SnO2 &a-MoO3core-shell nanobelts
Cite this:https://www.360docs.net/doc/422836779.html,mun .,2011,47,5205–5207SnO 2/a -MoO 3core-shell nanobelts and their extraordinarily high reversible capacity as lithium-ion battery anodes w
Xin-Yu Xue,*a Zhao-Hui Chen,a Li-Li Xing,a Shuang Yuan a and Yu-Jin Chen*b
Received 5th January 2011,Accepted 1st March 2011DOI:10.1039/c1cc00076d
Extraordinarily high reversible capacity of lithium-ion battery anodes is realized from SnO 2/a -MoO 3core-shell nanobelts.The reversible capacity is much higher than traditional theoretical results.Such behavior is attributed to a -MoO 3that makes extra Li 2O reversibly convert to Li +.
Lithium-ion batteries (LIBs)are one of the most important power sources for mobile electrical devices and electric vehicles due to their high energy density and long service life.1,2Although graphite is usually used in commercial LIB anodes for its high reversibility,its low capacity (372mAh g à1)can hardly meet the increasing demand for LIBs with higher energy density.2,3Therefore,many e?orts have been taken to explore new materials or design novel composite structures for LIB anodes.4,5Nowadays,various metal oxides such as SnO 2,which form Li-metal alloys have been regarded as promising candidates for anode materials of the next generation of LIBs due to their high capacity.6–8However,two serious problems greatly block their practical applications:(1)the capacity fades signi?cantly during discharge/charge cycles because their volume changes signi?cantly with Li insertion/extraction and the pulverization of materials usually occurs;6–9(2)the irreversible capacity caused by Li 2O formation is too high (take SnO 2as an example,the conversion reaction of Li 2O is irreversible,which results in low coulomb e?ciency and the waste of Li storage.7–9The ?rst problem has been e?ectively solved by introducing one-dimensional (1D)nanostructures or nano-composites because 1D nanostructures can accommodate the strain of volume e?ect,as shown by the previous reports of SnO 2nanowires,SnO 2/carbon nanotubes and SnO 2/graphene nanosheets.9–15On the other hand,the second problem remains a challenge and the irreversible capacity can not be activated due to the limitation of the electrochemical reaction.12,16If one can make the conversion process of Li 2O reversible,the reversible capacity can be greatly enhanced.
In this work,extraordinarily high reversible capacities of LIB anodes are realized from SnO 2/a -MoO 3core-shell nano-belts.Their reversible capacity is up to 2214mAh g à1at C/10rate (10h per half cycle)and maintains 1895mAh g à1after 50cycles.Such an extraordinarily high reversible capacity of SnO 2/a -MoO 3core-shell nanobelts is probably attributed to the distinct electrochemical activity of nanostructured a -MoO 3,which activates the irreversible capacity of SnO 2.
The synthesis process of SnO 2/a -MoO 3core-shell nanobelts is shown in the ESI.w The XRD pattern of SnO 2/a -MoO 3core-shell nanobelts is shown in Fig.S1,w indicating the crystallization of SnO 2/a -MoO 3core-shell nanobelts.
