魏 来，刘开宇，李 艳，吕美玉，钟剑剑

(中南大学化学化工学院，中国长沙 410083)

1 Introduction

Rechargeable lithium ion batteries have numerous advantages such as high energy density，good cycle life，sta-bility and good power performance comparing with other rechargeable batteries，and have been regarded as the key elements for electronic devices(EVs)and hybrid electric vehicles(HEVs)［1］.Nowadays，conventional commercial anode materials for lithium ion batteries are carbonaceous materials，such as graphite，modified carbonaceous sphere，soft carbon and so on［2］.Commercial graphite anode materials are limited by its gravimetric specific capacity(372 mAh· g－1)［3］，which can not satisfy the demand for high energy density devices in the future.In this regard，silicon-based materials have been considered as a promising candidate for anode materials due to their high theoretical gravimetric specific capacity of 4200 mAh· g－1(Li22Si5)［1］.However，the volume expansion(＞300%)of Si occurred in the electrode during the process of insertion/extraction of Li+leads to huge mechanical stress，resulting in the pulverization of the electrode and the loss of the contact between the electrode and current collector，as a result，the capacity decreases dramatically in several cycles and the electrode suffers from poor capacity retention.Moreover，the low intrinsic conductivity of Si could also lead to the polarization of the electrode，which limits its practical use.

To address these problems，many efforts have been taken to improve the overall electrochemical performance of Si-based electrode.The synthesis of novel nanostructure of Si have been studied，such as“porous Si”［4］，“Si nanowire”［5］“silicon-based thin films”［6］，“silicon nano spheres”［7］and “Si-carbon hollow core-shell”［8］.Other methods focus on the combination of Si with other components such as metals［9-10］or compounds［11-12］.Furthermore，the decoration of the surface of nano-Si［13］or the modifying of current collector［14］were also investigated for improving the cohesion force of binder and collector of Si electrode.Comparing with these studies，creating Si/C composites is a promising approach because of their relative mild preparation with stable electrochemical performance.Carbon materials have been frequently used as the active matrix because of its softness，good electronic conductivity and small volume change.Si/C composites were usually synthesized by the way of high energy ball milling with other components［15］or by the phrolysis of different organic carbon sources［16-17］.The types of carbon sources and the methods of preparation seem to be quite important for Si/C composites with good performance.

In this paper，Si/C composites were prepared by a facile method of dispersing nano-Si and graphite in the solution of glucose monohydrate，followed by carbonization in the high temperature at argon atmosphere.The microstructure，morphology and electrochemical performance of the as-prepared Si/C composites were also investigated by different methods as anode materials for lithium ion batteries，and this material exhibited obviously enhanced electrochemical performance comparing to pristine pure nano-Si and graphite.

2 Experimental

2.1 Preparation of the materials

The Si/C composites were synthesized as follows:Firstly，glucose monohydrate(C6H12O6·H2O，1.5g)was dissolved in 50 mL deionized water and ethanol(3∶1 in volume)solution with constant magnetic stirring.The nano-silicon powders(commercial available，＞99.9%，Shuitian Materials Technology Co，Ltd，Shanghai，China)and flake graphite powders(200 mesh)were mixed in a ratio of 3:7.Subsequently，the mixture was slowly added into the previous glucose solution with strong magnetic stirring for 12 h，then the solvent was evaporated at 80℃over night to get a solid blend，the obtained solid blend precursor was heated to 750℃ under nitrogen atmosphere in a furnace for 2 h(5 ℃·min－1)and cooled naturally to room temperature.The products were grounded and sieved by 200-mesh shifter to obtain the Si/C composites.

2.2 Structural and morphological characterization of the materials

The morphologies of the composites were investigated by scanning electron microscopy(SEM，Quanta-200).The phase components of the materials were confirmed by powder X-Ray diffraction(XRD，D/maxш，Rigaku)with Cu Kα radiation(10°～80°).The microstructures of the composites were examined by transmission electron microscope(TEM，JEOL-3010).

2.3 Electrochemical measurement

The composites were evaluated using CR2016 coin-type cells with pure lithium tablets as the counter electrode under the same conditions and instruments.A micro-porous polypropylene(PE)membrane was used as the separator and the electrolyte was LiPF6(1 M)in a mixture of ethylene carbonate(EC)and dimethyl carbonate(DMC)with a volume ratio of 3∶7.The working electrode was prepared by adding active materials(80 wt.%)，acetylene black(10 wt.%)as conducting agent and polyvinylidene fluoride(PVDF，10wt.%)as binder.The mixture was dispersed in N-methyl pyrrolidinone(NMP)and the obtained slurry was then spread uniformly on a copper foil and dried at 120℃ for 12 h.The electrodes were punched into round pellets with diameter of 14 mm and cells were assembled in an argon-filled glove box.The charging/discharging test of cells were carried out on the Land battery tester(CT2001，Wuhan)with the potential ranges of 0.005 V to 1.5 V(vs.Li+/Li)at room temperature.The cyclic voltammogram(CV)was measured with a RST electrochemical analyzer，the scanning rates was 0.1 mV·s－1.

