Materials Research Bulletin 44 (2009) 2155–2159

Synthesis of ribbon type carbon nanostructure using LiFePO4 catalyst and theirelectrochemical performance

Kuldeep Rana, Anjan Sil, S. Ray *

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247 667, Uttarakhand, India

A R T I C L E I N F O

Article history:

Received 3 February 2009

Received in revised form 11 August 2009

Accepted 23 August 2009

Available online 29 August 2009

Keywords:

A. Nanostructure

B. Vapor deposition

C. X-ray diffraction

C. Raman spectroscopy

D. Electrochemical properties

A B S T R A C T

Ribbon type of carbon nanostructure has been synthesized by chemical vapor deposition using a new

catalyst (LiFePO4) introduced for the first time and its electrochemical behavior has been determined

from charge/discharge characteristics. The synthesized material characterized by X-ray diffraction,

scanning electron microscopy, transmission electron microscopy and by Raman spectroscopy confirms

the graphitic structure and ribbon type morphology of material. The performance of the single cell using

purified carbon nanoribbon as the anode has been studied and the reversible lithium intercalation

capacity has been found about 345 mAh/g, of which 335 mAh/g remain after 14th cycle. The columbic

efficiency has been stabilized at approximately 98% from the 5th cycle.

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1. Introduction

Since the commercialization of lithium secondary batteries andtheir application in various portable electronic devices, theresearch on different anode materials has been a focus. Severalanode materials that have been investigated include graphiticcarbon, amorphous carbon, nitrides, tin oxide and tin-based alloys[1]. However, dominant and commercially used anodes are thegraphite based materials [2]. Several modifications and prepara-tions of carbon based anode materials have been reported inliterature [3]. Since the discovery of carbon nanotubes (CNTs) in1991 [4], carbon based nanomaterials have received attention andbeen synthesized with different morphologies for use in devicefabrication. These materials have exotic properties [5,6], and canbe used as anode material of lithium ion batteries. Multi-walledcarbon nanotubes (MWCNTs) produced by different methods andtheir structure modifications have been studied in detail [7–9]. Theslightly graphitized MWCNTs exhibit a high specific capacity of640 mAh/g [9]. However, lower capacity of 282 mAh/g duringdischarge is found with well-graphitized MWCNTs [9]. Lithium canreversibly intercalate into single-walled carbon nanotubes(SWCNTs), and the capacities range from 460 to 1000 mAh/g[10], due to defects introduced by ball-milling [11]. However,irreversible capacity in 1st cycle is very high, i.e. up to 1200 mAh/g.A similarly fascinating carbon system of different morphology is a

* Corresponding author. Tel.: +91 1332 285732; fax: +91 1332 285732.

E-mail address: surayfmt@iitr.ernet.in (S. Ray).

0025-5408/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2009.08.019

strip of a graphene sheet named carbon nanoribbons (CNRs), whichare strips of graphene sheets of nanometric size [12]. Theexperimental studies on CNRs have begun only recently [13,14].Theoretical studies of CNRs show that it is semi-metal at themicron scale or larger, but when one of the two directions getsconfined to less than 100 nm, electron confinement leads toopening of band gap [15,16]. The electronic properties of CNRs aregoverned by the ribbon width and geometry of atomic arrange-ment, viz. zig-zag or armchair along the edges. CNRs have attractedenormous attention due to versatile electronic properties that areexpected to be important for future application in nanoelctronicsas well in spintronics [17,18]. The CNR may also prove to be betteranode materials for lithium ion battery, as compared to CNT,because the ribbon has large areas of edge sites, which may lead tohigher lithium intercalation and enhanced charge capacity.However, potential application of these materials in large scaleis yet to be explored.

