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Self-assembly of Si entrapped graphene architecture for high-performance Li-ion batteries

Sang-Hoon Park a ,Hyun-Kyung Kim a ,Dong-Joon Ahn b ,Sang-Ick Lee b ,Kwang Chul Roh c ,⁎,Kwang-Bum Kim a ,⁎⁎

a Department of Materials Science and Engineering,Yonsei University,134Shinchon-Dong,Seodaemoon-Gu,Seoul 120-749,Republic of Korea

b Advanced Battery Materials Team,GS Energy,Daejeon 305-380,Republi

c of Korea

c

Energy and Environmental Division,Korea Institute of Ceramic Engineering &Technology,233-5Gasan-Dong,Guemcheon-Gu,Seoul 153-801,Republic of Korea

a b s t r a c t

a r t i c l e i n f o Article history:

Received 18April 2013

Received in revised form 25May 2013Accepted 27May 2013

Available online 31May 2013Keywords:Si

Graphene

Reduced graphene oxide (RGO)Si/RGO architecture Self-assembly Anode material

Si nanoparticles were successfully entrapped between graphene nanosheets by simple self-assembly of chemically modi fied graphene (RGO)without using any chemical/physical linkers.The resulting Si/RGO ar-chitecture possessed a more ef ficient conducting/buffering framework for Si nanoparticles when compared to the framework of the mechanically mixed Si/RGO product.The Si/RGO architecture exhibited an improved cyclability (1481mAh/g after 50cycles)and showed favorable high-rate capability.

©2013Elsevier B.V.All rights reserved.

1.Introduction

The exponential increase in the demand of electric vehicles,por-table electronics,and renewable energy storage facilities has trig-gered focus on high-energy-density rechargeable lithium-ion batteries with long cycle life as attractive energy storage devices [1].To obtain high-performance lithium-ion batteries,electrode ma-terials with high speci fic/volumetric capacity and satisfactory cycle life are eagerly sought after [2].Among the various electrode mate-rials available for lithium-ion batteries,silicon,owing to its highest theoretical speci fic capacity (4200mAh/g),low operating potential,natural abundance,and low cost,is one of the most attractive anode materials [3].However,its practical application is greatly hin-dered by the large volume changes (over 400%)that occur upon lithiation/delithiation;these large volume changes could cause dra-matic pulverization and loss of electrical contact between the active materials,leading to rapid capacity fading [4].In order to effectively exploit Si-based materials,recent research has focused on the syn-thesis of nanostructured Si (such as nanowires,nanotubes,and mesoporous Si)or Si/carbon composite materials [3–8].

Recently,graphene,a new nanocarbon with one-atom thickness and two-dimensional honeycomb lattice has attracted considerable attention as an excellent template to design Si-based composite ma-terials owing to its high electrical conductivity,mechanical flexibility,and large surface area (over 2600m 2/g)[9–16].In these composites,graphene acts as highly conductive network and buffering matrix for the volume variation of Si during lithiation/delithiation.Despite the improved electrochemical performance,the strategies for the synthe-sis of these composite materials have generally focused on homoge-neously mixing and dispersing Si nanoparticles onto the surface of the graphene nanosheet [12,13].Furthermore,for some Si/graphene composites,multi-step and the use of chemical or physical linkers are necessary to bind Si nanoparticles with the graphene nanosheet [14–16].

Here,we report a simple self-assembly process that uses chemi-cally modi fied graphene (RGO)for entrapping Si nanoparticles.Un-like previous studies,in our procedure,chemical/physical linkers or complex equipment are not required to assemble the Si nanoparticles between networks of RGO nanosheets.The Si entrapped RGO archi-tecture (denoted as Si/RGO architecture)prepared in this study pos-sesses a more ef ficient framework to buffer the large volume changes in Si caused during cycling,when compared to the frame-work of conventional Si/RGO electrodes obtained by mechanical mixing.Hence,the Si/RGO architecture is expected to exhibit signi fi-cantly improved electrochemical performance as an anode material in lithium-ion batteries.

Electrochemistry Communications 34(2013)117–120

⁎Corresponding author.Tel.:+82232822463.⁎⁎Corresponding author.Tel.:+8223657745.

E-mail addresses:rkc@kicet.re.kr (K.C.Roh),kbkim@yonsei.ac.kr (K.-B.

Kim).1388-2481/$–see front matter ©2013Elsevier B.V.All rights reserved.

http://dx.doi.org/10.1016/j.elecom.2013.05.028

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Electrochemistry Communications

j o ur n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c o m

2.Experimental

Graphite oxide was synthesized from graphite flakes (~45μm,99.99%,Aldrich)by a modi fied Hummers method with sulfuric acid (95%,Samchun Chemical),potassium permanganate (99%,Aldrich),and hydrogen peroxide (35wt.%in water)[16].Then,the as-prepared graphite oxide was exfoliated and dispersed in de-ionized water at a con-centration of 4.0mg/ml by ultrasonication to obtain graphene oxide (GO)suspension.Si nanoparticles (~100nm,GS-Energy)were also dis-persed in de-ionized water at a concentration of 1mg/ml by ultrasonication.The GO and Si suspensions were homogeneously mixed at a volume ratio of 1:2(mass ratio of GO:Si =2:1in the mixed suspension)Then,L-ascorbic acid (99%,Aldrich)as reducing agent was added to the Si-GO suspension,and the mixture was maintained at 70°C without stirring to allow the self-assembly of GO.The self-assembled Si/RGO hydrogel was washed and freeze-dried overnight to obtain the Si/RGO architecture (aerogel),which was heat-treated at 600°C under Ar gas.