3 Results and discussion

3.1 Structure and morphology of the materials

Fig.1 shows X-ray diffraction patterns of flake graphite，nano-Si，pyrolyzed carbon from glucose at the given conditions and the prepared Si/C composites.For the XRD pattern of the Si/C composites，it is clear to find the diffraction peaks of Si(28.4°，47.3°，56.1°，69.1°and 76.4°)and graphite(26.6°，42.5°，43.5°，54.7°and 77.6°)，indicating the presence of graphite and silicon，and both silicon and graphite retain its own crystalline structure during the synthesized process，and any other phases(such as SiC or SiO2)are not observed.As for the XRD pattern of pyrolyzed glucose，obvious peaks are not detected except a diffused broad peak around 2θ=23°(amplifying figure in Fig.1)，proving that the pyrolyzed carbon in the composites under given conditions was an amorphous phase.This results show that other inactive phases do not exist in the composites，and the composites are the blends of graphite，silicon and amorphous carbon pyrolyzed from glucose.

The morphology of the raw materials and as-prepared Si/C composites are presented in Fig.2 by SEM investigation.Fig.2(a)and Fig.2(b)are the morphology of pristine nano-Si and flake graphite，respectively，which were used to prepare Si/C composites.The nano-Si powders show uniform and nano sized spherical particles，and the average size of the particles is about 100 nm.The flake graphite has primary sizes around 50 μm，and the particles are thin and flat.The image of the as-prepared Si/C composites is shown in Fig.3(c)，indicating that the morphology of the composites is irregular.

Fig.1 X-ray diffraction patterns of(a)flake graphite，(b)nano-silicon，(c)glucose pyrolyzed carbon and(d)Si/C composites

Fig.2 SEM images of nano-Si(a)，flake graphite(b)and Si/C composites(c)

The TEM images of Si/C composites are presented in Fig.3.Fig.3(a)shows that the composites have fine sizes，nano-sized Si particles are bonded to the graphite sheets by the coating of disordered carbon from glucose.However，the Si particles consist of agglomerates of clusters，as particles may not be perfectly dispersed by pyrolysis process.Fig.3(b)clearly displays the figure of the carbon coated Si particle.It is evident that Si particles are uniformly coated by the carbon layer.The thickness of this layer forming a complete shell is around 10 nm.Fig.3(c)and Fig.3(d)are the HRTEM images of the material.The crystal plane spacing fit well with the number of Si(111)and flake graphite(002)，indicating that composites are composed of three phases，graphite，nano-Si and disordered carbon from pyrolyzed glucose.Nano-Si and graphite particles are dispersed into carbon networks from glucose，and the structure provides a buffer for Si particles to accommodate the huge stress and the and volume change during the Li+inserting and extracting processes［18］.

Fig.3 TEM images of Si/C composites(a)and(b);HRTEM of Si/C composites(c)and(d)

3.2 Electrochemical performance of the electrode materials

The charge-discharge curves of the as prepared Si/C composites at different cycles under current density of 50 mA g－1are shown in Fig.4(a).Obviously，there is a distinct potential platform during the first discharge curve from 0.1 to 0.9 V，which could mainly attribute to the formation of a solid electrolyte interphase(SEI)on the surface of electrode.During this process，a part of Li+in the electrolyte were consumed to the formation of SEI and the decomposition of the electrolyte，contributing to the irreversible capacity loss of the electrode.After the first cycle，the potential platform disappears，and the structure of crystal structure silicon transforms to amorphous phase，which can be proved from the shift of the subsequent discharge curves.The distinct charge potential platform around 0.4 V is due to the extraction of Li+from Si，while the slope ranging from 0.15 to 0.2 V can be related to the process of lithium ion extracting from the flake graphite［19］.As for the discharge curve，the straight potential platform below 0.2 V is mainly ascribed to the insertion of lithium ion for both silicon and flake graphite，as silicon and graphite possess similar discharge potential vs.Li+(0 ～0.1 V，0 ～ 0.2 V，respectively［1，19］).Fig.4(b)shows the first charge-discharge curves of nano-Si，flake graphite and the as prepared Si/C composites at the current rate of 50 mA· g－1.Visibly，the discharge platform around 0 ～0.2 V is the Li+inserting of active materials，including graphite and silicon，and the main extraction process there are several distinct potential platforms can be attributed to nano-Si anode(0.4 V)and flake graphite anode(0.15 V)can also be observed，although the first charge and discharge specific capacity of nano-Si are 1800.18 mAh· g－1and 3483.56 mAh· g－1，respectively，The initial columbic efficiency is only 51.72% ，which is similar to the previously reports of the nano-Si.The Si/C composites，however，exhibit a first charge capacity of 733.65 mAh· g－1and discharge capacity of 1048.27 mAh· g－1，along with an initial columbic efficiency of 69.98%，based on the ratio of graphite and nano-Si during the preparation and the theoretical calculating methods of the specific capacity of Si/graphite composites［20］，the initial charge and discharge capacity of the material are maintained within reasonable values.

