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1Carbon-coated SiO2 nanoparticles as anode material for lithium ion batteries

Journal of Power Sources 196 (2011) 10240–10243

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Journal of Power

Sources

j o u r 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 /j p o w s o u

Short communication

Carbon-coated SiO 2nanoparticles as anode material for lithium ion batteries

Yu Yao,Jingjing Zhang,Leigang Xue,Tao Huang,Aishui Yu ?

Department of Chemistry,Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,Institute of New Energy,Fudan University,Shanghai 200438,China

a r t i c l e

i n f o

Article history:

Received 28April 2011

Received in revised form 24June 2011Accepted 2August 2011

Available online 9 August 2011

Keywords:Silica

Lithium ion batteries Anode material Carbon coating

a b s t r a c t

A simple approach is proposed to prepare C-SiO 2composites as anode materials for lithium ion bat-teries.In this novel approach,nano-sized silica is soaked in sucrose solution and then heat treated at 900?C under nitrogen atmosphere.Transmission electron microscopy (TEM)and X-ray diffraction (XRD)analysis shows that SiO 2is embedded in amorphous carbon matrix.The electrochemical test results indicate that the electrochemical performance of the C-SiO 2composites relates to the SiO 2content of the composite.The C-SiO 2composite with 50.1%SiO 2shows the best reversible lithium storage perfor-mance.It delivers an initial discharge capacity of 536mAh g ?1and good cyclability with the capacity of above 500mAh g ?1at 50th cycle.Electrochemical impedance spectra (EIS)indicates that the carbon layer coated on SiO 2particles can diminish interfacial impedance,which leads to its good electrochemical performance.

1.Introduction

Lithium-ion battery (LIB)with high energy density is in great demand as an energy source for many applications.Carbon anode material has the advantages of long cycle life and low cost,however,its low Lithium-storage capacity cannot satisfy future require-ments of the electronic devices and electric vehicles.Silicon and some metals (e.g.Sn,Si,Sb)which could alloy with lithium have higher theoretical speci?c capacity.These materials were exploited as anode materials to replace the current carbon-based materi-als [1–8],but they have disadvantages of poor mechanical and cycling stability caused by drastic volume changes in charge and discharge processes.To solve this problem,metallic oxides and metallic oxide-based glasses have been proposed as alternative anode materials.Lithium insertion into metal oxides produces an inert Li 2O matrix,which could support and disperse active metal domains.The matrix can greatly decrease the volume change effect and improve the cyclability [9–15].It was reported that silicon monoxide (SiO)has a reversible capacity greatly higher than car-bon materials [16–18].The Si–C–O material was also prepared and showed high reversible capacity [19,20].Gao et al.claimed that commercial SiO 2nanoparticles (7nm in diameter)can react with Li between 0.0and 1.0V (vs.Li/Li +)with a reversible capacity of 400mAh g ?1[21].Guo et al.prepared a HC-SiO 2composite and their tests proved the electrochemical reduction of nano-SiO 2and the formation of Li 4SiO 4and Li 2O as well as Si in the ?rst-discharge process [22].

?Corresponding author.Tel.:+862151630320;fax:+862151630320.E-mail address:asyu@https://www.wendangku.net/doc/487102433.html, (A.Yu).

In this work,carbon-coated SiO 2was prepared by simply soak-ing the nano-SiO 2in sucrose solution,followed by drying and carbonization in an inert atmosphere.The pyrolysis product of sucrose coating on SiO 2particle formed a hard-carbon layer and then the C-SiO 2composites were investigated as anode materials for lithium ion battery.

2.Experimental

2.1.Preparation of C-SiO 2composites

Sucrose was dissolved in deionized water with continuous stir-ring.Subsequently,different quantities of SiO 2nanoparticles (7nm in diameter,AEROSIL)were added into the sucrose solution.The weight ratio of SiO 2to sucrose was ranging from 5:8to 1:8.The mixture was then steadily agitated to ensure that SiO 2disperses throughout the solution and evaporating water at 60?C to give a solid blend.The mixture was heated at 900?C in a nitrogen atmo-sphere for 3h and cooled down to room temperature naturally.The carbon-coated SiO 2nanoparticles with various carbon contents were obtained.

