lable at ScienceDirect

Current Applied Physics 11 (2011) 1159e1163

Contents lists avai

Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Improvement of the electrochemical properties of V3O7$H2O nanobelts for Libattery application through synthesis of V3O7@C core-shell nanostructuredcomposites

Yifu Zhang a, Min Zhou a, Meijuan Fan b, Chi Huang a,c, Chongxue Chen a, Yuliang Cao a,*, Houbin Li b,Xinghai Liu b,*

aCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Chinab School of Printing and Packaging, Wuhan University, Wuhan 430079, Chinac Engineering Research Center of Organosilicon Compound and Material, Ministry of Education of China, Wuhan 430072, China

a r t i c l e i n f o

Article history:Received 9 October 2010Received in revised form22 January 2011Accepted 14 February 2011Available online 23 February 2011

Keywords:V3O7$H2O@CV3O7@CCore-shell structuresChemical synthesisElectrochemical properties

* Corresponding authors. Tel.: þ86 27 68752701; faE-mail addresses: ylcao@whu.edu.cn (Y. Cao), liux

1567-1739/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.cap.2011.02.010

a b s t r a c t

V3O7$H2O@C core-shell structured composites have been successfully synthesized using V3O7$H2Onanobelts as the cores and glucose as the source of carbon in the presence of sodium lauryl sulfate (SDS).The as-obtained V3O7$H2O@C core-shell materials were characterized by X-ray powder diffraction (XRD),transmission electron microscopy (TEM), elemental analysis (EA), Fourier transform infrared spectros-copy (FT-IR) and Raman spectrum. The thickness of the carbon shell can be controlled by the hydro-thermal reaction time and the quantity of glucose. The surfactants have great influence on fabricatingV3O7$H2O@C core-shell composites, which have been discussed in detail. V3O7@C composites weresubsequently obtained through thermal treatment with V3O7$H2O@C. The electrochemical properties ofV3O7@C core-shell composites were studied, indicating that the discharge capacity is still 151.2 mAh/gafter 45 cycles, which is better than that of pure V3O7$H2O nanobelts.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, extensive efforts have beenmade to research on newmaterials applied to lithium-ion batteries, for the traditionalbatteries suffer from high cost and toxicity of LiCoO2 cathode.Specially, vanadium oxides and their derivative compounds haveattracted more and more attention because of their diverse physi-cochemical properties and potential applications as cathodematerials for lithium-ion batteries [1e5]. In recent years, vanadiumoxide hydrate (V3O7$H2O) as a new class of vanadium oxides hasbecome the focus of the investigation and a lot of researchers haveengaged in this field because of its novel catalytic [6,7], magnetic[8e11] and electrochemical [2,3,11e13] properties. V3O7$H2Onanobelts with high aspect ratio are promising cathode materialsfor reversible lithium-ion batteries [3,12,13], for example,V3O7$H2O nanobelts with a high initial discharge capacity of253 mAh/g [3], 323 mAh/g [13] and 373 mAh/g [14], have beenreported. From these data, we can deduce this novel materialexhibits a very high initial discharge capacity. However, according

x: þ86 27 68754067.h@whu.edu.cn (X. Liu).

All rights reserved.

to the reference [13] and our previous research [14], the cycleperformance of specific capacity is not good. Therefore, theimprovement of the cycle performance of V3O7$H2O nanobelts isstill a great challenge for material scientists.

Amorphous carbon has become one of the most importantmaterials in many potential applications owing to its chemicalinertness, biocompatibility, and high thermal conductivity [15e17].Recently, the amorphous carbon coated on the surface of metal ormetal oxides is regarded as an effective way to improve the elec-trochemical properties of many materials and has been introducedto the field of lithium-ion batteries [17e21]. These novel materialswith core-shell structures applied to lithium-ion batteries areexpected to combine the functions of both core and shell parts,which improves the general performance of the composites. Herein,a post-hydrothermal approach was adopted to form a very thincarbon shell on the surfaceofV3O7$H2Onanobelts andV3O7@Ccore-shell composites were subsequently obtained through thermaltreatment with V3O7$H2O@C, which have not been reported before.Furthermore, the electrochemical properties of V3O7@C core-shellcomposites have been investigated. The results indicate that thesenovel composites exhibit a high initial discharge capacity of200 mAh/g, which is much higher than the value of Li3V2(PO4)3@C

Fig. 1. XRD patterns of the as-obtained products: (a) the plots of JCPDS No. 85-2401;(b) the starting material of V3O7$H2O; (c) V3O7$H2O@C core-shell composites.

Y. Zhang et al. / Current Applied Physics 11 (2011) 1159e11631160

[20].Moreover, the discharge capacity of V3O7@C is still 151.2mAh/gafter 45 cycles,much higher than that of pure V3O7$H2O (100mAh/gafter 30 cycles) [14], achieving the aim of the improvement of theelectrochemical property of V3O7$H2O nanobelts.

