化学还原的RGO和碳球制备的三维锂离子阳极材料,JMC,2012

Composites of chemically-reduced graphene oxide sheets and carbon nanospheres with three-dimensional network structure as anode materials for lithium ion batteries ?

Yongqiang Yang,Ruiqing Pang,Xuejiao Zhou,Yan Zhang,Haixia Wu *and Shouwu Guo *

Received 23rd July 2012,Accepted 13th September 2012DOI:10.1039/c2jm34843h

It is challenging to develop lithium ion batteries (LIBs)possessing simultaneously large reversible capacity,high rate capability,and good cycling stability,which are in turn determined mainly by the component materials of batteries.We designed and synthesized a series of composites of chemically-reduced graphene oxide (CRG)sheets and carbon nanospheres (CNS).It was illustrated that within the as-obtained composites the CNSs were fully cladded and bridged with CRG sheets forming a three-dimensional (3D)network with cavities and pores.Coin cells using the anodes made of the as-obtained composites with appropriate composition exhibit large reversible capacity,high rate capability,and good cycling stability.The highest reversible speci?c capacity could reach up to 925mA h g à1and 604mA h g à1at charge–discharge current densities of 5A g à1and 10A g à1,respectively,and faint capacity and rate capability fades were detected even after 200charge–discharge cycles.The excellent

electrochemical performance of the anodes made of the as-obtained composites in the LIBs originates from the unique 3D network structure and the intrinsic properties of CRG and CNS that provide plenty of transportation pathways for electron and Li +,and suf?cient tolerant sites for Li/Li +.

Introduction

Rechargeable lithium ion batteries (LIBs)have been widely used in portable electronic devices such as cellular phones and lap-top computers,and hold great promises to the batteries for electric/hybrid vehicles and stationary electricity storage (called power LIBs).1–8However,as a prerequisite for practical application there are still several key issues for power LIBs that need to be improved,for example the speci?c capacity,rate capability,cycling stability,safety and cost.9–11From a materials science point of view these issues concerning power LIBs are closely related to the electrode (anode and cathode)and electrolyte materials utilized.Crystalline-layered LiCoO 2,spinel LiMn 2O 4,olivine LiFePO 4,LiFe(Mn)PO 4,LiCo 1/3Ni 1/3Mn 1/3O 2,and others have been intensively studied as cathode (positive elec-trode)materials and some of them have been found practically applicable.12–15However,it is still challenging to get cathode material with large speci?c capacity and high rate capability satisfying power LIBs.Similarly,graphite and other carbona-ceous materials have been utilized as anode (negative electrode)materials,but their electrochemical properties are not

satisfactory for power LIBs either.5,10,16–18Therefore,there has been an increasing demand for developing both cathode and anode materials with large reversible speci?c capacity and high rate capability.19

Graphene and chemically-reduced graphene oxide (CRG)sheets have been considered ideal anode materials due to their ultra-large speci?c surface area,excellent electrical conductivity,good chemical stability,and intriguing mechanical proper-ties.20–22More pronouncedly,as anode materials for LIBs,besides the incomparable electron transportation capability,graphene and CRG sheets show decent lithium ion (Li +)mobility ($10à7to 10à6S cm à1),and the great tolerant capability for lithium (Li)owing to the formation of Li 2C 6complex in prin-ciple.10,23,24It has been illustrated that some of graphene and CRG anodes exhibit high capacity of $1264mA h g à1at low charge/discharge rates of 0.05–0.1A g à1,but it drops rapidly with an increasing charge/discharge rate,and shows a poor cycling stability.25The reason might be that the graphene and CRG sheets aggregated in the anodes form a layered motif that is similar to the graphite in which the insertion and extraction of the lithium ions was blocked.24–29Therefore,numerous composites of graphene or CRG with different nanomaterials,such as CRG/carbon nanotubes (CNT),28,30–32CRG/full-erene(C 60),32CRG/inorganic nanoparticles,2,24,26–29,31,33–44have been prepared and used in anodes.By incorporation of the nanoparticles,the aggregation of graphene and CRG sheets could be suppressed essentially.Another advantage for the use of