Fig.4 (a)Charge-discharge profile of Si/C composites at different cycles;(b)initial charge-discharge curves of nano-Si，flake graphite and Si/C composites

Fig.5(a)compares the cycling performance of nano-Si，flake graphite and Si/C composites at 50 mA· g－1.Evidently，the pure nano-Si electrode exhibits high initial charge(1800.18 mAh· g－1)and discharge capacity(3483.56 mAh· g－1)，however，the capacity decays rapidly to 274.48 mAh· g－1after 20 cycles.It is well known that the capacity fade and large initial irreversible capacity for Si anode is owing to the large volume changes during the insertion and extraction processes of Li+，leading to the poor capacity retention of pure Si electrode.The flake graphite exhibits an initial discharge capacity of 433.54 mAh· g－1and keeps a steady capacity at about 380 mAh· g－1during the cycling，which is even higher than the theoretical specific capacity of graphite.This result may be due to the previous grinding process during the preparation of the half cells.It has been reported that graphite have a higher reversible capacity after grinding process，and the grinding process of crystalline graphite is essentially a non-graphitization process from a structural chemistry perspective［21］.In this regard，the as-prepared Si/C composites exhibit a relatively stable capacity during the cycling，capacity fading is significantly alleviated and the capacity of 586.98 mAh· g－1is reserved after 20 cycles with the capacity retention of 80.01%，while that of nano-Si is 15.21%.The cycling performance at different rates of Si/C composites are shown in Fig.5(b).As seen，at the current density of 150 mA· g－1，300 mA· g－1，and 600 mA· g－1，the initial capacities of Si/C composites are 664.57 mAh· g－1，625.35 mAh· g－1and 431.44 mAh· g－1，respectively，and the coulombic efficiencies of Si/C composites are 69.97%，69.85%and 69.56%，respectively.After 20 cycles，83.50%，77.26%and 85.38%of the initial capacity can be reserved.It is evident that improved capacity retention of the Si/C composites is achieved.The enhanced cycleability can be related to the following reasons:(1)Nano-Si and graphite are coated by the glucose-pyrolyzed carbon，providing the carbon network for the connection between Si particles and flake graphite and maintains stable electrical contact of nano-Si particles in the Si/C composites during the charge-discharge process，that is to say，nano-Si particles and graphite sheets are connected by the electronic con-ducting network from the glucose-pyrolyzed carbon.(2)The presence of coated carbon on the surface of active materials reduced the direct contact between electrode and electrolyte，which is beneficial for maintaining its mechanical stability by reliving stresses resulting from volume change from Si.(3)The volume change occurred in the Si electrode may lead to the fracture of SEI，resulting in increased Li+to the formation of new SEI on the surface of electrode during the subsequent processes，the addition of coated carbon and graphite can accommodate the volume change occurred in Si electrode and therefore the enough insertion/extraction of Li+in the electrolyte is guaranteed.

Fig.5 (a)Cycling performance of nano-Si，flake graphite and Si/C composites at 50 mA· g－1;(b)cycling performance of Si/C composites at different current densities

Fig.6 Cyclic voltammograms of the Si/C composites for first three cycles at scanning rate of 0.1 mV·s－1from 0 ～1.5 V

To further investigate the charge-discharge process of the Si/C composites，cyclic voltammograms(CV)was conducted.Fig.6 displays the first three CV cycles of the materials at the scanning rate of 0.1 mV·s－1.There is a broad cathodic platform ranging from 0.4 to 0.8 V during the first cycle，the platform corresponds to the formation of the SEI on the surface of the electrode，which can be the result of the decomposition of electrolyte，after the first curve，the platform disappears.The distinct cathodic peak below 0.15 V is due to the Li+insertion into the active material，including both Si and graphite.There are two anodic peaks during the charge process，the anodic peak between 0.15 and 0.3 V is mainly related to the Li+extraction from flake graphite，while the anodic peak around 0.45 V is related to the extraction of Li+from nano-Si.The other cathodic peak located at 0.2 V from the 2nd cycle corresponds to dealloying process of crystal Si to amorphous phase.It is evident that all the results are in agreement with the charge-discharge curves discussed above.

4 Conclusion

Si/C composites were successfully synthesized by steps of liquid solidification and subsequent pyrolysis process.The Si/C composites exhibit high reversible capacity of 733.65 mAh· g－1with an initial coulombic efficiency of 69.98%at the current of 50 mA· g－1，and improved capacity retention is achieved after 20 cycles at different current.The improved overall electrochemical performance can be attributed to the characters of the composites including the special structure and the uniformly carbon coating.This indicates that the composites may be a promising anode material for lithium ion batteries.However，further studies on optimizing the particle distribution of the raw materials in the composites and promoting the enhanced electrochemical performance of this material are still necessary.

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