Spherical silicongraphitecarbon composites as anode material for lithium-ion batteries.
Available online at https://www.sodocs.net/doc/0a7408878.html, Journal of Power Sources176(2008)
353–358
Short communication
Spherical silicon/graphite/carbon composites as anode
material for lithium-ion batteries
Jong-Hyuk Lee,Wan-Jun Kim,Jae-Youn Kim,
Sung-Hwan Lim,Sung-Man Lee?
Department of Advanced Materials Science and Engineering,Kangwon National University,Chuncheon,
Kangwon-Do200-701,Republic of Korea
Received28June2007;received in revised form22September2007;accepted28September2007
Available online18October2007
Abstract
A spherical nanostructured Si/graphite/carbon composite is synthesized by pelletizing a mixture of nano-Si/graphite/petroleum pitch powders, followed by heat treatment at1000?C under an argon atmosphere.The structure of the composite sphere is examined by transmission electron microscopy(TEM)and scanning electron microscopy(SEM)with energy dispersive X-ray analysis(EDAX).The resultant composite sphere consists of nanosized silicon and?aked graphite embedded in a carbon matrix pyrolyzed from petroleum pitch,in which the?aked graphite sheets are concentrically distributed in a parallel orientation.The composite material exhibits good electrochemical properties,a high reversible speci?c capacity of～700mAh g?1,a high coulombic ef?ciency of86%on the?rst cycle,and a stable capacity retention.The enhanced electrochemical performance is attributed to the structural stability of the composite sphere during the charging–discharging process.
?2007Elsevier B.V.All rights reserved.
Keywords:Silicon/graphite/carbon composite;Anode material;Lithium-ion battery;Spherical composite;Electrochemical performance;Speci?c capacity
1.Introduction
Lithium-ion batteries containing graphite anodes are now the most widely used power sources for portable electronic devices.For commercial applications,however,lithium-ion batteries with high speci?c energy density are in increasing demand.Recently,various anode materials have been proposed to overcome the limited capacity of graphite(372mAh g?1). Much attention has been given to silicon-based anode materials because silicon has the high speci?c capacity of4200mAh g?1. Unfortunately,silicon undergoes a change in volume change during lithium-ion insertion and extraction,and this result in mechanical instability and poor cycleability[1,2].There-fore,in order to obtain better cycleability for silicon-based anodes,the mechanical stresses induced by the large volumetric change should be buffered.Many attempts to solve this problem ?Corresponding author.Tel.:+82332506266;fax:+82332426256.
E-mail address:smlee@kangwon.ac.kr(S.-M.Lee).have been made[3–28].For example,silicon–graphite–carbon composite systems in which ultra?ne silicon particles are dispersed uniformly in a ductile matrix of graphite and the mixtures are coated with carbon,are quite promis-ing for practical applications[22–28].The electrochemical performance of such composites appears to be dependent on the preparation method and the resultant structure of the composite.Moreover,it is well known that spherically shaped anode material is preferable as a means of improv-ing the anode characteristics due to its high tapping density [29,30].
In this study,a spherical silicon–graphite–carbon composite anode material for lithium-ion batteries,is prepared and found to exhibit a large charge–discharge capacity and excellent cycle performance.
2.Experimental
Spherical nano-Si/graphite/carbon composite powders were fabricated as follows.Nano-Si(99.9%,100nm,Nanostructured
0378-7753/$–see front matter?2007Elsevier B.V.All rights reserved. doi:10.1016/j.jpowsour.2007.09.119
354J.-H.Lee et al./Journal of Power Sources 176(2008)
353–358
Fig.1.SEM images of (a)nano-silicon and (b)natural ?ake
graphite.
Fig.2.SEM images of (a)the composite precursor and (b)composite particles.