2.2.Characterization

The content of carbon in the composite was determined from the ignition loss of the sample at 800?C in air with a thermo gravi-metric and differential thermal (TGA/DTA)apparatus (DTG-60H,Shimadz).Fourier-transform infrared re?ection (FTIR)spectra were recorded on a Shimadz IRPrestige-21FTIR spectrometer using KBr pellet.The morphology of as-prepared carbon-coated SiO 2was observed by scanning electron microscopy (SEM,FE-SEMS-4800,

Y.Yao et al./Journal of Power Sources 196 (2011) 10240–10243

10241

500

1000

15002000e

d c

b T r a n s m i t t a n

c e (%)

ber(cm avenum W -1

)

a (a) SiO 2 : Sucrose 1:8(b) SiO 2 : Sucrose 2:8(c) SiO 2 : Sucrose 3:8(d) SiO 2 : Sucrose 4:8(e) SiO

2 : Sucrose 5:8

Fig.1.Fourier transform infrared spectroscopy of carbon coated SiO 2particles with

different carbon contents.

Hitachi)and transmission electron microscopy (TEM,JEM-2100F,Japan).X-ray diffraction (XRD)pattern was performed on a Brüker D8Advance and Davinci Design X-ray diffractometer using Cu K ?

radiation with a of 1.5406?A

at a scan rate of 4?min ?1from 10?to 90?.

2.3.Electrochemical measurement

The electrochemical performance of the as-prepared carbon-coated SiO 2materials was evaluated using coin cells assembled in an argon-?lled glove box (Mikarouna,Superstar 1220/750/900).An assembled coin cell was composed of lithium as the counter electrode and the working electrode prepared by active materi-als,super P carbon black and polyvinylidene ?uoride (PVDF)with a weight ratio of 80:10:10,using 1-methyl-2-pyrrolidinone (NMP)as the solvent onto a copper foil.The resulting ?lm was dried at 80?C in vacuum for 12h in order to evaporate the NMP.1M LiPF 6was dissolved in a mixed solvent of ethylene carbonate (EC)and dimethyl carbonate (DMC)(1:1in weight)to act as the electrolyte and microporous polypropylene ?lm (Celgard 2300)was used as separator.The total mass of active electrode material is about 4mg and electrode surface area is 1.54cm 2( 14mm).The electrochem-ical performance of the as-prepared materials was tested by cyclic voltammetry,galvanostatically charging and discharging tests and electrochemical impedance spectroscopy (EIS).Cyclic voltammetry was carried out in the potential range from open-circuit potential (OCP)to 0V (vs.Li/Li +),at a scan rate of 0.1mV s ?1on an electro-chemical workstation (CH Instrument 660A,CHI Company).The galvanostatic charge–discharge tests were performed on a battery test system (Land CT2001A,Wuhan Jinnuo Electronic Co.Ltd.)at a constant current density of 50mA g ?1in the potential range from 0to 3.0V.EIS measurements were measured for the fresh cells at open potential with an ac amplitude of 5mV over the frequency range from 100k to 0.01Hz.

3.Results and discussion

Fig.1is the FTIR spectra of carbon-coated SiO 2nanoparticles with different carbon contents.There are three characteristic peaks attributed to SiO 2.The peaks at 1102,798,472cm ?1can be assigned to asymmetry Si–O–Si bond stretching,SiO 4tetrahedron ring,and O–Si–O bond deformation,respectively.With the reduction of ini-tial ratio of SiO 2,the intensity of the three peaks decreased.

Thermal gravimetric results (shown in Table 1)give the actual content of SiO 2in the C-SiO https://www.wendangku.net/doc/487102433.html,pared with carbon-coated SiO 2nanoparticles,same proportion of carbon black (super P)which is similar to hard carbon pyrolysis from sucrose was mixed with SiO 2nanoparticles by milling.Electrochemical test of both

Table 1

Electrochemical performance of the carbon coated SiO 2and super P carbon black mixed SiO 2.

