Short Communication
Low pressure preparation of spherical Si@C@CNT@C anode material for lithium-ion
batteries
Lianyi Shao,Jie Shu ?,Kaiqiang Wu,Xiaoting Lin,Peng Li,Miao Shui ?,Dongjie Wang,Nengbing Long,Yuanlong Ren
Faculty of Materials Science and Chemical Engineering,Ningbo University,Ningbo 315211,Zhejiang Province,People’s Republic of China
a r t i c l e i n f o Article history:
Received 15April 2014
Received in revised form 22May 2014Accepted 26May 2014
Available online 2June 2014Keywords:
Si@C@CNT@C composite Pyrolysis
Chemical vapor deposition Anode material
Lithium-ion batteries
a b s t r a c t
Si@C@CNT@C composite is prepared by preliminary low-pressure forming Si@C and Si@C@CNT precur-sors from Si powder.After pyrolysis from glucose,acetylene and pitch,Si@C@CNT@C shows a spherical multi-phase composite structure.By using as lithium storage material,Si@C@CNT@C shows an initial dis-charge capacity of 620.5mA h g à1with an initial coulombic ef?ciency of 82.2%.After 60cycles,this spherical sample can maintain a reversible capacity of 563.5mA h g à1at 100mA g à1,corresponding to a capacity retention of 90.8%.For comparison,the reversible capacities for Si powder,Si@C and Si@C@CNT are 10.9,380.3and 494.6mA h g à1,respectively.Even cycled at 400mA g à1,Si@C@CNT@C can deliver a reversible lithium storage capacity of 389.2mA h g à1.It indicates that spherical Si@C@CNT@C can be used as a high performance anode material for lithium-ion batteries.
ó2014Elsevier B.V.All rights reserved.
1.Introduction
Graphite has been acted as commercial anode material in lith-ium-ion batteries for about thirty years.However,the reversible capacity of graphite is only 330mA h g à1,which makes it fail to follow the demands of portable electronic devices and electric vehicles.To solve this issue,high capacity anode materials have attracted the attention of material scientists from all over the world in the past decades.
Owing to high theoretical capacity (500–1200mA h g à1),metal oxides (such as Co 3O 4,SnO 2,NiO and Cr 2O 3)have become the promising candidates among the proposed lithium storage materi-als in recent years [1–4].For comparison,Si can accommodate a maximum value of 4.4Li per formula during lithiation process.It indicates that the theoretical capacity of Si is about 4200mA h g à1,which is much higher than that of metal oxides.Therefore,Si can be a promising high capacity lithium storage material [5–7].
Although Si displays tremendous potential as high capacity anode material,bare Si cannot be directly used as electrode mate-rial due to the huge volume change appeared during lithiation/delithiation process [8].The repeated volume change results in particle pulverization and electrode deterioration upon cycles.To improve the cycling properties,the size of Si particle was reduced by using different preparation techniques,such as ball-milling and laser-induced silane gas reaction [7].Besides,various carbon sources are used as structural buffers and conductive additives for Si anode [9–11].For instance,pyrolyzed carbon can form a shell to suppress the volume change of Si particles during electrochem-ical cycles.As a result,core–shell structure Si@C,Si@CNF and Si@CNT composites show high reversible capacity and good cycling lifetime [12–15].
In this work,spherical Si@C@CNT@C multiple composite is fab-ricated by a low pressure process to form high density precursor and then a following hydrolysis.The morphology and electrochem-ical property of Si@C@CNT@C and its derivates are described and compared.It is clear that spherical Si@C@CNT@C reveals a revers-ible capacity of 563.5mA h g à1after 60cycles.Moreover,it also shows outstanding rate performance compared to other Si–C composites.2.Experimental 2.1.Material preparation
Spherical Si@C@CNT@C multiple composite was prepared from Si powder.The detailed preparation process is described in Fig.1.Firstly,10g Si powder was put in a chamber and vacuumed for 1h.After that,100mL,500g L à1glucose solution was added and the mixture was pyrolyzed at 650°C for 5h in an argon
https://www.doczj.com/doc/2b6752480.html,/10.1016/j.jelechem.2014.05.0311572-6657/ó2014Elsevier B.V.All rights reserved.