The Si/RGO architecture was characterized by scanning electron microscopy (SEM)and transmission electron microscopy (TEM).X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),and estimation by the BET method were carried out to determine the structural/chemical properties and the surface area of the sam-ples.Thermogravimetric analysis (TGA)was carried out to evaluate the weight percent of Si in the Si/RGO architecture.

The electrochemical properties were investigated using 2032coin cells,with lithium foil as the counter electrode and 1M LiPF 6in ethyl carbonate/dimethyl carbonate (1:1v/v)as the electrolyte.The working electrode was prepared by slurry casting a mixture of

90wt.%Si/RGO architecture and 10wt.%binder onto Cu foil.For com-parison,we also prepared two different electrodes composed of me-chanically blended Si/carbon black (Si/CB)and Si/RGO (Si/RGO mixture).The weight ratios of Si,carbon (CB or RGO),and binder in the electrodes were fixed at 50,40,and 10wt.%,respectively.The speci fic capacities of all the electrodes were based on the total weight of Si and carbon.The electrochemical characterization was performed within 2.5–0.001V using a potentiostat/galvanostat (VMP3,Bio-logic).Electrochemical impedance spectroscopy was carried out using an impedance analyzer at a DC bias voltage of 0.2V at AC fre-quencies ranging from 200kHz to 10mHz.3.Results and discussion

Fig.1a shows the schematic of the self-assembly of the Si/RGO ar-chitecture from the Si/GO mixed suspension.Graphite oxide can be well dispersed into aqueous medium to form a stable GO suspension because of its hydrophilic nature attributed to the oxygen functional-ities [17].During reduction,the oxygen functionalities in the GO are diminished,increasing its hydrophobicity in the aqueous suspension.When reduction is carried out under homogeneous mechanical stirring,the RGO sheets do not assemble at a macroscopic scale and instead form micro-aggregated powders.However,when the reduc-tion occurs without any external disturbance,GO can self-assemble into a 3D RGO architecture owing to the hydrophobicity and π–πstacking,while maintaining the shape of the vessel that contains the dispersed GO suspension [18].A similar approach can be applied to assemble the Si/RGO architecture in the presence of Si nanoparticles homogeneously dispersed with GO in the suspension [19].

Interestingly,

Fig.1.(a)Photographic images depicting the preparation of Si/RGO architecture.(b)XRD patterns of graphite oxide,Si,and Si/RGO architecture.(c)XPS C1s spectra of graphite oxide and Si/RGO architecture.(d)Low-magni fication SEM,(e)high-magni fication SEM,and (f)TEM images of the Si/RGO architecture.

118S.-H.Park et al./Electrochemistry Communications 34(2013)117–120

during the reaction,the mixed suspension gradually turned transparent with the formation of the Si/RGO hydrogel,indicating that most of the homogeneously dispersed Si nanoparticles might have been entrapped in the assembled RGO nanosheets.(However,for GO:Si mass ratios ex-ceeding 2:3,Si/RGO hydrogels were not obtained,indicating that the highly concentrated Si might have interrupted the π–πstacking be-tween RGO nanosheets)After the reaction,the Si/RGO architecture (aerogel)was obtained from the hydrogel by freeze-drying.

The XRD pattern of as-prepared graphite oxide shows the main dif-fraction peak at 2θ=9°corresponding to the (002)plane (Fig.1b).The interlayer spacing (0.99nm)is higher than that of graphite (0.34nm),which can be attributed to introduction of oxygen functionalities by Hummers method [17].The peak positions in the XRD patterns of the Si/RGO architecture were identical to that of both RGO and Si,indi-cating that the Si nanoparticles were ef ficiently embedded during self-assembly (Fig.1b).The XPS C1s spectra of graphite oxide and Si/RGO architecture showed typical components arising from C =C/C-C (sp 2/sp 3,~284.6eV),C –O (hydroxyl/epoxy,~286.5eV),and C =O (carbonyl,~287.8eV)(Fig.1c),as established previously [15].After self-assembly of the Si/RGO architecture,both C –O and C =O peaks in the C1s spectrum signi ficantly decreased,which could be attributed to the removal of oxygen functionalities by the reduction of GO to RGO [18].The atomic concentrations of C,O,and Si in the Si/RGO archi-tecture obtained by wide-scan XPS were 85.2,8.7,and 3.9,

respectively,

Fig.2.(a)Charge –discharge pro files and (b)dQ/dV differential pro files of Si/RGO architecture.(c)Cyclability of the Si/CB,Si/RGO mixture,and Si/RGO architecture electrodes at a current density of 500mA/g.(d)Rate capability of the Si/RGO mixture and Si/RGO architecture electrodes.Nyquist plots for (e)Si/RGO mixture and (f)Si/RGO architecture elec-trodes during cycling.

