Electrochemistry Communications 13 (2011) 125–128

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Electrochemistry Communications

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Exfoliation from carbon nanotubes versus tube size on lithium insertion

Dawei Zhang a,b,c,⁎, Yongbin Zhao c, John B. Goodenough b,⁎, Yuhao Lu b, Chunhua Chen c,Long Wang c, Jianwei Liu c

a School of Chemical Engineering, Hefei University of Technology, Hefei 230009, PR Chinab Texas Materials Institute, The University of Texas at Austin, 1 University Station, C2201, Austin 78712, Texasc Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China

⁎ Corresponding authors. Zhang is to be contacteEngineering, Hefei University of Technology, Hefei 2300

E-mail addresses: zhangdw@ustc.edu.cn (D. Zhang),(J.B. Goodenough).

1388-2481/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.elecom.2010.11.031

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 November 2010Received in revised form 24 November 2010Accepted 24 November 2010Available online 2 December 2010

Keywords:Multiwalled carbon nanotubesLithium-ion batteriesElectrochemical insertionExfoliation

Exfoliation from multiwalled carbon nanotubes (MWCNTs) during electrochemical insertion of lithium isshown to depend on the inner diameter of the tubes and the wall thickness. Those with core diameters of10 nm and walls 5 nm thick showed no incorporation of solvated Li+ ions into the core and exfoliation as aresult of solvated Li+ ions entering the walls; those with core diameters of 40 nm with walls 30 nm thickshowed no exfoliation in an electrolyte based on ethylene carbonate (EC) and little exfoliation even inelectrolytes based on propylene carbonate (PC). Exfoliation of graphene layers from a MWCNT on lithiuminsertion depends not only on the size of the solvated Li+ ions, but also on the concentration of the solvatedLi+ ions between the graphene layers in the walls.

d at the School of Chemical09, PR China.jgoodenough@mail.utexas.edu

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Lithium-ion batteries are widely used as the power supply ofportable electronic devices, power tools, and electric vehicles. Variouscarbon materials have been studied as lithium-insertion host anodes,among which carbon nanotubes (CNTs) have attracted attentionbecause of their ability to retain their shape during lithium insertionas well as having a high electronic conductivity [1–7].

The multiwalled carbon nanotubes (MWCNTs) contain twointerstitial spaces for lithium insertion: the hollow core of the tubeand the spacing between the graphene layers of the tube walls [8,9].Intercalation of solvated Li+ ions into the tube walls can causegraphene layers to be peeled off. This process increases the anodesurface area for formation of a passivating solid–electrolyte interface(SEI) layer. Formation of the SEI layer absorbs Li+ ions irreversibly,thereby reducing the capacity of a cell [10–12]. It has been argued[9,13,14] that exfoliation, i.e. peeling off of graphene layers, occurswhere the stresses induced by intercalation between the graphenelayers in the walls become large enough to break the weak bondingbetween the layers. Therefore, emphasis has been placed on how thecritical concentration of inserted lithium for initiation of exfoliationdepends of the size of the solvated Li+ ions. In this study, we showthat the exfoliation phenomenon also depends on the concentration

of solvated Li+ ions between the layers of the MWCNTs and that this,in turn, depends on the morphology of the MWCNT.

2. Experiments

2.1. Synthesis

MWCNTs were synthesized by using an ethylether thermal-reduction process with metallic Mg or Na as the reducing agent asdescribed previously [4,15]. In a typical experiment, metallic Mg(0.95 g; 99%) and ethylether (40 mL) were mixed and placed in a50 mL stainless-steel autoclave. The sealed autoclave was held at600 °C for 12 h and then cooled naturally to room temperature. A darkprecipitate was collected and washed successively with ethanol,dilute HCl, and deionizedwater. Finally, the productwas dried at 65 °Cfor 24 h. Products obtained with Mg as the reducing agent werelabeled MWCNT1; those obtained with Na via a similar process werelabeled MWCNT2.