In this work, synthesis of carbon nanostructure was carried outwith a novel catalyst (LiFePO4) so as to have nanostructure withdifferent morphology. Iron and iron oxide based catalyst have beenwidely used for growth of CNTs/CNFs, such catalysts are preferredbecause of its yield is better than using other catalyst based onnickel and cobalt [19,20]. Lithium influences the carbon nanos-tructure when used along with Ni and Co in the catalyst [21]. Thenew catalyst used in present study combines the effect of both Liand iron. The nanostructure synthesized by chemical vapordecomposition using catalyst LiFePO4, was characterized withthe objective of application as an anode material for lithium ionbattery. The electrochemical behavior of these synthesized CNRs

K. Rana et al. / Materials Research Bulletin 44 (2009) 2155–21592156

was studied by preparing an electrode on copper foil and using it ina cell assembly.

2. Experimental procedure

The nanostructures were synthesized by chemical vapordeposition (CVD) of acetylene gas. The catalyst particles (LiFePO4)to be used for the synthesis of nanostructures were prepared bysol–gel route. Li(OH)�H2O, NH4H2PO4, FeC2O4�2H2O and citric acidwere used as starting materials to synthesize LiFePO4. All the saltswere dissolved in double distilled water at room temperatureseparately and then added together. The solution was homo-geneously mixed by using magnetic stirrer at 80 8C till theformation of gel takes place. The thick gel obtained have beenplaced into furnace and heated at 350 8C for 2 h in order to removeall organic contents and further calcined at 700 8C for 10 h in inert(Ar) atmosphere. The catalysts particles were dispersed in iso-propanol and spread onto anodized aluminum oxide (AAO)substrate. The AAO substrate was prepared by prior anodizationof pure aluminum. A thin strip of aluminum was first electro-polished in ethyl alcohol and perchloric acid (in ratio of 4:1 byvolume) bath by applying 20 V for 1 min at room temperature. Theelectro-polished aluminum strip was anodized at constant voltageof 40 V for 1 h in 3 wt.% oxalic acid electrolyte. The substrate(anodic aluminum oxide) with catalyst deposited on it was placedon an alumina boat, which in turn was placed in a tubular furnace.The ammonia gas was passed at the rate of 60 sccm (standard cubiccentimeter per minute) when temperature reached up to 530 8C at

Fig. 1. FE-SEM micrographs AAO surface (a), CNRs at magnific

an atmospheric pressure till temperature was raised to 650 8C.When the temperature of the furnace reached 650 8C, acetylene gaswas introduced along with the ammonia gas, flowing at 20 sccmfor 20 min. The carbonaceous nanostructures synthesized wereremoved from the substrate and purified in a solution of 6 M HNO3

in order to remove catalyst particles as well as amorphous carbon.The separated nanostructures were washed thoroughly withdistilled water and dried in a vacuum oven at 170 8C. Themorphology of the CNRs was examined under field emissionscanning electron microscope (FE-SEM) (FEI QUANTA 200 F) andsimultaneous elemental analysis was carried out using energydispersive X-ray analysis (EDX). Transmission electron microscopy(TEM) (FEI Technai G2-20-S-Twin microscope) of the samples wascarried out at an operating voltage of 200 kV. X-ray diffraction(XRD) study of the nanostructures was conducted by X-raydiffractometer (Bruker AXS, D8 advance) with CuKa radiation(1.5418 A) with a scanning speed of 18 per minute between2u = 158 and 658. Raman spectrum of the CNRs was recorded byspectrometer (Renishaw-system 1000 B) using 8 mW argon laserbeam having wavelength of 514.0 nm. The Lorentzian function hasbeen used for fitting the peaks in Raman spectrum of nanos-tructures. A Teflon cell was assembled in an Ar-filled glove box(MBRAUN-MB 200G) having oxygen and moisture level below0.1 ppm, for electrochemical measurements. The cell consists of apositive electrode which was prepared using a copper foil as acurrent collector and on one side of which, slurry of 90% activematerial, 7% binder of polyvinyl di-fluoride (PVDF) and 3%acetylene black was pasted by using doctor blade and dried at

ation 4k� (b), at 16k� (c) and EDX spectrum of CNRs (d).

