate (DEC) (1:1 by volume). Highlypure lithium foil was used as the counter electrode. The coincells were assembled in an argon-filled glove box (Mikrouna,super 1220) where the oxygen and moisture contents were lessthan 1 ppm.

The cells were galvanostatically discharged and charged intwo different potential windows (0.01–3.0 V and 1.0–3.0 V) usinga Battery Testing System (Neware Electronic Co., China). Cyclicvoltammetry (CV) measurements were carried out on an elec-trochemical workstation (Zahner IM6ex) over the potential rangeof 0.01–3.0 V and 1.0–3.0 V versus Li/Li+ at a scanning rate of0.2 mV s−1. Electrochemical impedance spectra (EIS) of the TiO2-graphene nanocomposite and TiO2 nanoparticles after 30 cycleswere measured at a discharged potential of 1.79 V versus Li/Li+

at the electrochemical workstation in the frequency range from10 mHz to 1 MHz, and the potential perturbation was 5 mV.

3. Results and discussion

3.1. Microstructural characterization

Fig. 1 presents the schematic diagram for the preparation pro-cess of TiO2-graphene nanocomposite. TiCl4 and graphene sheetsin EG solution was stored in the beaker, while aqueous ammo-nia solution (NH3·H2O) was placed in the autoclave liner outsidethe beaker. At the room temperature, the two solutions were sepa-rated by the beaker. At the elevated reaction temperature (180 ◦C),

evaporated ammonia reacted with Ti4+ at the gas/liquid interface toproduce Ti(OH)4, which in situ deposited onto the graphene sheets.Because NH3·H2O is a high volatile liquid and EG is not a good sol-vent for NH3, the reaction between NH3 and Ti4+ was limited at the

D. Cai et al. / Electrochimica

10 20 30 40 50 60

b

(21

1)

(10

5)

(20

0)

(00

4)

(10

1)

2θ / deg ree

a

Fa

gq

T

T

gaaCftinns

tatApspsitn

nAppeppUtst[

−1

ig. 2. XRD patterns of the as-synthesized (a) the TiO2-graphene nanocompositend (b) the bare TiO2 nanoparticles.

as/liquid interface [41,42]. Then the produced Ti(OH)4 could beuickly decomposed to TiO2 at 180 ◦C (Eqs. (1) and (2)).

iCl4 + 4NH3·H2O → Ti(OH)4 + 4NH4Cl (1)

i(OH)4 → TiO2 + 2H2O (2)

It should be pointed out that although the oxygen-containingroups on the graphene sheets has been remarkably reducedfter the heat treatment at the high temperature (1050 ◦C), therere still many residual functional groups including the OH andOOH on the surfaces of graphene sheets [26]. These residual

unctional groups interact strongly with the metal ions duringhe synthesis process of the TiO2-graphene nanocomposite, whichs great beneficial to achieve a homogeneous loading of TiO2anoparticles on the graphene sheets [43,44]. So the homoge-eous and stable TiO2-graphene nanocomposite can be prepareduccessfully.

Fig. 2 shows the XRD patterns of the as-prepared samples. Allhe strong diffraction peaks can be perfectly indexed to the typicalnatase phase structure (JCPDS No. 21-1272), indicating that crys-alline TiO2 could be formed by the gas/liquid interfacial reaction.ll the peaks in the XRD pattern of the TiO2-graphene nanocom-osite are also in good agreement with those reported in otherimilar literatures [31–35]. Furthermore, no obvious diffractioneak attributed to graphite is observed, which indicates that thetacking of graphene sheets in the TiO2-graphene nanocomposites disordered. The broad diffraction peaks of the bare TiO2 nanopar-icles and the TiO2-graphene nanocomposite suggest that the TiO2anoparticles are very small in size.