Key Laboratory for Thin Film and Microfabrication of the Ministry of Education,Research Institute of Micro/Nano Science and Technology,Shanghai Jiao Tong University,Shanghai 200240,P.R.China.E-mail:swguo@https://www.wendangku.net/doc/8d7290104.html,;haixiawu@https://www.wendangku.net/doc/8d7290104.html,

?Electronic supplementary information (ESI)available.See DOI:10.1039/c2jm34843h Dynamic Article Links C

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Materials Chemistry

Cite this:J.Mater.Chem.,2012,22,https://www.wendangku.net/doc/8d7290104.html,/materials

PAPER

P u b l i s h e d o n 13 S e p t e m b e r 2012. D o w n l o a d e d b y S h a n g h a i N o r m a l U n i v e r s i t y o n 18/07/2016 17:44:05.

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composites is that it can effectively prevent the volume expan-sion/contraction of the inorganic nanoparticles during the Li insertion and extraction processes.Indeed,it has been demon-strated that the anodes made of these composites have a rela-tively high capacity,but their cycling stabilities,rate capabilities,and cost for mass production remain to be improved.29,33,45–47Meanwhile,the carbon nanospheres (CNS)can be prepared from glucose and other carbon precursors on bulk scale,48and show manifest electrochemical performance as anode for LIBs.49Hence,CNSs could be another category that can be used for fabrication of graphene or CRG-based composites that could be used as anode materials for LIBs.Prospectively,having the synergistic effects of CNS and graphene or CRG,the as-designed all-carbon anode may have a large speci?c capacity and high rate capability that are suf?cient to power LIBs.

To this end,we describe herein a novel category of all-carbon composites of CRG/CNS as anode materials prepared through a facile and scalable procedure.The structure and morphology of as-synthesized all-carbon composites have been characterized using scanning and transmission electron microscopies (SEM and TEM),atomic force microscopy (AFM),X-ray powder diffraction (XRD),and other spectroscopies.It has been illus-trated that the CNSs have been fully cladded and bridged by the CRG sheets affording the composites with a 3D network struc-ture,which contains plenty of pores and cavities.The electro-chemical measurements on the composites have been carried out in coin cells with a Li foil as the counter electrode and 1M LiPF 6in 1:1ethylene carbonate (EC)and dimethyl carbonate (DMC)as the electrolyte.It has been demonstrated that the ?rst cycle reversible capacity of CRG/CNS can reach 925mA h g à1at a current density of 5A g à1,and remained as high as 700mA h g à1even after 200charge–discharge cycles,which is much higher than those of the anodes made of graphite and other carbona-ceous materials at such high a charge–discharge rate.

Experimental section

Preparations of graphene oxide (GO)sheets and CNSs

GO was prepared from graphite powder through a modi?ed Hummers method that we described previously.50,51CNSs were synthesized through a hydrothermal procedure using glucose and glutaric acid as precursors and mineralization reagent,respec-tively.In a typical experiment,5g of glucose and 0.1g of glutaric acid were dissolved in 40mL of deionized (DI)water.The solution was transferred into a Te?on autoclave,and then heated at 180 C for 5h.After being cooled naturally to room temperature,the solid product was separated by centrifugation and washed three times with mixture of DI water and ethanol (1:1in volume).The as-obtained CNSs were ?nally dispersed in water and kept at room temperature ($23 C)for further char-acterization and usage.

Fabrication of CRG/CNS composites

To prepare CRG/CNS composites,the CNSs and GO aqueous suspensions were mixed under ultrasonication with the ratios of GO to CNS of 1/1,5/2,5/1,10/1and 20/1in weight,respectively.After the water was slowly evaporated at 80 C in an oven,the composites with different compositions were obtained.Finally,

the composites were annealed at 900 C in an argon (Ar)gas ?ow containing 5%of H 2for 3h to reduce the GO to CRG and graphitize the CNSs.The as-obtained composites were named as CRG/CNS1-1,CRG/CNS5-2,CRG/CNS5-1,CRG/CNS10-1and CRG/CNS20-1,respectively,according to the original ratio of GO to CNS.For comparison,the bare CRG sheets and graphitized carbon nanospheres were annealed under the same conditions.Characterization

SEM images were acquired on an Ultra 55scanning electron microscope (Zeiss,Germany).TEM images were obtained using a JEM-2010transmission electron microscope (JEOL Ltd.,Japan).The FTIR spectra were recorded on an EQUINOX 55FTIR spectrometer (Bruker,Germany).The specimens for FTIR measurement were prepared by grinding the dried powder with KBr together and then compressed into thin pellets.Nitrogen adsorption/desorption isotherms were measured at 77K on an automated adsorption apparatus (Micromeritics,USA).The surface area was determined using the Brunauer–Emmett–Teller (BET)method.X-ray powder diffraction (XRD)patterns were obtained on a D/max-2600PC diffractometer (Rigaku,Japan)

using Cu/K a radiation (l ?1.55406 A).