2. Experimental details

2.1. Materials

Vanadium pentoxide (V2O5), ethanol (C2H5OH), D-(þ)-Glucose(C6H12O6$H2O), sodium lauryl sulfate (SDS) and hexadecyl tri-methyl ammonium bromide (CTAB) were purchased from Sino-pharm Chemical Reagent Co., Ltd and used without any furtherpurification.

2.2. Synthesis of V3O7$H2O nanobelts

The synthesis of V3O7$H2O nanobelts was based on our previousreport [14]. In a typical synthesis, 9.00 g of V2O5 powder wasdispersed into 100 mL of ethanol and 300 mL of deionized watermixed solutionwithmagnetic stirring. Then themixed solutionwastransferred into a 600mL stainless steel autoclave after the solutionbecame suspension. The autoclave was sealed and maintained at180 �C for 24 h and then cooled to room temperature naturally. Theproducts were filtered off, washedwith distilled water and absoluteethanol several times to remove any possible residues, and dried invacuum at 75 �C for future application. The morphologies of thestarting materials were shown in Fig. S1 (Supplementary data).

2.3. Synthesis of V3O7$H2O@C

In a typical procedure, 0.40 g of the as-obtained V3O7$H2Onanobelts and 0.01 g of SDS were dispersed into the glucose solu-tion (2.00 g of glucose and 45 mL of distilled water) in a 100 mLbeaker under ultrasonic for 20 min, and then the solution wasunder vigorous magnetic stirring for 1 h. After the solution becamesuspension, they were transferred into a 60 mL stainless steelautoclave, which was sealed and maintained at 180 �C for 3 h. Aftercooling to room temperature naturally, the products were filteredoff, washed with distilled water and absolute ethanol several timesto remove any possible residues, and dried in vacuum at 75 �C forfuture characterization and application.

2.4. Synthesis of V3O7@C

The hydrothermal products (V3O7$H2O@C composites) wereheated in a tube furnace with 5 �C/min heating rate under a flow ofargon gas at 400 �C for 2 h, and cooled to room temperature in theargon flow to prevent the oxidation of V3O7@C composites. V3O7@Ccore-shell nanocomposites were prepared for the electrochemicalcharacterization.

2.5. Characterization

X-ray powder diffraction (XRD) was carried out on D8 X-raydiffractometer equipment with Cu Ka radiation, l ¼ 1.54060 Å. Theelemental analysis (EA) of those samples was carried out usingVarioEL III (Germany) with a TCD detector to analyze the element ofC, H, N and S. The morphology of the products was observed bytransmission electron microscopy (TEM, JEM-100CXII). The Ramanspectrumwas taken on an RM-1000 spectrometer (Confocal RamanMicrospectroscopy) from 800 to 2000 cm�1 with an argon-ion laserat an excitationwavelength of 514.5 nm. Fourier transform infraredspectroscopy (FT-IR) pattern was recorded on a Nicolet 60-SXBspectrometer from 4000 to 400 cm�1 with a resolution of 4 cm�1.

2.6. Electrochemical measurements

The electrochemical properties of the as-obtained core-shellcomposites (V3O7@C) were tested in assembling experiment cellswith metallic lithium as the negative electrode. The working elec-trode was made by dispersing with 85 wt% active materials(V3O7@C), 10 wt% acetylene back carbon powder, and 5 wt% poly-vinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone(NMP) solvent to form a homogeneous slurry. The slurry was thenspreaded and pressed on Al foil. The coated electrodes were driedin vacuum at 125 �C for 18 h. The electrolyte was 1 mol Le1 LiPF6 ina mixture of ethylene carbonate (EC) and diethyl carbonate (DEC)(EC/DEC ¼ 1/1, v/v). The cells were assembled in a glove box underan argon atmosphere. Charge-discharge tests were carried out ina voltage range of 4.0e1.5 V and current density of 0.2 mA/cm2.

3. Results and discussion

3.1. Characterization of V3O7$H2O@C

The phase and composition of the resulting products weredetermined by X-ray powder diffraction (XRD), as shown in Fig. 1.All the peaks from the stating materials (Fig. 1b) can be readilyassigned to the orthorhombic crystalline phase (space group:Pnam) of V3O7$H2O, in agreement with the literature values (JCPDSNo. 85-2401) [22], whose plots are represented in Fig. 1a. After thecoating process, all the peaks from Fig. 1c can mainly be indexed tothe orthorhombic phase of V3O7$H2O. Naturally, some peaks ofV3O7$H2O are not detected, which indicates that somematerials areprobably coated on the surface of V3O7$H2O. Carbon is not detectedin the pattern of V3O7$H2O@C composites (Fig. 1c). However, thecarbon content in the composites is ca. 13.36% according to theelemental analysis. Usually carbon obtained from glucose underhydrothermal condition is amorphous and cannot be detected byXRD even with higher amounts [20].

