50 TheElectrochemicalSocietyInterface•Spring2010

ECS Science at Its BestJES Classics

Perspectives on Electrochemical Insertion of Lithium in Carbon Nanotubes

by Vito Sgobba and Dirk M. Guldi

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Carbon nanotubes (CNTs) areemerging materials with highpotential in several disciplines, in

particular electronics and photovoltaics.Moreover, the unique structuralfeatures, such as high aspect ratio, haveestablished CNTs as novel materialsin nanotechnology, especially incomposites. Importantly, thestructureof

asinglewallednanotube(SWNT)canbeconceived as wrapping a one-atom-thicklayerofgraphite,calledgraphene, intoaseamless cylinder with diameters in therange of 1-2 nm and lengths of severalhundreds micrometer. They form longcrystallinebundlesconsistingofafewtothousands of individual SWNTs packedin triangular lattices. The interstitialchannels with a size of 0.6 nm providethenecessaryintercalationsitesanalogousto those found in the interlayergalleriesin graphite or the tetrahedral/octahedralvacanciesinfccC60.

Lithium ion batteries are the mainpowersourceintoday’sportableelectronicdevices.Thesealsobearthegreatpotentialto take over as power sources of electric

vehicles and hybrid electric vehicles.Owing to the successful development ofcarbon based anodes to be incorporatedinto lithium ion batteries,1 lithiumintercalation into, for example, differentcarbonnanostructuredmaterialsemergedasaninterdisciplinarytopictoimprovetheoveralldeviceperformances.Additionally,the outstanding electrical conductivity,

the remarkable mechanical strength andthe lasting chemical stability of carbonnanotubes, ingeneral;andofsinglewallcarbonnanotubes,inparticular,triggeredaruntotestthemashighcapacitylithiumstorageanodematerials.2

In this featured article,3 for the firsttime the insertion of lithium into apurifiedandhighlycrystallineSWNTwasperformed. To this end, electrochemicalcycling with lithium was pursued asthe method of choice. The formation ofthe new SWNT/lithium systems werecomplementary monitored by in situgalvanostatic charge-discharge, cyclicvoltammetry, X-ray diffraction, as wellas resistivity measurements. Notably,the work necessitated the construction

of specially designed sample holdersand cells including a beryllium windowallowing the transmission of X-rays anda magnet operated reservoir. Overall,such a cell configuration assists inattainingfullcontrolovertheelectrolytelevelasameanstodopeand/ordedope.Simultaneously,X-raydataaccumulationisguaranteed.4

In contrast to the aforementionedcell configuration, in situ resistivitymeasurements are required to contactSWNTs with Cu wires—two as currentleads and two as voltage leads. SuchSWNT assemblies were placed betweenceramic plates screwed together toachieve optimum pressure contacts.While the current leads were connectedtoanacconstantcurrentsource,thetwovoltage leads were connected to a lock-inamplifiertomeasurethevoltagedrop.Furthermore, one of the current leadswas also connected to the potentiostatto polarize the SWNT films. The use ofac for resistance measurements and dcfor electrochemical control eliminatedinterference due to, for instance, cross

TheElectrochemicalSocietyInterface•Spring2010 51

coupling between the two circuits. Thebreakthrough in this approach haslatelybeenmanifested through itswideapplicabilityinfine-tuningtheelectronicstructure of semiconducting SWNTs.Equally important are contemporarycontributions toward thoroughestimations of electron affinities andionizationpotentials.5

This paper has received, since 2001,a steady number of citations averagingwell over 18 per year. The number ofcitationstoRef.3ispresentedinFig.1asafunctionofthepublicationyear.

OneofthemajoraccomplishmentsoftheworkbyClaye,et al.isthereversiblyconductedelectrochemicalinsertionofLiintoSWNTs.Here,galvanostaticcharge/discharge cycles were run in half-cellconfigurationsthatimplementedpurifiedSWNTbuckypaper,ononehand,andLimetalontheotherhand.Determinationof the total and irreversible specificcapacities of these materials requiredtheuseof a1MLiPF6 electrolyte in anethylene carbonate/dimethylcarbonatesolventmixture(1:1v/v).