Fig. 3 presents the SEM and TEM micrographs of the bare TiO2anoparticles, graphene and the TiO2-graphene nanocomposite.s shown in Fig. 3a, the TiO2 nanoparticles aggregate into largearticles. The resulting large particles would lead to the poor rateerformance as an anode material due to the long diffusion path forlectron and Li-ion transport during the Li-ion insertion/extractionrocess. The SEM images of graphene sheets with a curled mor-hology and wavy structures can be observed in Fig. 3b and e [26].ltrathin graphene sheets have been successfully prepared via the

hermal exfoliation method. As shown in Fig. 3c and f, the grapheneheets distributed between the TiO2 nanoparticles can preventhe aggregation of the bare TiO2 nanoparticles to a certain extent36], and improve the electrochemical performance. It should be

Acta 74 (2012) 65– 72 67

pointed out that the random hybridization of TiO2 nanoparticlesand ultrathin graphene sheets can form a three-dimensional porousstructure of the TiO2-graphene nanocomposite, which is great ben-eficial to the rate performance because the electrolyte can soak intothe material through the cavities of the nanocomposite [31,35].The porous structure of the composite can be further confirmedby the subsequent BET measurement. The TEM images (Fig. 3d andf) revealed that the average particle size of the TiO2 nanoparticlesin the both bare TiO2 nanoparticles and TiO2-graphene nanocom-posite is about 10 nm, which is consistent with the result from XRDpatterns. The small nanoparticles can provide short path lengths forelectron and Li-ion transport during the Li-ion insertion/extractionprocess, resulting in excellent rate capability.

Nitrogen isotherm adsorption/desorption curves and the poresize distributions of bare TiO2 nanoparticles and TiO2-graphenenanocomposite are shown in Fig. 4. TiO2-graphene nanocom-posite could be approximately categorized to type IV with atype-H3 hysteresis loop [45], indicating the presence of a meso-porous structure. Such a unique feature provides numerous openchannels for the access of electrolyte and facilitates the ultra-fast diffusion of Li-ion during the cycling processes, which mayimprove the rate performance of lithium-ion batteries. Based on theBarrett–Joyner–Halenda (BJH) equation, the main pore size (inset inFig. 4) in both TiO2-graphene nanocomposite and the TiO2 nanopar-ticles is approximately 10 nm [31,46]. The BET specific surface areasof bare TiO2 nanoparticles and TiO2-graphene nanocomposite areabout 131.4 and 159.4 m2 g−1, respectively. Obviously, the BET spe-cific surface area of TiO2-graphene nanocomposite is higher thanthat of bare TiO2 nanoparticles, which could lead to its increasedelectrochemical reactive activity [9,16,31,32].

3.2. Electrochemical properties of TiO2-graphene nanocompositeand TiO2 nanoparticles

The electrochemical performance of the TiO2-graphenenanocomposite was evaluated by using coin-type cells versusmetallic lithium in two different potential windows. The electro-chemical performance of the bare TiO2 nanoparticles was alsoinvestigated under the same conditions for a comparison.

Fig. 5 shows that the initial discharge (Li-ion insertion) andcharge (Li-ion extraction) curves of TiO2-graphene nanocompos-ite at a current density of 100 mA g−1 in two different potentialwindows of 1.0–3.0 V and 0.01–3.0 V. Two distinct voltage plateausare clearly visible at ∼1.75 V (discharge) and ∼1.92 V (charge) inthe above both potential windows. However, the composite showsa high specific charge capacity of 499 mAh g−1 in the enlargedpotential window (0.01–3.0 V). Such a high specific charge capacityof the composite is superior to that of previously reported TiO2-graphene nanocomposites. Graphene in the previously reportedcomposites just acted as a conductive agent, rather than a lithiumstorage material since they were studied only in the narrow poten-tial window of 1.0–3.0 V. Herein, the TiO2-graphene nanocompositein the enlarged potential window (0.01–3.0 V) possesses a highspecific charge capacity, which can be attributed to the fact thatgraphene not only acts as a conductive agent but also as a lithiumstorage material during Li-ion insertion/extraction process of TiO2-graphene nanocomposite.

Fig. 6 presents that the initial discharge/charge curves of theTiO2-graphene nanocomposite and the bare TiO2 nanoparticlesat a current density of 100 mA g−1 between 0.01 and 3.0 V. Thespecific charge capacity of the TiO2-graphene nanocomposite (ca.499 mAh g−1) is far higher than that of the bare TiO2 nanoparti-

cles (ca. 287 mAh g ) between 0.01 and 3.0 V. The higher specificcharge capacity of the TiO2-graphene nanocomposite than the bareTiO2 nanoparticles can be possibly explained by the followingtwo reasons. One reason is that the lithium storage performance