Raman spectroscopy measurements were performed on an Ar ion laser Micro-Raman spectroscope (Jobin Yvon LabRam HR 800UV,France)with an excitation laser beam wavelength of 514.5nm.

The electrochemical measurements have been carried out with coin cells.To prepare the working electrode for the coin cells,the as-fabricated CRG/CNS composite was mixed with poly-vinylidene ?uoride binder (90:10in weight)under mechanical grinding in an agate mortar to get a slurry mixture.The slurry mixture then was coated on Cu foil as a thin ?lm with thickness of $20m m,and dried at 120 C under vacuum for 12h.The amount of active material loaded onto the electrode ?lm is $1mg cm à2.The thin ?lm on Cu foil was ?nally cut into round disks of 12mm in diameter,which were dried at 100 C for 2h under vacuum and used as anodes for the coil cells.The coil cells were assembled in an argon-?lled glove box with less than 0.5ppm of oxygen and water,using lithium metal as the counter/reference electrode,a Celgard 2325membrane separator,and 1M of LiPF 6in 1:1ethylene carbonate (EC)and dimethyl carbonate (DMC)as the electrolyte.The as-assembled coin cells (usually called CR2025)were used for electrochemical measurements.Galvanostatic charge–discharge curves were recorded on a LAND CT2001A electrochemical workstation (Wuhan,China)at various current densities from 0.2to 10A g à1between 3.0and 0.01V versus Li +/Li at room temperature.The cyclic voltam-mograms (CV)were obtained from 0.01to 3.00V at a scanning rate of 0.1mV s à1,and electrochemical impedance spectroscopy (EIS)were carried out by applying a perturbation voltage of 5mV in a frequency range of 0.01Hz to 100KHz using an electrochemical working station (CHI 660C)(Shanghai,China).

Results and discussion

The morphology of the GO sheets used in this work was ?rst characterized.Fig.1a shows an AFM image of GO acquired using tapping mode.The average height of the ?at GO sheets is

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$1nm (Fig.1a,inset),demonstrating the single atomic layered feature.50,51The GO sheets have abundant surface oxygen-con-taining groups such as hydroxyl,epoxide,and carboxylic groups,illustrated by Fourier transform infrared spectroscopy (FTIR)(see Fig.S1A?),and thereby show moderate water solubility.The size and morphology of as-prepared CNSs in this work were characterized using SEM.As depicted in Fig.1b,the sizes of CNSs range from 200to 250nm in diameter with a modest size distribution.Different from other approaches of carbon nano-spheres preparation,48,49the glutaric acid was used as a miner-alization reagent in this work.Thus,the as-obtained CNSs have good aqueous dispersion property which might be due to the surface oxygen-containing groups introduced by glutaric acid and this has been veri?ed by FTIR (see Fig.S1B?).Because both GO sheets and CNSs have good aqueous dispersion capabilities,the GO/CNS composites were prepared by simply mixing them together ?rst with a weight ratio of 1:1in DI water under vigorous stirring.The solid product was obtained by evaporating the water under vacuum.As illustrated in Fig.2a,in the composites each CNS was fully cladded by GO sheets,which were bridged by additional GO sheets forming three dimensional (3D)networks.The formation of a 3D structure was further proved by TEM (see Fig.S2?).The formation of the 3D composite can be understood from several aspects.First of all,there are plenty of oxygen-containing groups on the GO sheets and CNSs surfaces.Thus,the strong interactions,such as the

