Journal of Power Sources

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1 Journl of Power Sources () 5 9 Contents lists ville t SciVerse ScienceDirect Journl of Power Sources jo ur nl homep ge: Short communiction Influence of the dimeter distriution on the rte cpility of silicon nnowires for lithium-ion tteries Aurélien Gohier, Brr Lïk,, Jen-Pierre Pereir-Rmos, Costel Sorin Cojocru, Pierre Trn-Vn c Lortoire de Physique des Interfces et des Couches Minces, École Polytechnique, Route de Scly, 98 Pliseu Cedex, Frnce Institut de Chimie et des Mtériux Pris-Est, ICMPE/GESMAT, UMR 78 CNRS-UPEC, à 8 rue Henry Dunnt, 94 Thiis, Frnce c Renult SAS, DREAM/DETA/SEE,, Avenue du Golf, 7888 Guyncourt, Frnce r t i c l e i n f o Article history: Received 6 Octoer Received in revised form 8 Novemer Accepted 9 Decemer Aville online 7 Decemer Keywords: Silicon nnowires Lithium-ion tteries Negtive electrode Rte cpility Cycling performnces s t r c t For the first time, the effect of the nnowires dimeter in terms of size nd distriution on electrochemicl properties of SiNWs grown y VLS ws investigted. The dimeter size ws tuned y using three different gold ctlyst film thicknesses. The crucil influence of this prmeter is evidenced through comprison of the chrge dischrge ehvior nd study of the rte cpility for the three smples. The rechrgele cpcity s well s the rte cpility is shown to e the est when the smllest dimeters (<65 nm) re used compred to lrger one (< nm nd <49 nm). High cpcity vlues of 5 mah g re otined for the smllest dimeters t C/5 rte ut still 5, 5 nd 5 mah g re recovered t C,.5 C nd 5 C. An excellent cycle life over 5 cycles is chieved t. C with cpcity of 5 mah g. This shows tht y tiloring the dimeter size nd distriution, SiNWs cn provide high power density nodes in lithium ion tteries. Elsevier B.V. All rights reserved.. Introduction Lithium-ion hs ecome the most promising system for portle electronic devices, implntle devices nd electric vehicles. Tody, gret efforts re eing mde to improve their energy density nd to prolong their service life y optimizing new host mterils for lithium insertion. Among node mterils, silicon is n ttrctive cndidte since it presents the highest known theoreticl chrge cpcity (579 mah g t room temperture [,]), which is ten times higher thn tht exhiited y the conventionl grphite electrode used for commercil ppliction. However, Si Li lloying process yields lrge volume chnge of silicon mteril, up to % []. The mechnicl stresses induced upon lithition/delithition cycles cn thus led to silicon mteril pulveriztion, resulting in loss of electricl contct etween prticles nd with the current collector nd hence cpcity fding. One possile wy to improve the silicon cycle life is to use rchitecture exhiiting significnt surfce/volume rtio in order to ccommodte lrge structurl strins. Thin film electrodes (-D mterils) hve een evluted to nswer this requirement. However, pprecile cyclility ws only ensured with thickness lower thn 5 nm [4,5]. Corresponding uthor. Tel.: ; fx: E-mil ddress: lik@icmpe.cnrs.fr (B. Lïk). Recently, lrge vriety of -D nnostructured silicon mterils hve ttrcted much ttention for lithium-ion tteries pplictions [6 4]. Silicon nnowires (SiNWs) hve een widely studied since they cn e directly grown on current collector [9,,5], thus providing good electronic contct. Their -D structure llows the electronic pthwy to e efficient nd to ccommodte the lrge volume chnges without crcking. Moreover, the need for inder or conducting dditive is eliminted. Since pioneer works [8,,], mny studies hve imed to improve SiNWs sed nodes performnce y studying the effect of potentil rnge for cycling [6], the nture of the collector [7] or silicon nnowires cotings [,8,9]. Surprisingly, the effect of SiNWs dimeter hs never een studied s experimentl prmeter on cycling properties while it is crucil prmeter which proly governs the ville storge cpcity, the rte cpility nd cyclility. Some rre uthors mention the vlues of tested SiNWs dimeters [4,6,] or more exctly the rnge in which SiNWs dimeters cn e found [8,] without ny comment out the rel dimeter distriution. Nevertheless, depending on processes nd synthesis conditions (HF etching nd CVD-VLS), SiNWs present vrious size with length m nd dimeter in the rnge 5 nm. From this disprity, the question rises out the impct of SiNWs morphologicl fetures on their electrochemicl ehvior, nd hence on node performnce. In other respects the disprity oserved in electrochemicl dt reported on SiNWs proly origintes from the wide dispersion in size dimeter nd length of the SiNWs /$ see front mtter Elsevier B.V. All rights reserved. doi:.6/j.jpowsour...