Considering that buckypapers aremechanically self-supporting andelectrically conductive, no need for abinder or carbon black evolved. This

Fig. 1.The number of papers citing Ref. 2 as a function of the year the paper was published.

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Carbon-basedsystemsarequitefamiliartoelectrochemistsastheyareanintegralpartofmanymoderndaysensors,catalysts,andpower sources. The recent emergence ofcarbonnanostructuresofferschallengestoexplorenew frontiers in electrochemistry.As a newcomer to the electrochemistrycommunity,fullerenesandcarbonnanotuberesearch has made a significant impact.Five of the most cited papers (as ofJanuary 2010) published in Journal of The Electrochemical Societyare listedasthefiveReferencesbelow.

One of the early challenges inunderstandingpropertiesoffullerenesandtheir derivatives was the characterizationof the reduction and oxidation states. Upto six reversible reductions of C60 wereestablishedusing cyclic voltammetry.Thespectroelectrochemical experiment byParkinsonandcoworkers1providedthefirstspectral fingerprint (IR absorption band ofC60-)andthislandmarkpaperhasservedtoestablishmanyinterestingredoxpropertiesoffullerenes.

The other area of interest is in theutilizationofcarbonnanotubesinstoragebatteries, fuel cells, and supercapacitors.EarlyworkofSmalleyandcoworkers2intheelectrochemical insertion of lithium ionsintopurifiedsinglewallcarbonnanotubesdemonstrated reversible capacities on the

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order of 460 mAh/g, corresponding to astoichiometry of Li1.23C6. Their studiesdemonstrated that the high lithiumcapacities in carbon nanotubes were notduetodoublelayercapacitanceeffects,buttoanactualioninsertion/extractionprocessinthebulkmaterial.

Wu, et al.3 also observed that thestructures of carbon nanotubes playmajor roles in both specific capacity andcycle life. A nanocomposite electrodeof single-walled carbon nanotube(SWNT) and polypyrrole (Ppy) exhibitedimproved specific capacitance of thesupercapacitor properties.4 Shao, et. al5report on the durability of multiwalledcarbon nanotubes supported Pt (Pt/CNTs)catalysts highlighted potential applicationof carbon nanostructures in polymerelectrolyte membrane fuel cells. Carbonnanostructures will continue to play animportantroleindesigningnextgenerationenergyconversionandstoragedevices.

About the Author

Prashant V. Kamat is a professor ofchemistry and biochemistry at theUniversity of Notre Dame (Indiana, USA).HeisapastChairoftheECSFNCNDivisionandanECSFellow.Hemaybe reachedatpkamat@nd.edu.

Prominent Contributions in Fullerenes and Carbon Nanotube Research by Prashant V. Kamat

References1. D.R.Lawson,D.L.Feldheim,C.A.Foss,

et al., “Near-IR Absorption-Spectra forthe Buckminsterfullerene Anions. AnExperimentalandTheoreticalStudy,”J. Electrochem. Soc.,139,L68(1992);cited112times.

2. S. Claye, J. E. Fischer, C. B. Huffman,et al., “Solid-State Electrochemistry ofthe Li Single Wall Carbon NanotubeSystem, J. Electrochem. Soc., 147, 2845(2000);cited160times.

3. G.T.Wu,C. S.Wang,X.B. Zhang, et al., “Structure and Lithium InsertionProperties of Carbon Nanotubes,” J. Electrochem. Soc., 146, 1696 (1999);cited108times.

4. K. H. An, K. K. Jeon, J. K. Heo,et al., “High-Capacitance Super-capacitor Using a NanocompositeElectrode of Single-walled CarbonNanotube and Polypyrrole,” J. Electrochem. Soc., 149, A1058 (2002);cited90times.

5. Y. Y. Shao, G. P. Yin, and Y. Z. Gao,“Durability studyofPt/CandPt/CNTsCatalysts under Simulated PEM FuelCell Conditions,” J. Electrochem. Soc.,153,A1093(2006);cited80times.