SiO 2:sucrose

Actual SiO 2content (wt.%)

First discharge capacity of carbon coated SiO 2(mAh g ?1)

First discharge capacity of carbon mixed with same proportion SiO 2(mAh g ?1)

5:881.0480.167.74:869.5476.8107.03:864.1519.9112.62:850.1535.9137.21:8

35.8

357.3

176.9

materials have been done and their ?rst discharge capacity data are shown in Table 1.It can be seen that ?rst discharge capacity of carbon-coated SiO 2nanoparticles increased slightly with the increase of carbon.But when the content of SiO 2is lower than 50%,the capacity begins to decrease.The sample with 50.1%SiO 2exhibits the highest speci?c capacity.For super P carbon black mixed SiO 2nanoparticles,the ?rst discharge capacity increases with the decreasing of SiO 2content,but is lower than the corre-sponding carbon-coated ones.

Fig.2a displays SEM images of the pure SiO 2nanoparticles.The diameter of

single particle is about 10nm.Fig.2b shows the carbon-coated SiO 2nanoparticles with the content of SiO 2content of 50.1%.

Fig.2.SEM images of SiO 2nanoparticles (a)and carbon coated SiO 2nanoparticles with 50.1%SiO 2(b).

10242Y.Yao et al./Journal of Power Sources 196 (2011) 10240–

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Fig.3.TEM image of carbon coated SiO 2nanoparticles with 50.1%SiO 2.

It can be seen that the diameter of SiO 2particles increases to 20nm which could be due to the carbon coating layer.

Fig.3shows TEM images of the carbon-coated SiO 2nanoparti-cles with 50.1%SiO 2.It can be clearly seen that the SiO 2particles are coated with carbon.The dark area in centre is the SiO 2particle,with diameter of about 10nm,which is consistent with the result of SEM image.

Fig.4a shows XRD pattern of pure SiO 2nanoparticles.The broad band indicates that the structure of SiO 2nanoparticles is amorphous.Fig.3b gives XRD pattern of carbon-coated SiO 2nanoparticles with 50.1%SiO 2.The wide peaks at about 22?(002),43?(100)are attributed to the carbon layer coated on SiO 2nanoparticles and the structure of carbon layer is also amorphous.There is no peak of silicon could be found,which manifests SiO 2was not reduced to Si.It was reported that SiO 2cannot be reduced to glass-like compounds such as SiO 2??or Si–C–O by carbon at 1000?C [22].

Impedance experiments were applied to explore the effect of carbon layer coated on SiO 2nanoparticles on the interfacial impedance of C-SiO 2composite.Fig.5shows Nyquist plots of the carbon-coated SiO 2particles and the carbon black mixed SiO 2parti-cles before cycling.The semicircle shows that interfacial impedance become smaller with the reduction of SiO 2content,which means SiO 2particles are coated more uniformly by carbon [23].The inter-

2θ(de gree)

I n t e n s i t y (a .u )

Fig.4.XRD patterns of SiO 2nanoparticles (a)and carbon coated SiO 2nanoparticles

with 50.1%SiO 2(b).

0% SiO

-Z "(o h m )

Z'(ohm)

-Z "(o h m )

Z'(ohm)

Fig.5.Electrochemical impedance spectra of carbon coated SiO 2nanoparticles (a)and super P black mixed SiO 2nanoparticles (b).

facial impedance of carbon coated SiO 2(Fig.5a)is lower than the corresponding carbon mixed one (Fig.5b).It means the car-bon coated samples have better effects in reducing interfacial impedance than the carbon mixed samples.The EIS plots account for the increase of the ?rst discharge capacity with the raise of carbon content,which is shown in Table 1.However,the further reduction of active material SiO 2led to discharge capacity fade,resulting in the decrease of discharge capacity when the SiO 2con-tent is below 50%.