K. Rana et al. / Materials Research Bulletin 44 (2009) 2155–2159 2157

140 8C for 24 h in a vacuum oven. Li metal foil has been used asnegative electrode and the electrolyte of 1 M LiPF6 dissolved in 1:1mixture by volume of ethylene carbonate (EC) and diethylcarbonate (DEC) was used in the cell. The cell was tested at20 8C by using computer-controlled cycler (Arbin BT 200) atconstant current of 100 mA in a voltage range 0.05 and 2.5 V.

3. Results and discussion

FE-SEM micrograph of AAO surface obtained after anodizationis shown in Fig. 1a, and it shows well ordered hexagonalarrangement of pores on the AAO surface which was used assubstrate. The estimated pore diameter varies in range of 30–80 nm. Fig. 2(a–c) shows the TEM images of nanostructuresobtained with catalyst LiFePO4 at different magnifications. TheTEM images confirm that the synthesized material consists ofcarbon nanoribbons (CNRs) mostly. The ribbons are relativelyuniform in width, ranging from 50 to 80 nm. The selected areadiffraction pattern of CNR, as given in the inset of Fig. 2b, showsspots from (0 0 2) plane having interlayer spacing of 3.396 A. TheFE-SEM image of the nanoribbons on the substrate is shown inFig. 1(b and c). There is also a small quantity of carbonaceousnanomaterial having spherical morphologies. Longer and flatnanoribbons are shown at higher magnification in Fig. 1c. Theenergy dispersive X-ray spectroscopy (EDX) spectrum of assynthesized CNRs is shown in Fig. 1d, which indicates that thegrowth of CNRs has started from the catalyst LiFePO4 particle as

Fig. 2. TEM micrographs of CNRs at different magnifica

evident from Fe and P detected in EDX at a point from where one ofthe CNR has emerged. Although ammonia gas was fed togetherwith acetylene during growth of nanostructure, there is noindication of nitrogen in EDX spectrum. Generally, most acceptedmechanism of growth involves Vapor–liquid–solid (VLS) process ofone-dimensional nanostructure proposed by Wagner and Ellis[22]. In the present study we observe long, thin (almosttransparent) CNRs formed from catalyst particles. The Catalystparticles in present case allow formation of graphene layer oflimited dimension growing from its faces without curling it up as atube or fiber (Fig. 2c). The syntheses of CNRs by using iron-basedcatalysts have been reported earlier [23,24]. It is possible thatunder prevailing reducing environment ferrous ions reduce to ironatom cluster, paving the way for its catalytic action for growthmechanism to operate [25]. But the presence of lithium or even Pmay have changed the surface characteristics to result in growth ofribbon in preference to nanotube or fiber.

Elemental analysis of purified CNRs show that chemicaldissolution of catalyst is not complete and 0.96 at.% of iron and1.93 at.% of phosphorous are still present in sample apart fromoxygen, which is present in the catalyst used and also in HNO3 usedfor purification.

Fig. 3 shows the XRD pattern of the purified CNRs and itconfirms the graphitic structure. The peaks in the pattern arereflections from (0 0 2) and (1 0 1) planes of hexagonal graphite atangles, 2u = 26.148 and 44.308, respectively. The broadened (1 0 1)peak reflect the turbostratic character of the material. The

tions and inset showing the SAD pattern of CNRs.

Fig. 3. Powder XRD pattern of CNRs grown with LiFePO4 catalyst.

Fig. 5. (a) Typical first charge–discharge profile of a cell Li/LiPF6 (in EC:DEC)/

electrode of CNRs and (b) discharge profile measured against lithium foil for 2nd to

K. Rana et al. / Materials Research Bulletin 44 (2009) 2155–21592158

interlayer distance calculated from pattern is d002 = 3.40 A, whichis in reasonable agreement with the value calculated (3.396 A)from selected area diffraction pattern under TEM, inset in Fig. 2b.