hydrogen bonding and electrostatic interaction,could be formed between them easily.Secondly,both GO sheets and CNSs assume good water solubility;hence,they precipitate synchro-nously during the water evaporation,resulting in a composite with homogeneous composition.Thirdly,due to a ?exible single atomic layer feature,the GO sheets could be easily wrinkled and folded.As a result,the CNSs were cladded by the GO sheets during the water evaporation.Finally,due to the relatively large lateral sizes of GO sheets (Fig.1a)and the strong p –p interac-tions among them,the GO-cladded CNSs could be bridged by the GO sheets.For comparison,GO/CNS composites with different GO to CNS ratios were prepared through the same procedure,and their morphologies are shown in Fig.2.With the GO to CNS ratio increasing,the density of CNSs in the composites decreased,but the 3D network structure features are preserved.Notably,the GO sheets in the composites were separated very well by CNSs which differed from the aggregate of pure GO (see Fig.2f).

Predictably,the oxygen-containing groups inherited from GO and CNS in composites (see Fig.S1C?)could affect their elec-trical and electrochemical properties.To reduce the number of oxygen-containing groups,the GO/CNS composites were ther-mally annealed.As illustrated in Fig.S1C,?the FT-IR spectra showed that most oxygen-containing groups have been removed after the annealing.In addition,as shown in Fig.S3,?the Raman spectra of the composites showed that the ratio of D to G band (I D /I G ),at 1350and 1580cm à1,respectively,increased after the annealing treatment,thus implying that the electron conjugate state of the GO was partially restored.11,24,26,52Nevertheless,the 3D network morphologies of the composites were preserved properly after the annealing process,see Fig.3a–f.To illustrate the crystalline state variation of the CNSs,X-ray powder diffraction (XRD)patterns of the CNSs before and after annealing were acquired.The diffraction peaks (Fig.S4?)appeared at $25 and $43.5 after the annealing could be indexed to (002)and (100)diffraction of graphite (JCPDS-ICDD Card no.41-1487).53,54This implied that the CNSs were graphitized and their crystallinities increased during the annealing.The speci?c surface area of the composite is $325m 2g à1as determined by nitrogen adsorption/desorption isotherm curves,Fig.S5.?The average pore size in

the

Fig.1(a)Tapping mode AFM image of graphene oxide sheets depos-ited on the ?eshly cleaved mica substrate.Inset shows the height pro?le of GO sheets.(b)SEM image of carbon

spheres.

Fig.2(a–e)SEM images of the GO/CNS composites with different GO to CNS ratios (in weight)of 1/1,5/2,5/1,10/1,and 20/1,respectively.(f)SEM image of pure GO.

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composite is $42nm.Summarizing,it can be concluded that after the annealing treatment at 900 C in Ar mixed with 5%of H 2(in volume)the GO sheets in the composites were reduced to CRG and the CNSs were graphitized ?nally forming the CRG/CNS composites.

As anode materials,CRG sheets and corresponding compos-ites have been studied immensely in recent years.11,24–26,32,45,52,55The theoretical speci?c capacity of CRG sheets,due to their unique two dimensional features,is more than two times higher than that of the graphite.However,the CRG anodes showed a relatively large speci?c capacity only under a low charge/discharge current density.With the increasing of the charge/discharge current density,their speci?c capacities decreased rapidly,and the cycling stability faded dramatically.To get insight into the electrochemical performance of as-obtained CRG/CNS composites as anode for LIBs,a series of coin cells (more strictly speaking half cells)were assembled,and their speci?c capacity and cycle performance were assessed.Fig.4a depicts a typical charge–discharge pro?le of a coin cell using the composite CRG/CNS10-1as anode and measured at a current density of 5A g à1.Even under such a high current density,the ?rst cycle reversible speci?c capacity still reached to 925mA h g à1,which is much larger than those of the anodes made of bare CRG or carbon nanospheres measured under relatively lower current density.25,45,52,55,56Moreover,the as-obtained anodes exhibit good cycling stability.As shown in Fig.4b,except for the initial 3to 5cycles,no obvious capacity fading was detected.The capacities were maintained at 709and 700mA h g à1after 50and 100cycles,respectively.It is worth noting that no obvious volume extension was observed after being cycled for 100times.These results indicate that the composite CRG/CNS10-1is a proper anode material candidate for power LIBs.