2 6 A. Gohier et l. / Journl of Power Sources () 5 9 In this work, we investigte the effect of SiNWs dimeter on the efficiency of electrochemicl lithition/delithition process. We descrie here the synthesis of three smples with 65 nm, nm nd 49 nm dimeters prepred y the VLS method, y controlling the thickness of the gold film used s ctlyst. Cycling performnces nd rte cpility re especilly investigted nd we show the est results re otined with the smllest dimeters.. Experimentl Silicon NnoWires (SiNWs) were grown on 5 m thick stinless steel foils using Hot-Filment Chemicl Vpour Deposition (HF-CVD). Steel sustrtes were copiously wshed with cetone nd isopropnol nd then covered y thin films of gold ( nm) using e-em evportor. CVD synthesis operted t 54 C nd 7.5 mr using SiH 4 /H (5 sccm for oth gs) mixture s precursor. Gold covered sustrtes were directly introduced in the growth tmosphere during min. At the erly stge of the process, gold thin film crcks nd forms ctlyst prticles onto which SiNWs nuclete nd grow. The size of gold ctlyst clusters determine the dimeters of the resulting SiNWs nd these cn e tuned y plying with the initil gold thickness s shown in the following section. Three sets of thin gold films were used to perform the SiNWs growth. They re chrcterized, using qurtz crystl microlnce, y thickness of nm, nm nd nm. Electrochemicl studies were crried out in two-electrode cell (Swgelok type) fricted out of SiNWs directly grown on the metllic current collector sustrte s working electrode nd lithium metl foil s oth reference nd counter electrode, s descried in detil elsewhere [9]. No inders or conducting cron ws used. The electrolyte ws commercil solution of mol L LiPF 6 in dimethylcronte (LP) otined from Merck. Glvnosttic mesurements were recorded using VMP pprtus (Biologic ). A constnt current density ws used for lithition/delithition experiments. The chrge/dischrge rte is clculted sed on the theoreticl mximl insertion, t room temperture, of.75 Li ions for one Si tom, corresponding to C =.58 A g.. Results nd discussion The growth of Silicon NnoWires (SiNWs) using Hot-Filment Chemicl Vpour Deposition (HF-CVD) technique requires the djustment of numerous experimentl prmeters to otin the desired SiNWs chrcteristics. Among them, it cn e cited without eing exhustive working pressure, temperture, SiH 4 flow rte, SiH 4 /H rtio, sustrte nture, thickness or pre-tretment. Former studies devoted to the optimiztion of these prmeters led us to operte t 54 C nd 7.5 mr using SiH 4 /H (5 sccm for oth gs) mixture s precursor. The sustrte is mde of stinless steel ecuse it does not lloy with lithium nd llows stisfctory shping of ctlyst clusters. A thickness of 5 m is chosen so tht the smple cn e cut, without eing deformed under the influence of the het. At lest, s gold is known to e the more efficient ctlyst nd s the electrochemicl contriution of the gold droplets still present t the top of the silicon wires fter synthesis is minor [], thin film of gold is used for the preprtion of the smples. At the erly stge of the process, the continuous gold film is trnsformed into three-dimensionl clusters under temperture effect. The initil gold film thickness hs n impct on the Auclusters size distriution, nd the dimeter of resulting SiNWs grown y Vpor Liquid Solid (VLS) method is dependent on the Au-prticles size. Hence, the thicker