52 TheElectrochemicalSocietyInterface•Spring2010

Sgobba and Guldi(continued from previous page)

approach is, in fact, so appealing andso unique that buckypapers as (1) freestanding films and (2) in all-solid-stateconfigurations paved the way to thefirst investigations focusing on in situCNT visible/near-infrared and Ramanspectroelectrochemistry.6,7

These pioneering investigations haveunambiguously demonstrated thatSWNTs give rise to reversible capacitiesoutperforming those theoreticallypredictedforgraphite,thatis,460versus372mAh/g,whichisequivalenttoaLi1.23C6stoichiometry.Asetbackisthatinitiallyalargeirreversiblecapacityof1200mAh/gevolved. In fact, the latter decrease upto 15% in the subsequently performedcycles. Implicit is that a significantamountofLi couldnotbe recoveredasthenumberofcycles increased.A likelyrationaleforthissetbackliesinthepost-treatment procedure of SWNT, namelyheat treatment at 1200°C. At 1200°Cthe SWNT caps/ends are presumedto be still intact, which prevents theefficient diffusion of Li inside theSWNTs. Reassuring is that in a follow-up study, which emphasized the use ofopened SWNT, the specific Li storagecapacity increased up to an impressive1000 mAh/g.8 The corresponding LiC3stoichiometry corroborates the notionthat Li is not anymore hampered todiffuseintotheinsideofSWNTs.

A close analysis of the resultinggalvanostatic charge/discharge featuresprompt to substantial losses due toirreversibility. It is very likely that thereduction of the electrolyte, which isfurther augmented by the formationof a solid electrolyte interphase right atthe surface of a SWNT, is the inceptionfor such effects. Well in line with suchan assumption is the fact that theintercalation and/or de-intercalationpotentialswerespreadoverafairlywiderange. Moreover, the high divergencebetween the potentials at which eitherinsertionorextractionoccurs,isthebasisfor a notable hysteresis. Recently, theoriginalhypothesis,thatis,formationofkeyintermediatesuchasC–H–Li,C–O–Li,etc.wasunmistakablyconfirmed.9,10Thelackofwell-definedpotentialsfortheLiinsertionorremovalwerealsoseeninthecyclicvoltammograms.

Arealassetoftheworkisthecorrelationbetweenthe3Dcrystallographicstructure,surfacechemistry,andtheelectrochemicalbehaviorof thesematerials,whichhavebeenelegantlyperformedbycomparingand contrasting in situ X-ray diffractionand high resolution transmissionelectronmicroscopy.Here,theimportantconclusion is that an irreversibledisorganizationofthepackingwithinthebundles dominates rather that a simpledisassembling.

Withregardtothechangeinresistance,as occurred during the doping and de-doping processes, use of an electrolyte

that is known to not to interfere—1MLiPF6 in THF—helped to decisivelysubstantiate charge transfer processesbetweenLiandC.11Here,anearly20-foldresistivity decrease was observed upondoping.

Finally, the Nyquist plots should bementioned, which were derived fromelectrochemicalimpedancespectroscopyatvariouspotentialsandmodeledwithinthe frame of Randles-type equivalentcircuits.Themostfarreachingconclusionisthatthechargetransferresistancehassignificant electronic contributions. Sincethe double layer capacitance was foundtobemuchlowerthanthatexpectedfora SWNT Brunauer-Emmett-Teller surfaceareaof 350m2/g,double layer chargingis ruled out as an origin of the large Licapacity. Instead, a closer inspection ofthelowerfrequencyEISportionconfirmsthatthelargeLicapacityoftheSWCTisduetoanactualioninsertion/extraction.Lately, theseNyquistplotswereutilizedasvaluablestandardstoassessthequalityofsimilarsamples.12,13

Conclusions

TheClaye,et al.contributionisgroundbreaking. In fact, this innovative andvisionary work comprises cutting-edgedevelopments in the renewable energysectorandhasoverthelastdecadeledtoacomprehensive, scientificunderstandingof electrochemically intercalating ionsin general into CNT buckypaper. Thishas shaped the scientific community’sview on technological advances in theemergingfieldsofenergystoragedevicesfocusing on Li-ion secondary batteries,supercapacitors,hydrogenstorage,etc.Italsounveilstheimpactoncontemporaryadvances and innovations in, forexample, electrochemical doping of theFermi level of CNT for photonic andnanoelectronicapplications.