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S.-H.Park et al./Electrochemistry Communications 34(2013)117–120

The Si/RGO architecture in Fig.2d clearly shows that the2D RGO nanosheets were interconnected in the3D macroporous architecture. The high-magnification SEM image(Fig.2e)shows that~100-nm sized Si nanoparticles were entrapped in the RGO nanosheets(red ar-rows),indicating that the nanoparticles could be efficiently entrapped during self-assembly.Similar to the SEM image,the TEM image of the individual nanosheets in the Si/RGO architecture(Fig.2f)reveals that the Si nanoparticles are covered with wrinkled RGO nanosheets (green arrows).

The above results demonstrate that Si nanoparticles were efficiently entrapped between the RGO nanosheets during self-assembly,possibly leading to improved electrochemical performance attributable to both conduction of electrons and efficient buffering of the volume change of Si during Li alloying-dealloying.Fig.2a shows the galvanostatic charge-discharge curves of the Si/RGO architecture electrode at a cur-rent density of500mA/g.Thefirst discharge and charge capacities were2438and1638mAh/g,respectively,and the irreversible capacity loss of the Si/RGO architecture could have resulted from the formation of a solid-electrolyte interphase and the reaction of Li with residual functionalities in RGO[11,12].After50cycles,the discharge capacity of the Si/RGO architecture was still as high as1481mAh/g,which corresponded to83.4%of the second discharge capacity,indicating ex-cellent cyclability.The dQ/dV profiles of the Si/RGO architecture elec-trode(Fig.2b)show that during thefirst discharge,a peak attributed to the formation of a Li-Si alloy phase appeared at0.07V,while the two peaks at0.31and0.50V during thefirst charge could be ascribed to dealloying of the Li-Si phase[12].A small peak at1.2V observed dur-ing thefirst discharge might be related to the reaction of the remaining oxygen functionalities in RGO[20].Fig.2c shows the discharge capaci-ties achieved during cycling for the mechanically blended Si/CB,Si/ RGO mixture,and the Si/RGO architecture electrodes.The initial dis-charge capacity and cycle performance of the Si/CB(1660and 1mAh/g after50cycles,respectively)was much lower than those shown by both Si/RGO mixture and Si/RGO architecture,indicating that the RGO can provide more efficient conducting/buffering network for the Si-based electrode.Thefirst and second discharge capacities of Si/RGO mixture electrode were2271and1594mAh/g,respectively, which were similar to those of Si/RGO architecture.However the capac-ity of the Si/RGO mixture electrode gradually decreased to769mAh/g after50cycles.Despite the similar amount of RGO in the Si/RGO mix-ture and Si/RGO architecture electrodes,the Si/RGO architecture elec-trode exhibited markedly better cyclability(1481mAh/g after 50cycles)than the Si/RGO mixture electrode.The Si/RGO architecture electrode exhibited good cyclability and high-rate capability(Fig.2d). The Si/RGO architecture electrode retained discharge capacities of 1420,1040,and705mAh/g,respectively,when the current densities increased from1000to2000,and to5000mA/g.These values are signif-icantly higher than that exhibited by the Si/RGO mixture electrode.Fur-thermore,the electrochemical performance of our Si/RGO architecture is one of the outstanding values reported for Si/RGO composite elec-trodes,indicating that this facile strategy of self-assembly afforded efficient buffering/conducting networks in the Si/RGO electrode,with-out use of any chemical/physical linkers or specific equipment to bind the RGO with Si nanoparticles[10–16].

Fig.2e and f show the Nyquist plots for the Si/RGO mixture elec-trode and the Si/RGO architecture electrode.Both Nyquist plots show a semicircle at high frequency and an inclined line at low fre-quency,which can be assigned to the charge-transfer resistance and semi-diffusion of lithium ions into the Si host[10,15].In the Si/RGO mixture electrode,the diameter of the semicircle clearly increased during cycling.However,the diameter of the semicircle shown by the Si/RGO architecture electrode increased marginally after30cy-cles,demonstrating that the Si/RGO architecture possessed a more stable conducting/buffering network than the Si/RGO mixture during lithiation/delithiation.

4.Conclusions

In conclusion,we have successfully synthesized Si/RGO architec-ture by the simple self-assembly of RGO.In comparison to the frame-work of the architecture of conventional Si/RGO mixtures,the Si/RGO architecture prepared in this study possessed a more efficient conducting/buffering framework for Si during lithiation/delithiation. The Si/RGO architecture electrode exhibited the discharge capacity of1481mAh/g after50cycles and high-rate capability of705mAh/g at5000mA/g.

Acknowledgements

This work was supported by the energy efficiency and resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP)grant funded by the Ministry of Knowledge Economy,Kore-an government(No:20122010100140).

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