68 D. Cai et al. / Electrochimica Acta 74 (2012) 65– 72

F d the T( e bare

oTrninc

ig. 3. SEM and TEM observation of the bare TiO2 nanoparticles, graphene sheets anb) graphene sheets and (c) the TiO2-graphene nanocomposite. TEM images of (d) th

f graphene is delivered during discharge/charge processes ofiO2-graphene nanocomposite between 0.01 and 3.0 V. Anothereason is that graphene sheets prevent the aggregation of the TiO2

anoparticles, increasing electrochemical reactive activity and thus

ncreasing the specific charge capacity of the TiO2 in TiO2-grapheneanocomposite. Considering the 19.49 wt.% (ω) graphene in theomposite, one can estimate the specific charge capacity of the TiO2

iO2-graphene nanocomposite: SEM micrographs of (a) the bare TiO2 nanoparticles, TiO2 nanoparticles, (e) graphene sheets and (f) the TiO2-graphene nanocomposite.

in TiO2-graphene nanocomposite is still as high as 350 mAh g−1

from Eq. (3).

CT = CTG − ω CG

1 − ω(3)

where CT, CTG, and CG are the specific charge capacities of the TiO2in TiO2-graphene nanocomposite, TiO2-graphene nanocomposite,

D. Cai et al. / Electrochimica Acta 74 (2012) 65– 72 69

0 20 40 60 80 10 0 12 0 140 160 180

0.00 0

0.00 5

0.01 0

0.01 5

0.02 0

0.02 5

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

Po

re v

olu

mn

/ c

m3g

-1n

m-1

Pore si ze / nm

Vo

lum

e A

dso

rbed

/ c

m3

g-1

Relative pressure , (P/P0)

SBET = 131 .4 m2 g

-1

a

0 20 40 60 80 100 120 140

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

350

Po

re v

olu

mn

/ c

m3g

-1n

m-1

Pore size / nm

SBET = 159.4 m2 g

-1

Vo

lum

e A

dso

rbed

/ c

m3

g-1

Relative pressure , (P/P0)

b

Fig. 4. Nitrogen isotherm adsorption/desorption curves of (a) TiO2 nanoparticlesat

ac

n33peiAcTogtoTifitstpl

0 20 0 40 0 60 0 80 0 100 0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Po

ten

tial

vs

. (L

i/L

i+)

/ V

Specific capacity / mAh g-1

0.01-3.0 V

1.0-3.0 V

potential window (0.01–3.0 V, 45.0%) shows lower coulombic effi-ciency in the first cycle than that in typical potential window(1.0–3.0 V, 71.8%). The relatively low coulombic efficiency in thefirst cycle is a general phenomenon for carbonaceous electrode

0 200 400 600 80 0 1000 12 00 14 00

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Po

ten

tial

vs.

(Li/

Li+

) /

V

Specific capacity / mAh g-1

TiO2-graphene nanocomposite

TiO2 nanoparticles

nd (b) TiO2-graphene nanocomposite. The insets show pore-size distributions ofhe corresponding samples.

nd graphene sheets, respectively. Among these, the specific chargeapacity of graphene sheets is 1116 mAh g−1 [41].

Fig. 7 presents the cyclic voltammograms of the TiO2-grapheneanocomposite and the bare TiO2 nanoparticles between 1.0 and.0 V, and the TiO2-graphene nanocomposite between 0.01 and.0 V at a scanning rate of 0.2 mV s−1. As shown in Fig. 7a and b, aair of cathodic (Li-ion insertion) and corresponding anodic (Li-ionxtraction) peaks was observed near 1.75 and 1.92 V, respectively,n accordance with those in the previous reports [11,16,32,36].s shown in Fig. 7, the cathodic and corresponding anodic peakshange significantly in amplitude and voltage positions for the bareiO2 nanoparticles during the subsequent cycles, while there is nobvious change in amplitude and voltage positions for the TiO2-raphene nanocomposite during the subsequent cycles, indicatinghat the TiO2-graphene nanocomposite shows higher reversibilityf the electrochemical reactions than the bare TiO2 nanoparticles.he broad current peaks at ∼0.5 V (Fig. 7c), are assigned to therreversible reduction of the electrolyte and the formation of SEIlm [19,39]. The current peaks at ∼0 V (Fig. 7c), are related withhe complicated insertion of Li-ion into the graphene matrix [19],uggesting graphene acts as an anode material for lithium-ion bat-

eries in the enlarged potential window during discharge/chargerocesses of the TiO2-graphene nanocomposite. So the increased

ithium storage sites of the TiO2-graphene nanocomposite partially

Fig. 5. The initial discharge/charge curves of TiO2-graphene nanocomposite at acurrent density of 100 mA g−1 in two different potential windows of 1.0–3.0 V and0.01–3.0 V.

depend on graphene between 0.01 and 3.0 V, which is consistentwith the result of the initial discharge/charge tests.

