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Electrochimica Acta 55 (2010) 5813–5818

Contents lists available at ScienceDirect

Electrochimica Acta

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raphene as a conductive additive to enhance the high-rate capabilities oflectrospun Li4Ti5O12 for lithium-ion batteries

an Zhua,1, Wen Liub,1, Mianqi Xuea, Zhuang Xiea, Dan Zhaoa, Meining Zhanga,itao Chenb,∗, Tingbing Caoa,∗∗

Department of Chemistry, Renmin University of China, Beijing 100872, PR ChinaBeijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China

r t i c l e i n f o

rticle history:eceived 6 March 2010eceived in revised form 27 April 2010

a b s t r a c t

Spinel Li4Ti5O12 (LTO) is a promising candidate anode material for Li-ion batteries due to its well-knownzero-strain merits. To improve the electronic properties of spinel LTO, which are intrinsically poor, weprocessed the material into a nanosized architecture to shorten the distance for Li-ion and electron

ccepted 6 May 2010vailable online 13 May 2010

eywords:lectrospinningraphene

transport using the versatile electrospinning method. Graphene was chosen as an effective carbon coatingto improve the surface conductivity of the nanocomposites. The as-prepared graphene-embedded LTOanode material showed improved discharging/charging and cycling properties, particularly at high rates,such as 22 C, which makes the nanocomposite an attractive anode material for applications in electricvehicles.

ithium-ion batteryigh-rate charging/discharging

. Introduction

Rechargeable Li-ion batteries (LIBs) have played an indispens-ble role in today’s mobile and information-rich society as keyomponents of portable entertainment, computing and telecom-unication devices [1]. The high energy density of LIBs has

nfluenced their current commercial success, but the low rateapability of LIBs has limited their use in important applicationsuch as hybrid electric vehicles (HEVs) and portable power toolshat require fast charging and discharging at high power rates2]. To improve the rate performance and reduce the cost of thelectrochemically active electrodes, nanostructured materials aresually adopted to solve the kinetic problems associated withhe solid-state diffusion of Li+ intercalation and electronic con-uctivity [3]. Soft-chemistry routes as well as template synthesesre generally used to prepare nanostructured electrode materi-ls [4–8]; however, during the subsequent processing procedures,

uch as high-temperature crystallization, some of the nanostruc-ured materials tend to aggregate, losing their high surface areasnd associated merits [6].

∗ Corresponding author. Tel.: +86 10 62514332; fax: +86 10 62516444.∗∗ Corresponding author. Tel.: +86 10 62754112; fax: +86 10 62757908.

E-mail addresses: chenjitao@pku.edu.cn (J. Chen), tcao@chem.ruc.edu.cnT. Cao).

1 These authors contributed equally to this work.

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.05.029

© 2010 Elsevier Ltd. All rights reserved.

To preserve the benefits of electrochemistry at nanoscale and toachieve high-rate capabilities, Martin et al. [7] grew nanostructuredactive insertion electrode materials onto a plane current collec-tor using a membrane as a template to achieve a nanostructuredelectrode design. Simon et al. [4] coated electrochemically activematerials onto a nanoporous current collector so that each particleof active material had its own current collector. Hammond et al.[8] assembled electrode materials into nanowires using virus as atemplate. Aside from these characteristic designs, one simple andversatile approach for constructing nanostructured electrodes iselectrospinning [9]. Recent developments in nanoelectrode materi-als for lithium-ion batteries by electrospinning [10–12] have showngreat advantages over other processing methods, such as highercharge/discharge rates resulting from higher electrode/electrolytecontact area, better accommodation of the strain of lithium inser-tion/extraction with improved cycle life and shorter path lengthsfor electron and Li+ transport [3,13].