Fig.1a and b are typical SEM images of a -MoO 3nanobelts and SnO 2/a -MoO 3core-shell nanobelts,showing their general morphologies.Both of them are highly dominated by the belt-like nanostructures.The surface of a -MoO 3nanobelts is cleaner than that of SnO 2/a -MoO 3core-shell nanobelts.Fig.1c is a TEM image of a -MoO 3nanobelts,showing their detailed morphologies.The widths of the a -MoO 3nanobelts are 140–200nm and the lengths are 6–8m m.Fig.1d is a TEM image of one single SnO 2/a -MoO 3core-shell nanobelt,showing that a -MoO 3nanobelts are uniformly coated with a thin layer.The width of the SnO 2/a -MoO 3core-shell nano-belts is about 220nm,larger than that of the a -MoO 3nanobelts.The thickness of the SnO 2nanoparticle layer is several tens of nanometres.Fig.1e is a high resolution TEM (HRTEM)image of the a -MoO 3nanobelts,clearly showing their single crystalline structures.Two sets of crystal lattice fringes (0.37and 0.40nm)correspond to a -MoO 3{001}and {100}atomic spacings,respectively.17Fig.1f is a HRTEM image of SnO 2/a -MoO 3core-shell nanobelts.SnO 2nano-particles are uniformly distributed on the whole surface of the a -MoO 3nanobelts and the SnO 2nanoparticles are single-crystalline structures.The spacing between two adjacent lattice planes of the nanoparticles is 0.32nm,which is in agreement with the SnO 2crystal.18,19The EDS spectra is shown in Fig.S2,w which con?rms that the single-crystal a -MoO 3nano-belts are uniformly coated with a thin SnO 2nanoparticle layer.Electrode fabrication and electrochemical measurement are shown in ESI.w The discharge/charge cycling performances of a -MoO 3nanobelts and SnO 2/a -MoO 3core-shell nanobelts at C/10rate are shown in Fig.2a and b,respectively.The 1st,2nd,10th,20th and 30th discharge capacities of the a -MoO 3nanobelts are 1955,1427,1406,1364and 1312mAh g à1,
a
College of Sciences,Northeastern University,Shenyang,China.E-mail:xuexinyu@https://www.360docs.net/doc/422836779.html, b
College of Sciences,Harbin Engineering University,Harbin,China.E-mail:chenyujin@https://www.360docs.net/doc/422836779.html,
w Electronic supplementary information (ESI)available:Experimental materials,Synthesis,XRD patterns,EDS spectrum,Electrode prepara-tion,SEM image analysis,Coulombic e?ciency,Con?rmation of reversible capacity,and Di?erential capacity curves.See DOI:10.1039/c1cc00076d
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respectively,which approach the theoretical capacity for a -MoO 3.20,21Those discharge capacities of the SnO 2/a -MoO 3core-shell nanobelts are up to 3104,2214,2172,2110and 2031mAh g à1,respectively.The reversible capacity of SnO 2/a -MoO 3core-shell nanobelts is extraordinarily high,much higher than that of a -MoO 3or SnO 2-based nanostructures.9–12,16
As shown in Fig.2c and d,both a -MoO 3nanobelts and SnO 2/a -MoO 3core-shell nanobelts have excellent cyclability
and stability not only at low current density but also at high current density.The reversible capacity of the a -MoO 3nano-belts after 50cycles maintains 1226mAh g à1(C/10)and 1067mAh g à1(C/2).For SnO 2/a -MoO 3core-shell nanobelts,the reversible capacity after 50cycles maintains 1895mAh g à1(C/10)and 1530mAh g à1(C/2).Such high cyclablities can be attributed to the fact that nanostructured a -MoO 3has a very high electron conductivity and Li +mobility,22and 1D nano-structures can accommodate the strain during Li insertion and extraction.9–15,23SEM images of SnO 2/a -MoO 3core-shell nanobelts before the cycling test,after the discharge process and after the charge process are shown in Fig.S3,w which demonstrate that 1D nanostructures can accommodate the strain during cycles and have high stability.
The coulombic e?ciency of a -MoO 3nanobelts and SnO 2/a -MoO 3core-shell nanobelts is shown in Fig.S4.w The low initial coulombic e?ciency is probably attributed to the formation of a solid electrolyte interphase (SEI)layer.24–26After the ?rst cycle,both types of nanobelt exhibit high coulombic e?ciency (496.0%).To con?rm the extraordinarily high reversible capacity,20di?erent LIBs were fabricated from a -MoO 3nanobelts and SnO 2/a -MoO 3core-shell nanobelts,respectively,ss shown in Fig.S5.w
In traditional theory,the capacity of physical mixtures can be calculated as follows:27
C SnO 2/a -MoO 3=C SnO 2?mass%SnO 2+C a -MoO 3
?mass%a -MoO 3
(1)
The theoretical maximum reversible capacity of SnO 2and a -MoO 3is 782and 1111mAh g à1,respectively.20,21,28–30As the values of mass%range from 0to 1,the theoretical reversible capacity of SnO 2/a -MoO 3composites should be 782–1111mAh g à1.However,the experimental reversible capacity of SnO 2/a -MoO 3core-shell nanobelts is about 2times higher than the traditional theoretical results.Thus,the traditional theory can not be applicable for SnO 2/a -MoO 3core-shell nano-belts,and eqn (1)is not applicable.