Fig. 8a compares the cycling performance and coulombic effi-ciencies of the TiO2-graphene nanocomposite and the bare TiO2nanoparticles at various current densities between 0.01 and 3.0 V.Compared to the bare TiO2 nanoparticles, the specific capacitiesof TiO2-graphene nanocomposite are substantially increased at allinvestigated discharge/charge current densities from 300 mA g−1

to 3000 mA g−1 between 0.01 and 3.0 V. For example, the specificcharge capacity of the composite is 150 mAh g−1 at a current densityof 3000 mA g−1 between 0.01 and 3.0 V, which is three times higherthan that of the bare TiO2 nanoparticles. Such an excellent rate per-formance of the TiO2-graphene nanocomposite may be attributedto high electronic conductivity of graphene and the porous struc-ture of the TiO2-graphene nanocomposite [9,11,31,35]. Moreover,after the high current density measurements, the specific chargecapacity of the TiO2-graphene nanocomposite can recover to theinitial value (ca. 411 mAh g−1), indicating an excellent cycle perfor-mance. However, the TiO2-graphene nanocomposite in an enlarged

Fig. 6. The initial discharge/charge curves of TiO2-graphene nanocomposite andnanoparticles at a current density of 100 mA g−1 between 0.01 and 3.0 V.

70 D. Cai et al. / Electrochimica Acta 74 (2012) 65– 72

1.0 1.5 2.0 2.5 3.0-1.0

-0.5

0.0

0.5

1.0

1.5 1

st

2nd

3rd

Sp

ec

ific

cu

rre

nt

/A g

-1

Pote ntial vs. (Li /Li+) /V

1.0 1.5 2.0 2.5 3.0-1.0

-0.5

0.0

0.5

1.0

1.5

1st

2nd

3rd

Sp

ec

ific

cu

rren

t /A

g-1

Potent ial vs. (Li/Li+) /V

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-0.4

-0.2

0.0

0.2

0.4 1st

2nd

3rd

Sp

ecif

ic c

urr

en

t /A

g-1

Potential vs. (Li/ Li+) /V

a

b

c

Fig. 7. Cyclic voltammograms of (a) TiO2-graphene nanocomposite, (b) TiO2

n0

mf

cnTid

Fig. 8. Comparison of the cycling performance of TiO2-graphene nanocompositeand TiO2 nanoparticles at various current densities in two different potential win-

graphene nanocomposite electrode is much lower than that of

anoparticles between 1.0 and 3.0 V, and (c) TiO2-graphene nanocomposite between.01 and 3.0 V at a scanning rate of 0.2 mV s−1.

aterials as the existence of irreversible Li insertion sites and theormation of a solid electrolyte interphase (SEI) [19].

Fig. 8b compares the cycling performance and coulombic effi-iencies of the TiO2-graphene nanocomposite and the bare TiO2anoparticles at various current densities between 1.0 and 3.0 V.

he TiO2-graphene nanocomposite delivers a higher specific capac-ty compared to the bare TiO2 nanoparticles at all investigatedischarge/charge current densities between 1.0 and 3.0 V which

dows of (a) 0.01–3.0 V and (b) 1.0–3.0 V, the insets show the coulombic efficiencies ofTiO2-graphene nanocomposite and TiO2 nanoparticles electrodes at various currentdensities in two different potential windows.

is similar with the results between 0.01 and 3.0 V. However, theTiO2-graphene nanocomposite possesses a higher capacity in theenlarged potential window of 0.01–3.0 V than that in the narrowpotential window of 1.0–3.0 V. This is because the lithium stor-age performance of graphene is delivered during discharge/chargeprocesses of the TiO2-graphene nanocomposite between 0.01 and3.0 V. In addition, the bare TiO2 nanoparticles shows lower coulom-bic efficiency in the first cycle between 0.01 and 3.0 V (40.1%),compared to potential window of 1.0 and 3.0 V (72.7%). The reasonof this phenomenon may be the existence of irreversible Li insertionsites below 1.0 V [39].