?Corresponding authors.Tel.:+8657487600787;fax:+8657487609987(J.Shu).
E-mail addresses:sergio_shu@https://www.doczj.com/doc/2b6752480.html, ,shujie@https://www.doczj.com/doc/2b6752480.html, (J.Shu),shuimiao@https://www.doczj.com/doc/2b6752480.html, (M.Shui).
Fig.1.Low pressure preparation process of(a)Si powder,(b)Si@C,(c)Si@C@CNT
and(d)Si@C@CNT@C.
SEM images of(a and b)Si powder,(c and d)Si@C,(e and f)Si@C@CNT and(g and h)Si@C@CNT@C.
diameter of15mm.The electrochemical performance of as-pre-pared samples were investigated by two-electrode coin-type cells, in which metal Li was acted as counter electrode and Whatman glass?ber was used as separator.The electrolyte was1mol Là1 LiPF6in a mixture of ethylene carbonate and dimethyl carbonate (1:1,v/v).Galvanostatic charge/discharge tests were performed on Land CT2001A multichannel battery test system in the potential range between0.0and2.0V.3.Results and discussion
It is known that the critical size of Si particles is about2.0l m to relieve the mechanical stress and volume change as lithium storage material[5,7].As the SEM images show in Fig.2a and b, pristine Si powder is composed of irregular particles with the size ranging from0.2to 2.0l m.It indicates that the Si powder is suitable for preparing Si@C composites.After a coating by
10L.Shao et al./Journal of Electroanalytical Chemistry727(2014)8–12
pyrolyzed-carbon from glucose,Si@C shows a homogenous carbon distribution over the Si particles as shown in Fig.2c.Based on the mass change,the carbon content in Si@C is54.0wt.%.Due to the connection by pyrolyzed carbon,some Si@C composites aggregate into bigger secondary particles with the size of10–20l m(Fig.2d). After CNT growth by CVD,the particle surface is tightly wrapped by cross-linking CNTs as revealed in Fig.2e and f,which is similar to the surface morphology of CNT/Si[15]or CNF/Si[16].The content of CNT in Si@C@CNT is4.7wt.%.This CNT coating will bring high electronic conductive network and strong buffer for relieving vol-ume change.After coating by pitch-pyrolyzed carbon and subse-quent granulation,Si@C@CNT@C shows spherical shape with the particle size ranging from5to20l m as shown in Fig.2g and h. The mass percent of pitch-pyrolyzed carbon in Si@C@CNT@C is 10.5wt.%.Moreover,this log-normal distribution of particle size is bene?cial to achieve high tap density for electrode preparation. Besides,the spherical shape of particles is convenient for high rate charge/discharge cycles.
Fig.3shows the charge/discharge curves of Si powder,Si@C, Si@C@CNT and Si@C@CNT@C during the initial15cycles.For bare Si particles,it can be found that a long and?at lithiation plateau appears at0.1V,which is related to the Li–Si alloying process to form LiSi,Li12Si7,Li7Si3,Li13Si4and Li22Si5[5,17].As a result,Si powder shows an initial discharge capacity of4004.7mA h gà1. This value is close to the theoretical capacity of4200mA h gà1 for a total formation of Li22Si5.Upon reverse delithiation,a charge plateau can be observed at0.42V,corresponding to the charge capacity of3005.2mA h gà1.However,this delithiation plateau shortens quickly in the following cycles due to the serious volume change of about300–400%for Si particles during lithiation/delith-iation process[5].After coating by glucose-pyrolyzed carbon,the discharge plateau for Si@C is maintained at0.1V and the initial the discharge capacity is reduced to1126.3mA h gà1.Furthermore, three charge plateaus can be detected at0.09,0.14and0.43V.The appearance of two short delithiation plateaus at0.09and0.14V is attributed to lithium storage in pyrolyzed carbon and de-alloying from Li22Si5alloy.The initial charge capacity is966.0mA h gà1. After15cycles,the reversible charge capacity can be kept at 791.3mA h gà1with a capacity retention of81.9%,which is much higher than the bare Si powder.However,it is still unsatis?ed to give an excellent cycling property.For comparison,Si@C@CNT shows similar electrochemical lithium storage behaviors but it
L.Shao et al./Journal of Electroanalytical Chemistry727(2014)8–1211
reveals better capacity retention than Si@C.After15cycles, Si@C@CNT exhibits a reversible charge capacity of610.0mA h gà1 with capacity retention of85.1%.After a further coating with pitch-pyrolyzed carbon,Si@C@CNT@C shows an outstanding elec-trochemical performance.The reversible capacity at15th cycle is 604.2mA h gà1with a high capacity retention of97.4%.It suggests that Si@C@CNT@C can be used as a promising high capacity anode material.