the gold thin film used for the synthesis, the lrger the dimeter of the SiNWs. Then, in order to compre the effect of the SiNWs dimeter on node performnce, Fig.. Scnning Electron Microscopy imges (tilt 45 ) of silicon nnowires grown during min on stinless steel foil using nm (), nm () nd nm (c) thick films of gold s ctlyst. set of three smples with significnt differences in terms of dimeter distriution were prepred. For this purpose, we used ctlytic thin films of gold with vrious thicknesses, nmely nm, nm nd nm. After eing heted under vcuum ove the Au Si eutectic melting temperture (54 C), SiH 4 diluted in H is injected in the furnce for min. As expected nd lredy presented elsewhere [9,], n uniform crpet of wires tht exhiits length up to 5 m, covers the sustrte. Both X-ry diffrction (XRD) pttern nd Rmn spectrum of the smples confirm silicon crystllinity. The comprison of SEM imges otined for the three smples (Fig. ) enles to confirm tht difference in the SiNWs dimeter distriution is chieved for ech smple. Indeed, the thicker the film from nm (Fig. ) to nm (Fig. ) nd nm (Fig. c), the lrger the dimeters of the wires. With the im to finely estimte SiNWs dimeter distriution nd ssuming in ech cse, dimeter does not increse in time, the growth of new set of smples ws stopped fter min. In this wy, ll SiNWs cn e distinguished y SEM, counted, individully mesured in dimeter in order to estimte their weights nd to e tken into ccount in the sttistics. For the three smples, the dimeters tke vlues scttered in rnge, ll the wider s the film is thick. To illustrte this result, the cumultive weight of silicon nnowires is shown in Fig. s function of the wires dimeters. For instnce, when the gold thickness is out nm (lck dt in the figure), 8 wt% of the silicon mteril correspond to wires with dimeter size <65 nm. Besides, 5 wt% of the silicon correspond to wires with dimeter lower thn 4 nm wheres the other 5 wt% re spred etween 4 nd nm. For smple nd smple, when or nm of gold were used, those dimeter vlues re significntly higher (Tle ). In the following of the pper, smples will e nmed y the vlue otined when 8 wt% of silicon re

3 A. Gohier et l. / Journl of Power Sources () Fig.. Histogrms of cumultive weight of silicon nnowires s function of the initil gold thin film thickness (lck: nm; white: nm; gry: nm). reched, nmely 65 nm, nm nd 49 nm for nm, nm nd nm of gold film thickness respectively. The electrochemicl properties of the SiNWs s function of their dimeters hve een investigted y glvnosttic cycling experiments with cutoff voltges of. V nd V vs. Li + /Li. The first typicl cycle nd the second lithition step otined t C/ rte re displyed in Fig.. The profiles presented in Fig. nd recorded for the thinnest dimeters (65 nm, smple ) re consistent with previous studies. Indeed, the first chrge (lithition) exhiits long flt plteu locted t round. V corresponding to lithited silicon compound formed from crystlline silicon. For higher potentils, the electrochemicl processes minly involve the Solid Electrolyte Interphse (SEI) formtion tht is responsile of sustntil irreversile cpcity. Susequent chrges re different from the first one ut look the sme: in ccordnce with the literture, there re two lloying steps t round. V nd. V respectively ssigned to lithition of morphous nd crystlline silicon [8,,]. The contriution t higher potentil is henceforth