To understand the improved rate capability, EIS on the cellcomprising the TiO2-graphene nanocomposite or the bare TiO2nanoparticles electrode as the working electrodes versus metalliclithium were carried out after 30 cycles at a discharged potentialof 1.79 V. The semicircle in the high frequency range is relatedto the charge transfer resistance [31,32]. As the Nyquist plotsshown in Fig. 9, the diameter of the semicircle for the TiO2-graphene nanocomposite in the high-medium frequency regionis much smaller than that of the bare TiO2 nanoparticles. Thisresult indicates that the charge transfer resistance of the TiO2-

the bare TiO2 nanoparticles electrode. The lower charge transferresistance can contribute to the higher rate capability of the TiO2-graphene nanocomposite compared to the bare TiO2 nanoparticles.

D. Cai et al. / Electrochimica Acta 74 (2012) 65– 72 71

Table 1Fitted impedance parameters of the TiO2-graphene nanocomposite and the bare TiO2 nanoparticles.

Samples R� (�) R1 (�) R2 (�) Rct (�) �w (� cm2 s−1/2)

TiO2-graphene nanocomposite 0.82 14.28

TiO2 nanoparticles 3.91 11.28

Fi

Tr

wRstatiratitigTttnbatin

4

geec(dpb

[

[[

[

[[[

[[[

[[[

[

[[[

[

[[

[

ig. 9. Nyquist plots of TiO2-graphene nanocomposite and TiO2 nanoparticles, thenset shows equivalent circuit for Nyquist plots.

herefore, the TiO2-graphene nanocomposite exhibits an excellentate performance.

To further understand the electrochemical processes, EIS resultsere fitted using an equivalent circuit. In the equivalent circuit,

� indicates the uncompensated bulk resistance of the electrolyte,eparator and electrode; the (C1R1) parallel element correspondso the SEI film, electrode roughness and inhomogeneous reactiont the surface; the (C2R2) parallel element might be attributedo the possible breakdown of electrolyte and electrode materialnternal microstructures; Rct is attributed to the charge-transferesistance at the active material interface; Cd is the constant phasengle element, involving double layer capacitance; W representshe Warburg impedance related to the diffusion of lithium-ionnto the bulk electrodes [32,47]. The fitted impedance parame-ers of the equivalent circuit are listed in Table 1. The Warburgmpedance coefficient (�w) is 81.17 � cm2 s−1/2 for the TiO2-raphene nanocomposite, which is lower than that of the bareiO2 nanoparticles (209.38 � cm2 s−1/2). Furthermore, the values ofhe uncompensated bulk resistance (R�) and charge-transfer resis-ance (Rct) are 0.82 and 36.34 �, respectively, for TiO2-grapheneanocomposite, which are significantly lower than those of theare TiO2 nanoparticles (3.91 and 67.40 �). Both the R� and Rct

re related to the conductivity of the electrode [32]. Therefore,he results demonstrate that the TiO2-graphene nanocompos-te exhibits much higher conductivity than the bare TiO2anoparticles.

. Conclusions

TiO2-graphene nanocomposite was first synthesized by a facileas/liquid interface reaction. The TiO2-graphene nanocompositexhibits a high capacity and excellent rate performance in thenlarged potential window of 0.01–3.0 V. The excellent electro-hemical performance can be attributed to the following reasons:

1) the lithium storage performance of graphene is delivereduring discharge/charge processes of the TiO2-graphene nanocom-osite between 0.01 and 3.0 V; (2) graphene sheets distributedetween the TiO2 nanoparticles can minimize the aggregation of

[

[[

41.87 36.34 81.17146.70 67.40 209.38

the TiO2 nanoparticles; (3) the excellent electronic conductivity ofgraphene can facilitate electron transfer; (4) mesoporous structureof the TiO2-graphene nanocomposite provide short path length forboth electron and Li-ion transports and large electrode/electrolytecontract area. The electrochemical test results indicate that theTiO2-graphene nanocomposite prepared by the gas/liquid interfa-cial method is a promising anode material for lithium-ion batterieswith high specific capacity and excellent rate performance.

Acknowledgements

This work was financially supported by the National Natural Sci-ence Foundation of China (No. 20936001), the Cooperation Projectin Industry, Education and Research of Guangdong Province andMinistry of Education of China (No. 2010B090400518) and theFundamental Research Funds for the Central Universities, SCUT(2009220038).

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