2.2. Characterization

The morphology of the products was imaged with a field-emissionscanning electron microscope (FESEM, JEOL-7500B) and by TEM.Electrochemical tests were conducted in coin cells (CB2032 size). Aslurry of MWCNTs and polyvinylene difluoride (PVDF) in the weightratio 9:1 was pasted onto Cu foil and dried for 2 h in a vacuum dry boxat 120 °C. The coin cells were assembled in an argon-filled glove box(MBraun Labmaster 130) with Li metal as counter electrodes andeither LB302 (1 mol% LiPF6 in ethylene carbonate, diethyl carbonate

126 D. Zhang et al. / Electrochemistry Communications 13 (2011) 125–128

(EC, DEC) w/w=1:1) or LB306 (1 mol% LiPF6 in propylene carbonate,diethyl carbonate (PC, DEC) w/w=1:1) as the electrolyte. Theseparator was a porous polypropylene membrane (Celgard 2400).The cells were cycled in the test system NEWARE BTS-610 at a currentdensity of 65 μAcm−2. Cyclic voltammograms and AC impedencespectra were measured with a CHI 604B electrochemical work station.

3. Results

Fig. 1 shows the FESEM images of the as-prepared MWCNT1 andMWCNT2 products. The inner and outer diameters of the MWCNT1product are about 40 nm and 100 nm, respectively, giving a wallthickness of about 30 nm. TheMWCNT2 product is composed of muchsmaller tubes. They have inner and outer diameters of only about10 nm and 20 nm, respectively, with a wall thickness of about 5 nm.The distance between the graphene layers of a MWCNT is 0.340 nm, alittle larger than that in planar graphite (0.335 nm), indicating aweaker bonding between the curved graphene layers in MWCNTs[13,14].

Fig. 2 shows the cyclic voltammograms (CVs) of the MWCNTscycled in LB302 and LB306 electrolytes. All the curves show a processoccurring at 1.3 V, which is more pronounced for the MWCNT2product than for the MWCNT1. This process has been interpreted asthe initiation of a passivating solid–electrolyte interface (SEI) layer[4,8,16]. But it occurs at a higher voltage than the 0.8 V found for agraphite anode [17]. Significantly, the CV curves for the MWCNT2product show a pronounced reduction peak at about 0.5 V. Thisprocess, which has been suggested to signal a peeling off of graphenelayers [18,19], is greater in the LB306 electrolyte where the solvatedLi+ ions are larger. The CV curves for the MWCNT1 product shows amuch reduced process in LB306 and a different process below0.8 V inLB302.

Fig. 3 shows the TEM images of the MWCNTs before and afterdischarge/charge cycling. The surfaces of the larger MWCNT1 remainsmooth with evidence, at most, of only a trace of exfoliation in theLB306 electrolyte whereas the smaller MWCNT2 show obviousexfoliation, indicated by arrows in Fig. 3e and f, in both LB306 andLB302 electrolytes. Moreover, the smallerMWCNT2 showno evidence

Fig. 1. FESEM images of MWCNT1, (a) an

of lithium insertion into the cores of the tubes, but residual lithium inthe walls. Also we should point out that no obvious SEI layer wasobserved in the TEM images after cycling, which should be due to thethin SEI film in nano scale[20] and the low contrast between carbonand SEI under electron beam.

The separators of the lithium-ion cells were examined after 50cycles. As shown in the TEM image inset, the separators of theMWCNT1 cells are white without any dark powder, even after cyclingin LB306, whereas the separators of the MWCNT2 cells contain aconsiderable amount of dark powder after cycling in both LB302 andLB306 electrolytes.

Galvanostatic discharge/charge curves in the LB302 electrolyte areshown in Fig. 4. Cells with the larger MWCNT1 electrode show aprecipitous drop in voltage to 0.5 V on the initial discharge with anirreversible capacity below 0.5 V. Cells with the smaller MWCNT2electrode show an extensive irreversible capacity setting in at about0.75 V. The irreversible capacity is indicative of lithium capture in aforming SEI layer [4,8,16,20]. In the first cycle, the MWCNT1 andMWCNT2 electrodes both deliver a capacity of about 340 mAhg−1. Butin subsequent cycles, the larger MWCNT1 electrodes give a reversiblecapacity of 210 mAhg−1 and a coulombic efficiency of about 62%whereas the smaller MWCNT2 electrodes give a reversible capacity ofabout 170 mAhg−1 with a capacity retention of only 50%. For theMWCNT2 electrodes there is a capacity increase from 2nd to 10th and100th cycles. That's an interesting phenomenon for anode materials[21]. As we know, in the first discharge process, the lithium ions willinsert into the carbon nanotubes. But in the first charge process somelithium ions won't extract from the inner space of the carbonnanotubes at once due to the flexible curly structure of the carbonnanotubes. These “dead” lithium ions will gradually extract from theinner space of the carbon nanotubes with the improved ion diffusionpaths in the following cycles. That's the possible reason for thecapacity increase.