The first order Raman spectrum of CNRs is shown in Fig. 4. TheRaman band appearing at 1589 cm�1 is noted as G-band (E2g2

graphite mode), which is related to the C–C vibration of the carbonmaterial with a sp2 orbital structure [26]. The band appearing at1348 cm�1 is noted as D-band (a defect induced mode), which iscontributed by the disorder-induced vibration of C–C bond [27].The full width at half maximum (FWHM) of G-band is estimated as44 cm�1, which is nearly double that of the width of about 20–22 cm�1 in MWCNTs grown with other catalyst and in graphite.The up-shift in G-band and larger FWHM for the CNRs, compared tothat of graphite and MWCNTs grown with other catalyst, areperhaps due to lithium bearing catalyst, through its contribution tostructural modification of the CNRs or charge transfer to carbonatoms and due to lithium insertion [28,29]. The relative intensityratio of D-band to G-band is an index to determine the graphiticstructure. In addition, the intensity ratio of the two Raman peaks

Fig. 4. Raman spectrum of CNRs grown with catalyst of LiFePO4.

14th cycles, and inset showing variation of columbic efficiency with number of

cycles in cells with electrode made of CNRs.

(ID/IG) has been calculated as 0.51, which is nearly same asobserved for the pristine and heat treated sample of CNT [30].

After purification of CNRs by removing the catalyst particle andamorphous carbonaceous materials, electrochemical measure-ment has been carried out. Fig. 5a shows the first charge anddischarge voltage profile of CNRs as anode in lithium ion cell. Thevoltage plateau around 0.9 V closely relate to electrolyte decom-position and the formation of a solid–electrolyte interface (SEI)[30]. The average discharge plateau is below 0.6 V, which is properfor application as anode material. The first discharge (lithiuminsertion) capacity as calculated from Fig. 5a is 725 mAh/g. Theopen edges of ribbons are suitable for Li-insertion and this may bethe reason for high discharge capacity observed. This capacity isnearly similar to that of MWCNTs grown with conventionalcatalysts (e.g. Co and Fe). The discharge profiles for 2nd to 14thcycle are shown in Fig. 5b. The reversible capacity at roomtemperature has been estimated to be about 340 mAh/g andreduces to 335 mAh/g at 14th cycle, up to which the measurementswere made. The plot of capacity vs. cycle number as an inset isshown in Fig. 5b, which indicates the fluctuation in capacity

K. Rana et al. / Materials Research Bulletin 44 (2009) 2155–2159 2159

initially, as it becomes 500 mAh/g in 3rd cycle and again drops to380 mAh/g in the 4th cycle. The columbic efficiency calculated asthe ratio of charge to discharge capacities has been stabilized atabout 98% from the 5th cycle onward. The columbic efficienciesreported from studies up to 5 cycles for SWCNTs is 84% [31] and forMWCNTs grown with iron particle catalyst is 83–95% [32]. Theseresults show that the CNRs synthesized with a novel catalyst haslittle higher reversible capacity as well as capacity retention asanode, demonstrating its potential for application in Li-ion battery.

4. Conclusion

In this study, it has been demonstrated that CNRs can besynthesized using LiFePO4 as a catalyst by thermal CVD ofacetylene at 650 8C. The TEM and FE-SEM images show that thematerials have ribbon type morphology having width of 50–80 nm.The SAD pattern and XRD results of the synthesized materials areconsistent. The first order Raman spectrum of CNRs grown withLiFePO4 shows that up-shift in G-band of graphite indicatinghigher crystallinity compared to MWCNTs grown with conven-tional catalysts. The electrochemical properties of CNRs show goodreversible capacity and good capacity retention up to 14th cycle. Tothe best of our knowledge, the synthesis of CNRs using catalystLiFePO4 and its potential for application as anode material for Li-ion battery are being reported for the first time.

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