About the Authors

Vito sgobba is currently a seniorresearcher at the Friedrich-AlexanderUniversity in Erlangen (Germany). Hereceivedhis ‘‘Laurea’’ inchemistryfromtheUniversityofBari(Italy)in2000underthesupervisionofF.NasoandhisPhDinmaterialsengineeringfromtheUniversityofLecce(Italy)in2003withG.Vasapollo.Inearly2005,hejoinedthegroupofD.M.Guldi.Hehasalsoco-authoredwithD.M.Guldithreebookchaptersoncarbonnanotubes chemistry, electrochemistry,andphotophysics.His research interestsare synthesis, characterization, andassembly of hybrid (organic–inorganic)nanostructuredmaterialsforsolarenergyconversiondevices.Hemaybereachedatvito.sgobba@chemie.uni-erlangen.de.

DirK m. gulDi graduated from theUniversity of Cologne (Germany) in1988, from where he also received hisPhD in 1990. Since 2004, he is a fullprofessor at the Institute of Physical

Chemistry at the Friedrich-AlexanderUniversityinErlangen(Germany).HewasawardedwiththeHeisenberg-Prize(1999,Deutsche Forschungsgemeinschaft);Grammaticakis-Neumann-Prize (2000,Swiss Society for Photochemistry andPhotophysics); JSPS Fellowship (2003,The Japan Society for the Promotion ofScience); JPP-Award (2004, Society ofPorphyrins and Phthalocyanines); andthe Elhuyar-Goldschmidt Award (2009,Royal Spanish Chemical Society). Prof.Guldi is Editor-in-Chief of Fullerenes, Nanotubes, and Carbon Nanostructures(since 2001). Currently he is chair ofthe ECS Fullerenes, Nanotubes, andCarbon Nanostructures Division. Hisresearch interests are widespread inthe field of carbon nanostructures. Inparticular, primary research activitiesare in the areas of charge separationin donor-acceptor ensembles andconstruction of nanostructured thinfilms for photoenergy conversion. Hemay be reached at dirk.guldi@chemie.uni-erlangen.de

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U.S. Patent,4959281(1990).2. Z.W.Zhao,Z.P.Guo,H.K.Liu,and

S. X. Dou, ECS Transactions, 6(25),205(2008).

3. A. S. Claye, J. E. Fischer, C. B.Huffman, A. G. Rinzler, and R.E.Smalley, J. Electrochem. Soc., 147,2845(2000).

4. J. E. Fischer, Acc. Chem. Res., 35,1079(2002).

5. S.Kazaoui,N.Minami,N.Matsuda,H. Kataura, and Y. Achiba, Appl. Phys. Lett.,78,3433(2001).

6. L. Kavan, P. Rapta, L. Dunsch, M.J. Bronikowski, P. Willis, and R. E.SmalleyJ. Phys. Chem. B, 105, 10764 (2001).

7. L.KavanandL.DunschChem. Phys. Chem.,8,974(2007).

8. H. Shimoda, B. Gao, X. P. Tang, A.Kleinhammes, L. Fleming, Y. Wu,and O. Zhou, Phys. Rev. Lett., 88,015502(2002).

9. J.Y.Eom,D.Y.Kim,andH.S.Kwon,J. Power Sources,157,507(2006).

10. S. Yang, J. Huo, H. Song, and X.Chen, Electrochim. Acta, 53, 2238(2008).

11. L. W. Shacklette, J. E. Toth, N. S.Murthy, and R. H. Baughman, J. Electrochem. Soc.,132,1529(1985).

12. S.H.Ng, J.Wang,Z. P.Guo,G.X.Wang, and H. K. Liu, Electrochim. Acta,51,23(2005).

13. S. Venkatachalam, H. Zhu, C.Masarapu, K. Hung, Z. Liu, K.Suenaga, and B. Wei, ACS Nano, 3,2177(2009).