Notably,a large irreversible capacity loss was detected during the ?rst cycle,Fig. 4.This may attribute to the electrolyte decomposition and/or the solid electrolyte interphase (SEI)?lm formation on the electrode surface.24,26,52To prove this assumption,cyclic voltammetry (CV)experiments were con-ducted on the coin cells.Fig.4c illustrates the CV curves of the coin cell using CRG/CNS10-1as anode material recorded at the scanning rate of 0.1mV s à1.Two peaks at $0.02and 0.65V were detected in the ?rst discharging cycle,and the other two peaks at

0.12and 1.15V were observed in the ?rst charging cycle.Inter-estingly,during the second discharging cycle the peak at 0.65V disappeared.In fact,a plateau appeared at the potential around 0.65V on the galvanostatic discharge pro?le for the ?rst cycle in Fig.3a.This observation leads us to believe that irreversible reactions occurred in the coin cell,and most probably the SEI ?lm formation on the anode results in the large irreversible capacity.The peaks at 0.02and 0.12V mainly correspond to the lithium ions insertion/extraction in the CRG sheets and carbon spheres.26,41,55The peak at 1.15V should be related to the potential hysteresis voltage of a special lithium storage mecha-nism of CRG/CNS10-1which might be due to the cavities and pores which exist in the composites.26,35

The kinetics of an anode process is of importance to its performance.The Nyquist complex plane impedance plots of the anode made of CRG/CNS10-1after 1,2,50and 100charge–discharge cycles were acquired with an open circuit voltage of coin cells of 2.4V and is presented in Fig.4d.In principle,the high-frequency semicircle is attributed to SEI ?lm and/or contact resistances;the semicircle in the medium-frequency region is assigned to the charge-transfer impedance at the electrode/elec-trolyte interface;and the inclined line at an approximate 45 angle to real axis corresponds to the lithium-diffusion process within electrodes.35,55,57As illustrated in Fig.4d,the diameters of the semicircles in both high and medium frequency areas become smaller evidently after 50cycles compared to the ?rst two cycles revealing the decrease of the contact and charge-transfer impedances,which are similar to the previously reported carbonaceous anode materials.16,58Notably,the impedance plots after 50and 100cycles are almost identical re?ecting a good cycling stability of the anode.This is in agreement with the aforementioned result of the charge–discharge measurement.Additionally,the impedance data of pure CRG and CNS were acquired and compared with that of CRG/CNS10-1,Fig.S6.?It shows that the electric properties CNS are improved through the formation of the composites with CRG.

To evaluate the rate capability,the charge–discharge experi-ments of the coin cell with CRG/CNS10-1as anode were con-ducted at current densities of 0.2,0.5,1,2,5,and 10A g à1.As shown in Fig.5a,at each current density the coin cell showed good cycling stability,higher Coulombic ef?ciency,and

large

Fig.3(a–e)SEM images of the GO/CNS composites after being annealed at 900 C in Ar mixed with 5%of H 2for 3hours,named correspondingly as CRG/CNS1-1,CRG/CNS5-2,CRG/CNS5-1,CRG/CNS10-1,and CRG/CNS20-1.(f)SEM image of pure CRG.

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reversible speci?c capacity.For instance,even at the highest charge–discharge current density of 10A g à1,the speci?c capacity of the coin cell still maintains at $604mA h g à1,which is,to the best of our knowledge,much larger than those of bare CRG and CNS measured at a similar current density.25,45,52,55,56Furthermore,the capacity can be recovered to 1300mA h g à1once the charge–discharge rate decreased from 10to 0.2A g à1.These results demonstrated the CRG/CNS10-1anode possesses not only a very good rate capability,but also a decent revers-ibility,which should be suitable for the power LIBs.

To explore the effect of the composition of the composites on their electrochemical performances,the coin cells with CRG,CRG/CNS1-1,CRG/CNS5-2,CRG/CNS5-1,CRG/CNS20-1,and CNS as anodes were assembled and their electrochemical properties were studied systematically.As shown in Fig.5b,the coin cell with CRG/CNS1-1as anode has the lowest

reversible

Fig.4The electrochemical performance of CRG/CNS10-1electrode assessed at a charge–discharge current density of 5A g à1.(a)Galvanostatic charge–discharge curves,(b)cycle performance and Coulombic ef?ciency,(c)the cyclic voltammograms at a sweep rate of 0.1mV s à1,and (d)AC impedance spectra of the electrode after 1,2,50,and 100charge–discharge

cycles.