4. Discussion

The MWCNT samples are distinguished by two features, the size ofthe inner diameter of the tube and the thickness of the walls. We are

d (b), and of MWCNT2, (c) and (d).

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

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Fig. 2. Cyclic voltammograms of as-synthesized MWCNT1 in LB302, (a), in LB306, (b), and MWCNT2 in LB302, (c), in LB306, (d).

127D. Zhang et al. / Electrochemistry Communications 13 (2011) 125–128

interested here in processes that distinguish the behaviors of twodifferent types of MWCNT. The process at 1.3 V manifest in the CVcurves of Fig. 2 appears to be common to both types of MWCNT. In

Fig. 3. TEM images of MWCNT1 (a: before cycling; b: after cycling in LB302; and c: after cyccycling in LB306) before and after cycling at 0.1 mVs−1 in LB302 and LB306, respectively. S

liquid carbonate electrolytes, SEI formation sets in below 1 V versusLi0 for all materials [20]. From the CV curves of Fig. 2 together with thegalvanostatic curves of Fig. 4, we may conclude that an additional

ling in LB306) and of MWCNT2 (d: before cycling; e: after cycling in LB302; and f: aftereparators after 50 cycles are inserted in the corner.

-50 0 50 100 150 200 250 300 350

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Fig. 4. Galvanostatic discharge/charge curves of (a) Li/LB302/MWCNT1 cell at a currentdensity of 0.065 mAcm−2; and (b) Li/LB302/MWCNT2 cell at a current density of0.032 mAcm−2.

128 D. Zhang et al. / Electrochemistry Communications 13 (2011) 125–128

volume of the SEI layer occurs in the vicinity of 0.5 V on the MWCNT2electrodes in both LB302 and LB306 electrolytes. However, theMWCNT1 electrode does not exhibit the sharp feature in the CVcurve at 0.5 V in both LB302 and LB306 electrolytes. Given the TEMimages of significant exfoliation from the MWCNT2 sample, but nonefrom the MWCNT1 sample, we may associate the sharp CV feature atabout 0.5 V versus Li0 with exfoliation. The peeled-off graphene layersincrease the electrode surface and, therefore, increase the area of theSEI passivation layer.

We also note that the solvated Li+ ions have been argued to belarger in a PC-based electrolyte (LB306) than in an EC-basedelectrolyte (LB302) [9,10], which means that insertion of lithiuminto the walls of the MWCNTs will place more stress on the interlayerbond in LB306 than in LB302. If lithium is being inserted into theMWCNT walls at 0.5 V, the interlayer stress will be greater in theLB306 than in the LB302 electrolyte. Since exfoliation requires a stressin excess of the interlayer bonding strength, the MWCNT2 can beexpected to showmore exfoliation in LB306 than in LB302. Consistentwith this reason is a larger sharp peak at 0.5 V in the CV curve in LB306than in LB302. Moreover, the appearance of carbon fragments on the

Cellgard separator after cycling a cell with a MWCNT2 electrode, butnot with a MWCNT1 electrode is consistent with this analysis.

Exfoliation is the result of intercalation of solvated Li+ ionsbetween the graphene sheets of the walls of a MWCNT.With a smallercore and thinner walls, the concentration of Li+ ions in any giveninterlayer space will be greater, thereby increasing any giveninterlayer stress. It follows that exfoliation should be more probablefor the smaller MWCNT2 with thinner walls.

5. Conclusion

We have shown that MWCNTs having thicker walls and largerinner diameter as in MWCNT1 do not exfoliate graphene sheets evenin a PC-based carbonate electrolyte and that they offer goodcyclability after the first charge/discharge cycle. However, for theMWCNTwith the smaller diameter, it can even be peeled off in the EC-based electrolyte and therefore a large irreversible capacity loss in thefirst cycle. The morphology of MWCNT anodes as well as the size ofthe solvated Li+ guest ions is an important determinant of theirperformance in a Li-ion battery. Our data indicate that a wall thicknessof at least 30 nm is needed to prevent exfoliation.

Acknowledgements

Financial support from the National Science Foundation of China(grant No. 20703013) and the Robert A. Welch Foundation (grant #F-1066), are gratefully acknowledged. D.Z also thanks the PostdoctorScience Foundation of China (grant No. 20070410219) and thesupport from the China Scholarship Council.

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