When pursuing high-rate capabilities for LIBs using nanostruc-tured materials, the issue of safety remains a top priority becausethese electrode materials are expected to perform in much roughercircumstances in HEVs than in portable electronic devices. Bulkgraphite and other metals or intermetallics are capable of thereversible accommodation of lithium and show very high charge

densities as promising anode materials for LIBs in HEVs. However,the huge volume changes during the Li+ insertion/extraction cyclewould inevitability lead to cracking and crumbling of the electrode,and dendritic lithium growth on a graphite-based anode surface athigh charging current raises safety concerns due to the formation of

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nternal short circuits [14], hindering their application in recharge-ble batteries for HEVs. Recently, spinel-type Li4Ti5O12 (LTO) hasttracted increasing attention as an anode material for LIBs largelyue to its zero-strain characteristics [15,16] and the absence ofurface lithium formation [3] during lithium insertion/extractionhen compared with other anode materials [17]. The state-of-

he-art Li-ion battery electrode material combining good capacity,xcellent cyclability and high-rate capability is expected to be theesired safe anode of LIBs for HEVs. Nevertheless, the intrinsi-ally poor conductivity of LTO at room temperature (<10−13 S cm−1)18,19] is a barrier to its practical applications.

In addition to the surface coating of conductive materials, thereparation of submicron or nanosized LTO [5] significantly short-ning the diffusion length of Li+ has been widely used for improvinghe electronic conductivity of this material [14,18]. Some hybridanostructures, like nanocomposites with nanoparticles embed-ed in a carbon matrix [20,21], have demonstrated an increasedapacity at high charge/discharge rates. While the nanocompositesffer significant advantages, perfect surface coatings and desiredixtures are often very difficult to achieve.Graphene, a two-dimensional macromolecular sheet of car-

on atoms with a honeycomb structure, has excellent electroniconductivity and mechanical properties, and may be the idealonductive additive for hybrid nanostructured electrodes [22–26].raphene-based materials have superior electronic conductiv-

ty compared to graphitic carbon, a high surface area of over600 m2 g−1, chemical tolerance and a broad electrochemical win-ow that could be very advantageous for applications in energyechnologies [23,24]. At the same time, the specific charge capacityf graphene (540 mAh g−1) is much larger than that of graphite, andhe graphene families obtained to date show very large reversibleapacities attributed to the increased basal spacing [23]. The usesf graphene to form composite materials with SnO2 [24] and TiO225] have also been reported to improve the capacity and cyclictability of anode materials.

In this investigation, we chose electrospinning as a generalethod to prepare graphene/LTO nanocomposites to improve the

lectrochemical properties of spinel LTO in LIBs. The BET surfacerea test, discharging/charging and other related measurementshowed that electrospun LTO had greatly improved surface areaver the sol–gel prepared material, which easily aggregates duringalcination at high temperature. Additionally, graphene can signif-cantly enhance the electronic conductivity of the anode material,

hich leads to much higher rate capabilities than other types ofybrid nanocomposites.

. Experimental

All materials and chemicals were purchased commercially andsed as received. Graphite oxide was prepared from graphite pow-er following the method described by Hummers and Offeman27]. Titanium (IV) isopropoxide (Ti[OCH(CH3)2]4, 98%) was pur-hased from Acros Organics. Lithium acetylacetonate ([C5H8O2]Li,7%) was purchased from Aldrich. Poly(vinylpyrrolidone) (PVP,90, Mw = 360,000) was purchased from Beijing Yili Fine Chemicalo., Ltd. Other chemical reagents were purchased from Sinopharmhemical Reagent Beijing Co. The high-voltage supply was pur-hased from BGG, Bmei. Co., Ltd., China. The syringe pump was aS2-60 Dual-syringe Pump from Baoding Longer Precision Pumpo., Ltd., China.

.1. Fabrication of graphene-embedded LTO nanofibers bylectrospinning

First, to prepare the electrospinning solution [28], 0.43 g ofitanium (IV) isopropoxide and 0.141 g of lithium acetylacetonate

ta 55 (2010) 5813–5818

(controlling the molar ratio of Li:Ti to 4.4: 5) were mixed with 2 mLof ethanol (prior to drying over molecular sieves) and 1 mL of aceticacid. Following, this solution was added to 2 mL of ethanol solutioncontaining 0.178 g of PVP, followed by ultrasonic stirring until thesolution was clarified. The freshly synthesized graphite oxide wasthen added to the above mixture (1 wt.% of the theoretical LTO) andtreated with ultrasonication. The mixture was immediately loadedinto a plastic syringe equipped with an N6-gauge needle made ofstainless steel. The needle was connected to a high-voltage supplythat was generating a DC voltage of 12.5 kV. The feeding rate for theprecursor solution was 0.3 mL h−1 using a syringe pump. A piece offlat aluminum foil was placed around 13 cm below the tip of theneedle to collect the nanofibers. The electrospinning process wasconducted in air.