&Amorphous Materials Inc.)and natural graphite (～5?m)powders were mixed in a weight ratio of 30:70.The ele-mental powders were mixed with a tetrahydrofuran solution in which a pitch (carbon yield,76%,provided by Carbonix Inc.,South Korea)as a carbon precursor was dissolved in 33wt.%of the mixture,agitated using a ball milling method,and ?nally vacuum-dried at 100?C for 6h.The dried composite was inserted into an agglomerator to prepare the ?rst spheri-cal composite particles.Then,the ?rst composite particles and petroleum-based pitch powders were mixed in a weight ratio of 87:13,followed by pelletizing and heating under an
argon
Fig.3.XRD pattern of the composite material.
atmosphere at 1000?C.The pelletizing process was performed in an air atmosphere.The particle morphology and electrode cross-section were examined by SEM with EDAX equipment.A TEM analysis was performed to observe the structure and dispersion of the components in the nano-Si/graphite/carbon composite particles.The sample for TEM analysis was pre-pared by means of a focused ion beam (FIB)technique.The phase components of the composite material were investi-gated by powder X-ray diffractometry (XRD)with Cu K ?radiation.
The electrodes were prepared by pasting an aqueous slurry containing 90wt.%composite powder,5wt.%carbon black,3.3wt.%styrene butadiene rubber (SBR)as a binder,and 1.7wt.%carboxymethyl cellulose (CMC)as a surface-active agent,on to a copper foil of 10?m thickness.The electrodes were then dried at 120?C for 12h under vacuum and subse-quently pressed.The electrolyte was 1M LiPF 6in a mixture of ethylene carbonate (EC)and diethyl carbonate (DEC)(1:1,v/v,provided by Cheil Industries Inc.,South Korea).2016coin-type half-cells were fabricated using a metallic lithium foil as a counter electrode.
The cycling performance of the cells was evaluated in a constant current–constant–voltage (CC–CV)mode for the initial two cycles and thereafter in constant–current (CC)mode.The CC–CV test was performed by holding the cur-rent at 0.2mA cm ?2until the potential dropped to 0.02V and then maintaining this potential until the current decreased to 0.1mA cm ?2.For the CC test,the charge–discharge current was 0.2mA cm ?2and the cut-off voltage was set between 0.02and 1.5V .
J.-H.Lee et al./Journal of Power Sources 176(2008)353–358355
Fig.4.Cross-section SEM images of silicon/graphite/carbon composite sphere (a and b)and EDAX maps of (c)silicon and (d)carbon.Note that the sample for (b)is obtained using a FIB workstation.
3.Results and discussion
The SEM images of the nano-silicon and natural ?ake graphite,used to prepare the Si/graphite/carbon composite mate-rials,are shown in Fig.1.The Si powders show an uniform particle-size distribution between 50and 100nm.The overall particle shape of graphite was thin and ?at,and the median diameter was 5?m.
The composite material was prepared by pelletizing a mix-ture of nano-silicon/graphite/petroleum pitch in a weight ratio of 20:47:33,using a method which is similar to that for making spheres from ?aky graphite [31],followed by a second pelletiz-ing of the ?rst pelletized powders and petroleum pitch powders in a weight ratio of 87:13.The resultant composite precursor was heat-treated at 1000?C for 1h in an argon atmosphere to obtain the nano-silicon/graphite/carbon composite.Fig.2shows the SEM images of the composite precursor and composite parti-cles.It is seen that spherical particles of the composite precursor are obtained,and that this shape is maintained after heat treat-ment at 1000?C.The XRD pattern of the composite material is shown in Fig.3.There exists only the crystalline diffraction peaks of the silicon and graphite in addition to a broadened diffused peak at around 2θ=26?that is attributable to the disor-dered pitch carbon.Notably,no other impurity phases,such as SiC,are detected.
In order to examine the microstructure of the composite particles,SEM and TEM analyses were conducted.Typical cross-section SEM images of the Si/graphite/carbon composite spheres are presented in Fig.4a and b.It should be noted that the image in Fig.4b was obtained using a FIB workstation,and in the
Fig. 5.Cross-sectional TEM images of silicon/graphite/carbon composite sphere.