Such extraordinarily high reversible capacity of SnO 2/a -MoO 3core-shell nanobelts can probably be attributed to the distinct electrochemical activity of nanostructured a -MoO 3.22A schematic illustration of the Li insertion/extraction mechanism for SnO 2/a -MoO 3core-shell nanobelts is shown in Scheme 1.During the ?rst Li insertion,the a -MoO 3nanobelts undergo an irreversible structural change from the a -phase to an amorphous phase followed by a conversion reaction:22,31
MoO 3+6Li ++6e à23Li 2O +Mo
(2)
This reaction yields nanodisperse particles of Mo metal.32It has been demonstrated that small size e?ect of Mo nano-particles,arising from an increased total number of surface atoms,can greatly increase the electrochemical reactivity and make the conversion between Li +and Li 2O reversible.1,22The experimental reversible capacity of a -MoO 3nanobelts is about 1100mAh g à1,which approaches the theoretical maximum insertion of Li +(6Li +/MoO 3).Moreover,?rst-principles molecular dynamics calculations in a previous report show that nanostructured a -MoO 3can contain more oxygen atoms than bulk a -MoO 3in order to passivate the surface dangling
Fig.2(a)The 1st,2nd,10th,20th and 30th discharge/charge cycles of a -MoO 3nanobelts at C/10rate.(b)The 1st,2nd,10th,20th and 30th discharge/charge cycles of SnO 2/a -MoO 3core-shell nanobelts at C/10rate.(c)Discharge capacity dependence on cycle number of a -MoO 3nanobelts at C/10and C/2rates.(d)Discharge capacity dependence on cycle number of SnO 2/a -MoO 3core-shell nanobelts at C/10and C/2rates.
Fig.1(a)SEM image of a -MoO 3nanobelts.(b)SEM image of SnO 2/a -MoO 3core-shell nanobelts.(c)TEM image of a -MoO 3nano-belts.(d)TEM image of one single SnO 2/a -MoO 3core-shell nanobelt.(e)High resolution TEM (HRTEM)image of a -MoO 3nanobelts.(f)HRTEM image of SnO 2/a -MoO 3core-shell nanobelts.
P u b l i s h e d o n 16 M a r c h 2011. D o w n l o a d e d b y C e n t r a l S o u t h U n i v e r s i t y o n 09/12/2013 13:08:17.
bonds.33Therefore,nanodispersed Mo metal clusters can probably make extra Li 2O reversibly convert to Li +if there is extra Li 2O in the environment.The SnO 2nanoparticle layer can just provide extra Li 2O by the following irreversible initial reaction:6,8,9
SnO 2+4Li ++4e à-2Li 2O +Sn
(3)
Thus,nanodispersed Mo metal particles probably make extra Li 2O (provided by SnO 2)reversibly convert to Li +,which results in an extraordinarily high reversible capacity of SnO 2/a -MoO 3core-shell nanobelts.As shown in Fig.S6w and S7,w the XRD patterns of SnO 2/a -MoO 3core-shell nanobelts after Li insertion and extraction and their di?erential capacity curves (dC/dV vs.voltage)con?rm that Mo metal clusters make Li 2O reversibly convert to Li +.At the same time,part of reversible capacity is contributed to by the formation of Li–Sn alloys.6,8–10Their reversible reactions are as follows:
MoO 3+x +(6+2x )Li ++(6+2x )e à2Mo +3Li 2O (MoO 3)+x Li 2O (SnO 2)(4)Sn +4.4Li ++4.4e à2Li 4.4Sn
(5)
In summary,extraordinarily high reversible capacities of LIB anodes were realized from SnO 2/a -MoO 3core-shell nano-belts synthesized by a wet chemical method.Their rever-sible capacity was much higher than that of a -MoO 3-or SnO 2-based nanostructures,and also about two times higher than traditional theoretical results.Such behavior was probably attributed to the distinct electrochemical activity of nano-structured a -MoO 3.Our results indicated that alloy-type metal oxide/a -MoO 3core-shell 1D nanostructures have potential applications in high performance LIB anodes.
This work is partly supported from the Fundamental Research Funds for the Central Universities (N090405017,N100405109),Liaoning Natural Science Foundation (20091027),Specialized Research Fund for the Doctoral Program of Higher Education of China (20090042120025),National Natural Science Foundation of China (51072038).
Notes and references
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Scheme 1Schematic illustration of Li insertion/extraction mechanism for SnO 2/a -MoO 3core-shell nanobelts.
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