2.2. Characterization of the structure and physical parameters ofthe nanofibers

The graphite oxide in the electrospun nanofibers was reducedto graphene using hydrazine hydrate vapor at 100 ◦C for 24 h [29].After the reduction process, there was a considerable increase of theC/O atomic ratio in the reduced material (10.3) compared to thatin the starting graphite oxide (2.7). The results of Raman measure-ments also demonstrated that the graphite oxide had been at leastpartially deoxygenated [29–31]. The prepared nanofibers were cal-cined at 550 ◦C for 3 h in a protective atmosphere of N2 with a rampof 5 ◦C min−1 from room temperature to 550 ◦C.

The SEM images were recorded with a JEOL 7401 microscope,the AFM images were recorded using a Veeco D3100 instrumentand TEM images were recorded with a JEOL JEM-2010F. Specificsurface areas and pore size distributions of the porous nanofiberswere evaluated using the Brunauer–Emmett–Teller (BET) equa-tion (ASAP2020, Micromeritics, USA) after preheating the samplesto 150 ◦C for 2 h to eliminate adsorbed water. The X-ray diffrac-tion (XRD) patterns were recorded with a MAXima-X XRD-7000,Shimadzu, Japan. The Carbon-Sulfur Analytical Instrument was pro-duced by Elemetar Vario MICRO CUBE.

2.3. Electrochemical measurements

The charge–discharge capacities were measured with atwo-electrode cell. Graphene-embedded LTO nanofibers, SuperPand Teflon (poly(tetrafluoroethylene), PTFE) binder were mixed(weight ratio 70:20:10), ground into a paste and formed into acircle ∼1 cm in diameter. The anode was dried at 120 ◦C undervacuum overnight. The active material we fabricated was used asthe working electrode, and Li foil was used for both counter andreference electrodes with 1.0 M LiClO4 in an electrolyte solution(EC/DEC = 1/1, v/v); the cell was then assembled as a coin cell. Thecell assembly was carried out in a glove box filled with high-purityargon gas.

Galvanostatic discharge–charge measurements (CT2001, LAND,China) were performed over a potential range of 1.0–3.0 V vs.Li+/Li at different current densities. Also, AC impedance measure-ments of the unit cell were performed in the frequency rangeof 10 mHz–100 kHz using an electrochemical impedance analyzer(Autolab PG302, Metrohm AG).

3. Results and discussion

Fig. 1 schematically illustrates the procedure used to pre-

pare the electrospun LTO nanofibers and graphene-embeddedLTO nanocomposites. The precursors of LTO were mixed with apoly(vinylpyrrolidone) (PVP) ethanol solution and transformedinto a white nanofiber web using an electrospinning setup. Toharvest the graphene-embedded nanocomposites, graphite oxide

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ig. 1. Schematic diagram of the laboratory setup for electrospinning and the illus-ration of the as-prepared graphene-embedded LTO nanofibers.

repared from graphite powder using the method describedy Hummers and Offeman [27] was added into the LTO/PVPolution prior to electrospinning. After electrospinning, graphitexide in the nanofibers was reduced to graphene nanosheets byydrazine hydrate vapor [29] at 100 ◦C to gain superior electroniconductivity.

The carbon content of electrospun LTO/graphene nanocompos-tes after calcination in an N2 atmosphere was 7.2%, as measured byhe “Carbon-Sulfur Analytical Instrument”. The carbon content ofhe electrospun LTO nanocomposites without graphene was mea-ured as 6.3%. The results above are in good agreement with thewt.% graphene added to the electrospun nanocomposites.