minor, even if SEI is thought to e formed constntly during the cycles [ 5]. Whtever the cycle, the dischrge step (delloying) retins the sme profile, with single plteu t round 45 mv corresponding to the delithition into morphous silicon []. Moreover, in this cse, the hysteresis etween chrge nd dischrge curves is out only mv which mkes silicon mteril with n ttrctive working potentil. The shpe of the cycling curves otined in the sme experimentl conditions, with thicker dimeters, nmely nm (Fig. ) nd 49 nm (Fig. c), re totlly similr, ut cpcities re shown to strongly depend on dimeter: the smller the dimeter, the higher the specific cpcity. Indeed the first lithition cpcity ws 5698 mah g, 678 mah g nd 68 mah g for dimeters of 65 nm, nm nd 49 nm respectively. For the three smples, delithition cpcity is shrply lower (7 mah g, 59 mah g nd 86 mah g for 65 nm, nm nd 49 nm respectively), reveling quite high irreversile cpcity. The lithition process in the second cycles show the sme trend s oserved in first cycles since the cpcity increses s function of dimeter with 5 mah g, 667 mah g nd 8 mah g for respectively c 4 6 Q / mah g Q / mah g Q / mah g - Fig.. Voltge profiles of the first cycle nd second lithition step t C/ s function of SiNWs dimeter distriution (respectively (), () nd (c) for smple, nd ). Tle 8 wt% nd 5 wt% of silicon re distriuted in nnowires of dimeters lower thn the vlue presented in the tle. SiNWs dimeter, nm 8 wt% 5 wt% Smple 65 4 Smple Smple smples, nd. However, the irreversile cpcity is considerly reduced from the second cycle, suggesting tht SEI formtion minly occurs during the first reduction step t C/ rte. One my note tht irreversile cpcity gets higher when decresing the SiNWs dimeter due to the correltive increse of ctive surfce re.

4 8 A. Gohier et l. / Journl of Power Sources () 5 9 Tle Specific cpcities t C/ rte s function of SiNWs dimeter distriution. Q, mah g First lithition First delithition Second lithition Smple, nm As shown ove, only smll dimeter SiNWs (65 nm) exhiit high specific lithition cpcity (5 mah g ) close to theoreticl vlue (58 mah g ) t C/ rte. Using lrger dimeter SiNWs, cpcities do not exceed 6 mah g t sme rte (Tle ). We ssume tht this significnt difference is due to the shorter diffusion pthwy which in the cse of smllest dimeters proly llows quntittive lithition of smll dimeter SiNWs. We hve lso exmined the influence of the current density on the different groups of SiNWs in order to get n evlution of the rte cpility for ech smple. Rtes vrying etween C/5 nd 5 C hve een pplied. Fig. 4 displys the specific cpcities vs. cycle numer s function of the lithition/delithition rte for the smple with the smllest dimeters. One typicl chrge/dischrge cycle otined for ech rte re gthered in Fig. 4 d respectively for smples, nd. For smple, the cpcity of mah g otined for the first four cycles t C/5, is close to the theoreticl one. It corresponds to 86% of the initil mss of silicon implied in the electrochemicl processes, suggesting tht n internl core mde of c-silicon is conserved during cycling, even if tiny prt is systemticlly involved in ech new cycle. It cn e thought tht this crystlline frmework further ensures good mechnicl properties of the SiNWs. As long s it exists, ville cpcities do not decrese. Fig. 4 evidences tht the lower the rte, the higher the ville cpcities oth in lithition