356J.-H.Lee et al./Journal of Power Sources 176(2008)
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Fig.6.High-magni?cation TEM micrographs of silicon/graphite/carbon composite:(a)bright-?eld,(b)dark-?eld (Si),(c)dark-?eld (graphite)and (d)selected area diffraction pattern.
process,part of the sample was broken.A microscale hole and cracks inside the composite sphere are found.The EDAX ele-mental mapping images shown in Fig.4c and d indicate that the silicon content is higher in the centre region than in the surface region,which originates from the composite sphere making pro-cess,that is conducted in sequential steps as described in Section 2.The composite particles consist of concentric ?aked graphite in a parallel orientation,which combined to form spherical par-ticles as shown in Fig.5.Magni?ed cross-sectional TEM
images
Fig.7.High-resolution TEM image of composite.
are given in Fig.6.The bright-and dark-?eld images reveal that Si nanocrystallites and thin graphite sheets of nanosize thickness are ?nely distributed in the carbon matrix pyrolyzed from the petroleum pitch.The selected area diffraction pattern (SADP)exhibits well-de?ned rings with spots that can be indexed to crystalline Si as well as diffused rings corresponding to nanosize graphite,There is no indication of the presence of SiC.It is also found that the ?aked graphite sheets are very thin (50–100nm).A high-resolution image of the composite is given in https://www.sodocs.net/doc/0a7408878.html,t-tice fringes corresponding to the nanocrystalline Si and nanosize graphitic stacks are visible.The nanocrystalline Si is surrounded with amorphous carbon but without forming any compound such as SiC.
Fig.8shows the charge (Li insertion)–discharge (Li extrac-tion)curves of the Si/graphite/carbon composite for the ?rst and second cycles,charged in CC–CV mode and discharged in CC mode.The reversible capacity obtained by the CC mode is ～700mAh g ?1which is maintained during cycling up to 50cycles.The reversible capacity and the coulombic ef?ciency of the composite electrode as a function of cycle number are pre-sented in Fig.9.The composite exhibits very stable capacity retention behavior and high coulombic ef?ciency,with a ?rst-cycle ef?ciency of 86and >99%ef?ciency for the subsequent cycles.The morphological changes of the composite electrode before and after 50cycles were examined by SEM (Fig.10).They reveal a similar surface morphology.Notably,after 50cycles,no cracks are formed.These results indicate that the microstructure of the nano-Si/graphite/carbon composite sphere is very effective in buffering the volume expansion of Si during
J.-H.Lee et al./Journal of Power Sources 176(2008)353–358
357
Fig.10.Morphology of composite electrode:(a)before cycling and (b)after 50
cycles.
Fig.8.Charge–discharge curves of Si/graphite/carbon composite for ?rst and second cycles.
cycling.Moreover,since spherical particles lead to an improved tap density,the resultant spherically shaped composites are highly desirable as anode materials for lithium-ion batteries.It is also worthwhile to the note that the present composite sphere is fabricated by a simple process,which is similar to that used to obtain spheroidal shapes from ?aky natural graphites [31].Higher capacity of the nano-si/graphite/carbon composite can be achieved by increasing the Si content,but if the Si content is increased,the fabrication conditions including the weight
ratio
Fig.9.Charge–discharge capacities and coulombic ef?ciency of composite electrode as function of cycle number.
of the composite components and the size of the composite par-ticles,need to be optimized in order to obtain an enhanced cycle performance.Relevant studies are currently in progress,and the results will be reported in subsequent publications.4.Conclusions
A nano-Si/graphite/carbon composite of spherical shape is synthesized by pelletizing a mixture of nano-Si/graphite/petroleum pitch powders,followed by pyrolysis at 1000?C under an argon atmosphere.The composite sphere,as indicated by SEM–EDAX and TEM analyses,consisted of nanosized sili-con and ?aked graphite embedded in a carbon matrix pyrolyzed from petroleum pitch,in which the thin graphite sheets are con-centrically distributed in a parallel orientation.The composite shows a high reversible capacity of ～700mAh g ?1,along with an initial coulombic ef?ciency of 86%and good cycleabil-ity.The good electrochemical performance is attributed to the enhanced structural stability of the composite sphere,in which the stress induced by the volume change of the silicon phase during cycling is effectively buffered.Acknowledgement
This work was supported by the Ministry of Information &Communications,Korea,under the Information Technology Research Center (ITRC)Support Program.References
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