.1. Morphology of the graphene-embedded LTO nanocomposites

To compare the priority of the electrospinning method withhe traditional process, we synthesized the LTO powder shown in

ig. 2. SEM observation of sol–gel LTO powders, electrospun LTO and graphene-embeddeC and D) electrospun LTO nanofibers before and after calcination; (E and F) graphene-em

agnification images of (D) and (F), respectively.

ta 55 (2010) 5813–5818 5815

Fig. 2A and B using the conventional sol–gel method. From the SEMimages, we could clearly investigate the agglomeration of sol–gelLTO powders before and after calcination at 550 ◦C in an N2 atmo-sphere. Spheroid, micrometer-, submicron- or nanometer-sizedLTO particles tended to aggregate into massive chunks, particularlyduring high-temperature crystallization. Fig. 2C and D shows theSEM images of electrospun LTO nanofibers before and after calci-nation at 550 ◦C in an N2 atmosphere. From these images, we foundimproved fibrous morphologies with uniform diameters below1 �m. Although the high-temperature calcination reduced the longnanofibers into shorter branches, the agglomeration phenomenonwas effectively eliminated through the electrospinning process.Graphene-embedded LTO nanocomposites, as shown in Fig. 2E andF, had morphologies similar to the electrospun LTO nanofibers, andthe high-temperature calcination process altered the morphologylittle, whereas under higher magnification (as shown in Fig. 2G andH) the electrospun LTO nanofibers exhibited some nanoparticle-like branches on the fiber surfaces, unlike the graphene-embeddedLTO nanocomposites, which had a much smoother fibrous mor-phology. We concluded that electrospun graphene-embedded LTOnanocomposites exhibited a noticeably smooth surface due tographene encapsulation, which prevented the LTO from formingnanoparticle-like crystals outside the surface of the electrospunfibers.

3.2. Physical properties and characterization of thegraphene-embedded LTO nanocomposites

High-temperature calcination is a necessary step for the crys-tallization of LTO before assembling the material as an anode

in LIBs. The X-ray diffraction (XRD) patterns of the sol–gel LTOpowder, electrospun LTO nanofibers and graphene-embedded LTOnanocomposites are shown in Fig. 3A. According to the relevantliterature [32], the diffraction peaks located at 2� of 18.4◦, 35.6◦,43.3◦, 57.3◦ and 62.8◦ can be indexed to the crystal faces (1 1 1),

d LTO nanocomposites. (A and B) Sol–gel LTO powders before and after calcination;bedded LTO nanocomposites before and after calcination. (G) and (H) show high

5816 N. Zhu et al. / Electrochimica Acta 55 (2010) 5813–5818

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ig. 3. (A) XRD patterns of the graphene-embedded LTO nanocomposites (I), electromage of graphene nanosheets with the measured thickness. (C and D) HRTEM ima

3 1 1), (4 0 0), (3 3 3) and (4 4 0) of spinel LTO, respectively. Theesults indicate that the graphene-embedded LTO nanocompos-tes were very well crystallized after calcination at 550 ◦C in an

2 atmosphere [33]. In contrast, small diffraction peaks locatedt 2� of 27.5◦ and 54.4◦ were observed in electrospun LTO andol–gel LTO, which were indexed to the (1 1 0) and (2 1 1) planes,espectively, of rutile TiO2 (generated from the precursors of LTOhen the sintering temperature was set at 550 ◦C). Generally,igh-purity LTO is prepared by firing the sol–gel derived pre-ursors at 700–750 ◦C; however, at very high temperatures, its too difficult for the electrospun LTO and graphene-embeddedanocomposites to retain their nanofibrous morphology. Addition-lly, LTO is apt to aggregate into chunks just as with the sol–gelpecies, while graphene will easily transform into graphite at ele-ated temperatures [25]. We found that the graphene-embeddedanocomposites could be crystallized at much lower temperatureshan sol–gel LTO, with no peaks indexed to rutile TiO2. We speculatehat graphene-embedded LTO nanocomposites have smaller grainize and thus undergo crystal phase transformations more readilyhan those prepared by the sol–gel method or by electrospinning

nly. We systemically tested different calcination temperaturesor graphene-embedded LTO nanocomposites between 450 and50 ◦C, and found that 550 ◦C was the optimal condition to enableheir complete crystallization while retaining their nanofibrous

orphologies.