The1st,20th,40th and60th differential capacitance curves of Si@C@CNT@C and its derivates are shown in Fig.4.It is clear that the redox peaks almost disappear after60cycles in the d Q/d V curves of pristine Si powder,indicating the deteriorated cycling property resulted from particle pulverization and structural break-down[17].Coating by glucose-pyrolyzed carbon,the capacity loss is greatly suppressed.As a result,the reduction peaks at0.07,0.10, 0.20,0.23V and the oxidation peaks at0.11,0.15,0.44V can be maintained after60cycles.This glucose-pyrolyzed carbon layer also improves the initial coulombic ef?ciency from75.0%to 85.8%,which can reduce the extra lithium consumption for cath-ode materials.Coating by CNT and pitch-pyrolyzed carbon,the decrease of redox peaks is further relieved.It is obvious that Si@C@CNT@C reveals slight changes for redox peak and presents a high initial coulombic ef?ciency of82.2%.
Fig.5shows cycling and rate properties of Si powder,Si@C, Si@C@CNT and Si@C@CNT@C.The reversible capacity of bare Si powder is only10.9mA h gà1at a current density of100mA gà1 after60cycles.In contrast,the reversible lithium storage capacities of Si@C,Si@C@CNT and Si@C@CNT@C are improved to380.3,494.6 and563.5mA h gà1,respectively.After60cycles,the correspond-ing capacity retention for Si@C,Si@C@CNT and Si@C@CNT@C are 39.4%,69.0%and90.8%,respectively.It indicates that the cycling performance of Si particles can be obviously enhanced by coating carbon layer,especially by using a multi-phase composite carbon structure.Fig.6compares the1st charge/discharge curves of Si@C@CNT and Si@C@CNT@C at different current density.With an increase of current density to200mA gà1,Si@C@CNT@C and Si@C@CNT exhibit similar charge capacities of566.6and 565.4mA h gà1,respectively.After10cycles,Si@C@CNT@C shows higher reversible capacity(543.7mA h gà1)than Si@C@CNT (511.1mA h gà1)as shown in Fig.5b.It tells that pitch-pyrolyzed carbon layer can further improve the structural stability during repeated cycles.Even cycled at400mA gà1,Si@C@CNT@C can deli-ver a reversible lithium storage capacity of389.2mA h gà1.It sug-gests that this micro-sphere structure Si@C@CNT@C can reach the same electrochemical properties as that displayed by one-dimen-sional nanostructure Si samples[17–19].Therefore,spherical Si@C@CNT@C can be a suitable anode material to replace graphite in rechargeable lithium-ion batteries.
4.Conclusions
In this paper,a series of Si–C samples are prepared by forming precursors under vacuum condition and subsequent hydrolysis.Si@C@CNT@C is fabricated by preliminary coating glucose-pyro-lyzed carbon on Si particles,and then growing cross-linking CNTs on the surface and a?nal deposition of pitch-pyrolyzed carbon layer.It shows spherical shape with the particle size ranging from 5to20l m.Electrochemical results show that Si@C@CNT@C can deliver higher reversible capacity and better rate properties than Si powder,Si@C and Si@C@CNT.Cycled at100mA gà1, Si@C@CNT@C shows a reversible capacity of563.5mA h gà1after 60cycles.Even cycled at400mA gà1,it can still maintain a revers-ible charge capacity of389.2mA h gà1.All these evidences suggest that Si@C@CNT@C has high lithium storage capability and outstanding rate property.
Con?ict of interest
There is no con?ict of interest.
Acknowledgements
This work is sponsored by National Natural Science Foundation of China(No.51104092),K.C.Wong Magna Fund and the Out-standing Dissertation Growth Fundation of Ningbo University (No.PY2013003).
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