The charge–discharge pro?les of carbon coated SiO 2nanoparti-cles with 50.1%SiO 2is shown in Fig.6a.The ?rst discharge capacity of the composite is 536mAh g ?1,which is much higher than car-bonaceous materials.The charging pro?le is very steep at potentials exceeding 1.5V,but not ?at as graphite.It owning to the large polarization of the glassy material derived from the SiO 2.The Si generated during the ?rst conversion cycle has poor conductivity and hinders the kinetics of Li alloying,though the active material particle size distribution is around 20nm.There are two plateaus at 0.69V and 0.15V in ?rst discharge curve (Fig.6a),correspond-ing to the two peaks in cyclic voltammogram (Fig.6b).The peak at 0.69V is associated with the electrolyte decomposition and the formation of the solid electrolyte interface (SEI)layer,which con-tributes about 100mAh g ?1to the total irreversible capacity of the cell (ca.355mAh g ?1).The other peak at 0.15V is related to electro-chemical reactions between lithium ions and SiO 2in the composite and contributing the other irreversible capacity.This electrochemi-

Y.Yao et al./Journal of Power Sources 196 (2011) 10240–10243

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100

200

300

400

600

700

800

900

1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

v o l t a g e (v )

Capacity(mAh/g)

(a)

0.0

0.5

1.0

1.5

2.0 2.5

3.0

3.5

-1.4

-1.2-1.0-0.8-0.6-0.4-0.2

0.00.20.4 Fi rst cycle

Second cycle

C u r r e n t (m A )

Potential (v)(b)

Fig.6.The charge–discharge pro?les (a)and cyclic voltammogram (b)of carbon coated SiO 2nanoparticles with 50.1%SiO 2of ?rst two cycles.

cal reaction involves amorphous nano-SiO 2being reduced to Si and forming amorphous Li 2O or crystalline Li 4SiO 4[22].These reactions can be summarized as follows:

SiO 2+4Li ++4e ?→2Li 2O +Si

(1a)

2SiO 2+4Li ++4e ?→Li 4SiO 4+Si

(1b)Si +x Li ++x e ??Li x Si

(2)

As both reaction (1a)and (1b)were irreversible,the formation of SEI layer and SiO 2reduction accounted for irreversible capacity.Fig.7shows cycling performance of carbon-coated SiO 2parti-cles with different carbon contents.It is found that the cycle life improved with the increase of carbon content,because the car-bon matrix can provide a stable structure to buffer the drastic volume changes during discharge and charge process.The com-posite with 50.1%SiO 2shows the best storage capacity,which remains above 500mAh g ?1at 50th cycle.The reasons for the good cyclability of the composite could be attributed to that nano-sized of particle greatly decreases the volume changes in lithium ion insertion and extraction,and meanwhile,Li 2O or Li 4SiO 4formed in the ?rst discharge process results in an inert matrix which can support and disperse active Si domains,and accommodate volume changes during the alloying and dealloy-ing reactions with lithium ions.The coated carbon layer might also act as the constrained force for volume change during cycling.

SiO 2 35 .8%SiO 2 50 .1%.1%.5%.0%

S p e c i f i c c a p a c i t y (m A h /g )

Cycle nu mbe r

Fig.7.Cycling performance of carbon coated SiO 2particles with different carbon

contents.

4.Conclusions

Carbon-coated SiO 2nanoparticles prepared by nesting nano-sized SiO 2particles in carbon cage.It showed high storage capacity

and good cyclability.The results indicate that coated carbon could lead to lower interfacial impedance and higher storage capacity.The composite with 50.1%SiO 2gives the largest storage capacity,which remains above 500mAh g ?1at 50th cycle.The good cyclabil-ity of the C-SiO 2composite attributes to that nano-sized of particle and Li 2O or Li 4SiO 4formed during ?rst discharge process alleviates the volume change in lithium ion insertion and extraction.Besides,the coated carbon layer restrain electrode pulverized during charge and discharge processes.

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