LTO nanofibers (II) and sol–gel LTO powder (III). (B) Atomic force microscope (AFM)electrospun LTO and graphene-embedded LTO nanocomposites.

Fig. 3B shows an atomic force microscope (AFM) image ofas-prepared graphene nanosheets with a measured thickness ofapproximately 1 nm, indicating the presence of less than fivelayers of graphene. To further investigate the morphology ofgraphene in LTO nanocomposites, high-resolution transmissionelectron microscopy (HRTEM) was used to characterize the struc-ture of electrospun LTO and graphene-embedded LTO nanofibers(as shown in Fig. 3C and D, respectively). Here, crystallized LTOcan be clearly observed in the HRTEM image of the electrospunnanofibers (Fig. 3C), which were indexed to the crystal faces(1 1 1), (3 1 1) and (4 4 0), respectively. From the HRTEM image ofgraphene-embedded LTO nanocomposites shown in Fig. 3D, wefound submicron or nanosized crystalline LTO clusters that werecoated by a uniform graphene sheet adjacent to their crystal phase.The graphene nanosheets were predominantly in the form of a fewlayers of accumulation, with some single layers that could be distin-guished from the nanocomposites; single layer corresponded to theplane face (0 0 2). In good agreement with the SEM image shown inFig. 2H, the HRTEM image indicates that graphene led to the smoothsurface of the electrospun LTO nanocomposites. The graphene con-

tent was very low in the nanocomposites (only 1 wt.%) of the weightof the whole active material, which resulted in the difficulty in iden-tifying graphene in Fig. 3D. To clarify that the surface layer of theparticles was graphene, PVP nanofibers were also fabricated sepa-rately by electrospinning, and the pyrolysis carbon obtained from

N. Zhu et al. / Electrochimica Ac

Fig. 4. (A) Nitrogen adsorption-desorption isotherms for graphene-embedded LTOnanofibers (black), electrospun LTO nanofibers (blue) and sol–gel LTO powder (red).(B) AC impedance measurements of graphene-embedded LTO nanocomposites,electrospun LTO nanofibers and sol–gel LTO powders (measured in the frequencyrEt

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model). The highly conductive graphene remarkably enhanced thesurface conductivity of the LTO nanocomposites, which directly ledto improved high-rate capabilities of the anode material.

ange from 0.01 Hz to 100 kHz). The inset figure shows the equivalent circuit for theIS measurement. (For interpretation of the references to color in this figure legend,he reader is referred to the web version of the article.)

he electrospun PVP after calcination at 550 ◦C had an amorphoustructure as determined by X-ray diffraction (XRD).

.3. Analysis of graphene-embedded LTO nanocomposites by BETnd EIS

Electrospinning is a simple and powerful method for prepar-ng nanomaterials with remarkable enhancements in porosity.o characterize the surface area of the anode materials, nitro-en adsorption-desorption measurements were carried out onhe graphene-embedded LTO nanocomposites together withlectrospun LTO and the sol–gel prepared powder. From therunauer–Emmett–Teller (BET) surface area of the three mate-ials (Fig. 4A), we found that the electrospun LTO and theraphene-embedded nanocomposite both had surface areas ofround 170 m2 g−1, which was approximately hundreds timesigher than that of the sol–gel LTO powder. These results were inood agreement with the SEM and TEM images of the noticeableanometer-sized diameters and morphologies.

Nanostructures cannot only boost the surface area of LTO

anocomposites, but also shorten the path lengths for electron andi+ transport. Simultaneously, graphene also enhances the conduc-ivity of as-prepared LTO nanocomposites. To further understandhe high-rate performance of these anode materials, electrochemi-al impedance spectroscopy measurements (EIS) of the three types

ta 55 (2010) 5813–5818 5817

of LTO hybrids were performed, and the results are shown in Fig. 4B.The measurements were operated after the cells had cycled fivetimes and were equalized at 1.56 V for 20 h [19].