nd delithition processes. This result cn e understood insofr s the lithium diffusion in the silicon nnowire is the limiting process. Moreover, when the electron trnsfer ecomes too fst, it cn e thought tht only the externl prt of the wire is involved in the electrochemicl process, leding to reduced cpcity. Concerning electrochemicl performnces s function of current density, smll dimeter SiNWs exhiit the est results with specific lloying cpcity decresing from 4 mah g to 5 mah g when the C rte vries from C/5 to 5 C (Fig. 4). With smples nd, lthough the chrge dischrge profiles of glvnosttic curves re the sme, the cpcities otined t C/5 rte re lower nd the decrese is fster with the rte (Fig. 4c nd d), so tht with the lrgest dimeters, no cpcity is delivered t 5 C. Conversely, the high cpcity vlue of mah g chieved for smple t C rte (Fig. 4), i.e. twice the cpcity of smples nd (Fig. 4 nd c) clerly outlines the interest of the smllest dimeters. These results demonstrte the etter rte cpility provided y the use of smll SiNWs dimeter. The rte limiting process eing Li diffusion, the smll size of dimeters promotes more quntittive, more homogeneous nd fster Li lloying delloying process. To check this ssumption, the cyclility of smll dimeter SiNWs node ws investigted t. C rte upon 5 cycles (Fig. 5). The specific cpcity for delithition rpidly increses to rech roughly stle cpcity during the first 6 cycles up to 9 mah g. During the following lithition/delithition cycles, 4 Q / mah g - C/5 C/ delithition lithition C cycle numer.. C C/ C/5 4 Q / mah g - c d. C C/ C/5 4 Q / mah g -. C C/ C/5 4 Q / mah g - Fig. 4. () Influence of the current density, from C/5 to 5 C, on lithition (lck squres) nd delithition (white squres) for smple ; (), (c), (d) evolution of typicl cycling curves with regime, respectively for smple, smple nd smple.

5 A. Gohier et l. / Journl of Power Sources () Q / mah g efficiency / % delithition lithition 4 5 cycle numer Q / mah g - Fig. 5. () Lithition (lck mrkers), delithition (white mrkers) nd coulomic efficiency (crosses) of smll dimeter SiNWs (smple ) vs. cycle numer t. C rte; () evolution of typicl cycling curves corresponding to cycles 5, 5 nd 5. only wek cpcity decy is oserved, reveling good cpcity retention with still 67 mah g fter 5 cycles. This cpcity evolution vs. cycles t high rte of. C (j = 4.6 A g ) is very close to tht reported y Ruffo et l. [6] with n optimized potentil rnge ut t moderte rte only C/5 rte (j =.84 A g ). This finding clerly highlights the significnt improvement of the rte cpility provided y smll dimeter SiNWs. In other respects, the only one cpcity vlues reported in literture t high rte,.6 C, re otined for cycling experiments in reduced potentil rnge []. They re found to e much lower, y fctor two, thn tht reported in Fig. 5, for sme cutoff voltge of.5 V. To our knowledge, 5 C is remrkle high rte for SiNWs tht hs never een reported in the literture. 