Fig.5(a)Rate capabilities and cycle performances of the CRG/CNS10-1electrode at the current densities of 0.2,0.5,1,2,5,and 10A g à1.(b)Cycling performances of the electrodes CRG,CRG/CNS1-1,CRG/CNS5-2,CRG/CNS5-1,CRG/CNS10-1,CRG/CNS2-1,and CNS measured at the charge–discharge current densities of 5and 10A g à1,respectively.

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speci?c capacities of 140and 90mA h g à1at the charge–discharge current densities of 5and 10A g à1,respectively.When the CRG to CNS ratio was increased from 1:1to 10:1,the reversible speci?c capacities of the corresponding coin cells increased from 140to 925mA h g à1at the charge–discharge current density of 5A g à1,and even increased from 90to 600mA h g à1at a relatively high charge–discharge current density of 10A g à1.However,further increasing the CRG to CNS ratio led to a decrease of the reversible speci?c capacity of the coin cell.For instance,the reversible speci?c capacity of the coin cell with CRG/CNS20-1composite as an anode is 600and 500mA h g à1at the charge–discharge current density of 5and 10A g à1,respectively,which are much lower than those of the CRG/CNS10-1.It is likely that within the anodes made of the composites with a low CRG to CNS ratio,the CRG sheets could be separated readily by CNSs which is preferable for Li +toler-ating and moving (insertion and extraction)resulting in the large capacity and high rate capability.When the CRG content is lower,the electrochemical properties of the corresponding coin cells will be dominated by CNS.As shown in Fig.5b,the inherent reversible speci?c capacity of coin cells with CNS as anode is $50mA h g à1at the current density of 5A g à1.Thus,the coin cells with anode made of composites with low CRG to CNS ratios,such as CRG/CNS1-1and CRG/CNS2-1have relatively lower reversible speci?c capacities,too.When the CRG to CNS ratio gets higher,for instance,larger than 20:1,the CRG sheets were aggregated due to the p –p stacking.Therefore,the toler-ating sites for the lithium ion were suppressed and the insertion and extraction of the lithium ions were blocked somehow.As a result,the anodes show a low speci?c capacity that is similar to the coin cells with the pure CRG as an anode (Fig.5b).On the other hand,in the composite with an appropriate CRG to CNS ratio,such as CRG/CNS10-1,the CNS separates the CRG sheets properly resulting in numerous cavities and pores,which provide plenty of anchoring sites or spaces and pathways for lithium ions binding and diffusion,hence the anode of CRG/CNS10-1assumes the largest reversible speci?c capacity.Additionally,it is worth noting that the as-prepared composites with proper CRG to CNS ratios have unique 3D network structures in which the CNSs were cladded and bridged by CRG sheets forming 3D electron and Li +conduction pathways.The 3D network struc-ture is also favorable to the electrolyte immersing in the anode.As a result,the anodes made of CRG/CNS composites ensure good rate capabilities.Moreover,due to the rigid 3D network feature,the anodes showed also good cycling stability.As shown in Fig.5b,even after 200charge–discharges cycles with the current densities of 5and 10A g à1,no obvious capacity fading was detected.Taking these results together,we speculate that the CRG/CNS composites with appropriate CRG to CNS ratio,such as CRG/CNS10-1,should be potential candidate anode materials for power LIBs.

Conclusion

In summary,a novel category of all-carbon composites (CRG/CNS)was designed and fabricated as candidate anode materials for power LIBs.It has been illustrated that the as-prepared composites had 3D network structures in which the CNSs were cladded and bridged with CRG sheets properly forming lots of

the cavities and pores.The unique structure and the inherent properties of CRG and CNS afford synergistically exceptional electrochemical properties for the composites.We demonstrated that the coin cell with anodes made of the composites had large reversible speci?c capacities,high rate capabilities,and good cycling stabilities in comparison with bare CRG and CNS anodes.Considering the simple and low cost processes for the composite preparation,we envisage that the CRG/CNS composites with appropriate compositions should ?nd practical applications in power LIBs.

Acknowledgements

This work was supported by the NSFC (nos.91123011,90923041),the National ‘‘973Program’’(no.2010CB933900).

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