Fig. 4B shows the Nyquist plot of the data, which includes twooverlapping semicircles at high frequency, including both Rct andRSEI. Such EIS patterns can be fitted into an equivalent circuit,as shown in the inset figure [18]. A simplified equivalent circuitmodel was constructed to analyze the impedance spectra using the“Zview 2.0” software [34]. The fitting goodness between the exper-imental (white circles) and calculated (black lines) values are alsoshown in Fig. 4B; here, we found that the Rct of the hybrid materialLTO obtained using the electrospinning method was slightly betterthan that of the sol–gel prepared LTO, considering that pyrolysiscarbon obtained from electrospun PVP can also act as a conduc-tive matrix. Furthermore, only small addition of graphene (1 wt.%)to the nanocomposites dramatically reduced the Rct from above212 ohm to only 36 ohm (as calculated from the equivalent circuit

Fig. 5. Electrochemical characterization of graphene-embedded LTO nanocompos-ites, electrospun LTO nanofibers and sol–gel LTO powders between 1 and 3 V vs.Li+/Li. (A) Rate capabilities of the three materials at different charge/discharge rates(0.2, 1, 2, 5, 8, 10 and 22 C). (B) Cycling performance of graphene-embedded LTOnanofibers up to 1300 cycles at 22 C; the inset shows the corresponding specificcapacity profile.

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.4. Electrochemical properties of the graphene-embedded LTOanocomposites

To examine the effectiveness of graphene and nanostruc-ures in improving the rate capability of the anode electrode,e investigated the specific capacity and cycling performance

f graphene-embedded LTO nanocomposites, electrospun LTOanofibers and the sol–gel LTO powder at different dis-harge/charge rates. As shown in Fig. 5A, with the incorporation ofraphene in LTO nanofibers, the specific capacity of the nanocom-osites increased at different discharge/charge rates in comparisonith the electrospun LTO and sol–gel prepared powder. The rel-

tive increment in specific capacity was particularly larger atigher charging and discharging rates. For instance, at a dischargingnd charging rate of 22 C, the specific capacity of the graphene-mbedded LTO nanocomposites was around 110 mAh g−1, whichs more than twice the rate capacity (47 mAh g−1) of the electro-pun LTO and three times the rate capacity (34 mAh g−1) of theol–gel LTO powder. The rate performance of graphene-embeddedTO nanocomposites calcined at 750 ◦C was also tested. Fromhe results, it was evident that the graphene-embedded LTOanocomposites had better electrochemical performance whenreated at 550 ◦C than at 750 ◦C, especially at high charge/dischargeate. The specific capacities at 0.2 C and 8 C were 164 mAh g−1

nd 137 mAh g−1 (83.5% retained) at 550 ◦C while they were45 mAh g−1 and 94 mAh g−1 (64.8% retained) at 750 ◦C.

Fig. 5B shows the cycle performance of graphene-embeddedTO nanocomposites at high rates. After 1300 cycles of dischargingnd charging at 22 C, the graphene-embedded LTO nanocompos-te material showed a ∼91% retention (101 mAh g−1) of the initialapacity, which is far better than retentions recently reported forimilar anode materials. It is worth noting that the electrospin-ing setup in this study is not an industrially effective method forrocessing nanomaterials because of its low product yield; we areorking on solving this problem by using parallel needles in an

mproved electrospinning setup.

. Conclusions

We combined two effective approaches to improve the elec-ronic conductivity of the Li4Ti5O12 (LTO) anode material for Li-ionatteries. Electrospinning can process the material into nano-ized architectures to shorten the electron and Li+ transport path,

nd graphene can be embedded into the LTO anode material toreatly improve the surface conductivity of the nanocomposites.he high-rate properties obtained from these graphene-embeddedTO nanocomposites are of great importance for possible appli-ations in hybrid electric vehicles and power tools, because their

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ta 55 (2010) 5813–5818

performance is the best reported to date using similar anode mate-rials. The simple and versatile electrospinning method and highlyconductive graphene additive, together with the zero-strain LTOmaterial, makes this nanocomposite an attractive anode candidatefor energy storage.

Acknowledgment

This research was supported by the NSF of China under grantnos. 20674096, 20733001 and 50773092.

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