4. Conclusion Silicon nnowires present ttrctive properties s node mteril in lithium-ion tteries nd re widely studied. However, the role of SiNWs dimeter size hs een never ddressed nd in most cses not specified. Our SEM study nd electrochemicl experiments show tht the SiNWs dimeter size nd distriution constitute crucil requirement for optimizing the chrge dischrge properties. By tiloring the gold ctlyst film thickness for SiNWs growth, we hve demonstrted different dimeters re otined with systemtic distriution in dimeter size nd not men size vlue s usully reported in literture. For three gold thicknesses of, nd nm, 8 wt% of the electroctive silicon consist respectively in wires with dimeter size <65, nd 49 nm. The cpcity delivered y SiNWs hs een proved to e dependent on the wires dimeter: the smller the dimeter, the higher the cpcity. For instnce, t C/ rte, vlues of 5, 7 nd 4 mah g re respectively otined in the second lithition process for 65, nd 49 nm. The smple with smllest dimeter vlues exhiits the est rte cpility nd cn sustin high rtes up to 5 C with interesting cpcities of 5, 5 nd 5 mah g t C,.5 C nd 5 C respectively. Such high cpcity vlues for SiNWs t high rte hve never een reported s yet. These results cn e understood y the shorter diffusion pthwy for lithium in SiNWs with smll size dimeter which ensures fst nd homogeneous lithition. As consequence, in the cse of smllest dimeters (<65 nm) we show n excellent cycle life with stle cpcity of mah g t reltively high rte. C. This first contriution to the understnding on how the morphology of silicon nnowires impcts their electrochemicl performnce clerly shows the need for rigorous control of the SiNWs dimeter during their growth. Therefore the use of n pproprite templte step must e the next step for further optimiztion. References [] M.N. Orovc, L. Christensen, Electrochem. Solid Stte Lett. 5 (4) A9 A96. [] J. Li, J.R. Dhn, J. Electrochem. Soc. (7) A56 A6. [] B.A. Boukmp, G.C. Lesh, R.A. Huggins, J. Electrochem. Soc. 4 (98) [4] J.P. Mrnchi, A.F. Hepp, A.G. Evns, N.T. Nuhfer, P.N. Kumt, J. Electrochem. Soc. 6 (6) A46 A5. [5] H. Jung, M. Prk, Y.-G. Yoon, G.-B. Kim, S.-K. Joo, J. Power Sources () [6] S.H. Ng, J.Z. Wng, D. Wexler, K. Konstntinov, Z.P. Guo, H.K. Liu, Angew. Chem. Int. Ed. 4 (6) [7] Y. Yo, M.T. McDowell, I. Ryu, H. Wu, N.A. Liu, L.B. Hu, et l., Nno Lett. 7 () [8] C.K. Chn, H. Peng, G. Liu, K. McIlwrth, X.F. Zhng, R.A. Huggins, et l., Nt. Nno (8) 5. [9] B. Lik, D. Ung, A. Cillrd, C.C. Sorin, D. Prit, J.-P. Pereir-Rmos, J. Solid Stte Electrochem. () [] B. Lïk, L. Eude, J.-P. Pereir-Rmos, C.S. Cojocru, D. Prit, E. Rouvière, Electrochim. Act 7 (8) [] L.-F. Cui, R. Ruffo, C.K. Chn, H. Peng, Y. Cui, Nno Lett. (9) [] K. Peng, J. Jie, W. Zhng, S.-T. Lee, Appl. Phys. Lett. (8), [] M.H. Prk, M.G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, et l., Nno Lett. (9) [4] C.K. Chn, R.N. Ptel, M.J. O Connell, B.A. Korgel, Y. Cui, Acs Nno () [5] C.K. Chn, H.L. Peng, G. Liu, K. McIlwrth, X.F. Zhng, R.A. Huggins, et l., Nt. Nnotechnol. (8). [6] R. Ruffo, S.S. Hong, C.K. Chn, R.A. Huggins, Y. Cui, J. Phys. Chem. C 6 (9) [7] J.W. Choi, L.B. Hu, L.F. Cui, J.R. McDonough, Y. Cui, J. Power Sources 4 () [8] Y.J. Cho, H.S. Kim, H. Im, Y. Myung, G.B. Jung, C.W. Lee, et l., J. Phys. Chem. C 9 () [9] R. Hung, X. Fn, W.C. Shen, J. Zhu, Appl. Phys. Lett. (9). [] K. Kng, H.S. Lee, D.W. Hn, G.S. Kim, D. Lee, G. Lee, et l., Appl. Phys. Lett. 5 (). [] Z. Wen, M. Cheng, J. Sun, L. Wng, Electrochim. Act () [] M.N. Orovc, L.J. Kruse, J. Electrochem. Soc. (7) A A8. [] L. Bggetto, J.F.M. Oudenhoven, T. vn Dongen, J.H. Klootwijk, M. Mulder, R.A.H. Niessen, et l., J. Power Sources (9) 4 4. [4] C.K. Chn, R. Ruffo, S.S. Hong, Y. Cui, J. Power Sources (9) 4. [5] L. Chen, K. Wng, X. Xie, J. Xie, J. Power Sources (7)