Further information about the structure of the carbon shell wascollected from Raman investigation. A typical Raman spectrum ofV3O7$H2O@C core-shell composites (Fig. 2) exhibits two mainpeaks centered at 1393 cme1 (Deband) and 1596 cme1 (Geband),which are in agreement with those of amorphous carbon [18,23].

Fig. 2. Raman spectrum of V3O7$H2O@C.Fig. 4. FT-IR of the resulting products: (a) V3O7$H2O@C; (b) V3O7@C.

Y. Zhang et al. / Current Applied Physics 11 (2011) 1159e1163 1161

According to the previous reports, the peak at 1393 cme1 (Deband)is usually corresponding to the vibrations of carbon atoms withdangling bonds for the in-plane terminations of disorderedgraphite, and the peak at 1596 cme1 (Geband) (corresponding tothe E2g mode) is associated with the vibration in all sp2ebonded

Fig. 3. TEM images of V3O7$H2O@C: (a)e(c) 2.00 g of glucose, 3 h; (d) an

carbon atoms in a twoedimensional hexagonal lattice [18,23,24].The intensity ratio of the Ge and Debands was IG/ID ¼ 1.52 for theresulting product. The relatively high intensity of Depeak furtherproves that the coating comprises disordered carbon. Therefore, the

d (e) 4.00 g of glucose, 4.5 h. Other parameters were kept constant.

Fig. 5. TEM images of V3O7@C.

Y. Zhang et al. / Current Applied Physics 11 (2011) 1159e11631162

Raman spectrum further confirms that the carbon in the resultingsamples is disordered.

To get a clear insight into the core-shell structure of the as-obtained products, the corresponding TEM measurements werecarried out, as shown in Fig. 3. After the coating process, thesamples consist of well-defined nanoblets with length up to severalmicrometers, and the contrast grade between cores and shellscould even be observed, indicating that each V3O7$H2O core isencapsulated into amorphous carbon shell, which is similar tocarbon coated Fe3O4 or Li3V2(PO4)3 [20,23]. The thickness of thecarbon shell can be controlled by the hydrothermal reaction timeand the quantity of glucose in our synthetic route. The averagethickness of the shells was ca. 8.1 nm collected from V3O7$H2O@C(Fig. 3aec), which were obtained at the conditions: 2.00 g ofglucose, 3 h and keeping other parameters unchanged. While theconditions were changed to 4.00 g of glucose and 4.5 h, the averagethickness of the shells was about 40 nm, as shown in Fig. 3d and e.

3.2. The effect of surfactants

It was found that the surfactants played a significant role incontrolling the thickness of the amorphous carbon coated on thesurface of V3O7$H2O nanobelts. The experiments with anionicsurfactant (SDS), surfactantefree and cationic surfactant (CTAB)added to the system were carried out with other parametersconstant, and their corresponding results were represented inFig. S2 (Supplementary data). The digital photos of the suspension

Fig. 6. The initial galvanostatic discharge/charge curve and the cycle performanc

(Fig. S2a,d and g) were taken after 1 h without reaction or stirring.The suspensionwashomogeneouswith surfactantefree (Fig. S2d) orSDS (Fig. S2a). However, the blue-green solid precipitated at thebottomof the beakerwhen CTABwas added to the system (Fig. S2g).These results indicated that the surface of V3O7$H2O nanobeltsaccompanied with some negative charges, such as hydroxyl groups,in agreement with our previous report [14]. This property mayfacilitate the amorphous carbon coated on the surface of V3O7$H2O.The resulting products consisted of welledefined V3O7$H2O@Ccore-shell composites with SDS or without surfactant, as shown inFig. S2b,c,e and f. The thicknesses of the shells are about 8.1 nmwithSDS and 27 nm without surfactant, respectively. However, theproducts were severely aggregated with CTAB used in the experi-ment (Fig. S2h). Many V3O7$H2O nanobelts could not be encapsu-lated into carbon shells, although some V3O7$H2O@C core-shellstructures were occasionally seen, as shown in Fig. S2i. Therefore,the surfactantshadgreat influenceon fabricatingV3O7$H2O@Ccore-shell composites.

3.3. Electrochemical property of V3O7@C

As is well known, V3O7$H2O@C core-shell materials synthesizedby this post-synthesis technology contain some organic groups,whichwere proved by FT-IR test, as shown in Fig. 4a. It was reportedthat the influence of a significant amount of water or organic func-tional groups in the materials on the electrochemical properties isvital, which may react with lithium to limit the cycling life andcapacity of the batteries [13,25]. It has been demonstrated that thecapacity and cycling performance can be remarkably improved bythermal treatment. What’s more, the capacity of V3O7$H2O@C isalmost zero without heating treatment according to our investiga-tion. This is why we did not directly measure the electrochemicalbehavior of V3O7$H2O@C but synthesize V3O7@C. According to theFT-IR spectrum after the thermal treatment (Fig. 4b), the organicgroups are comparatively reduced, such as C]O. The TEM images ofthe post-annealing sample are shown in Fig. 5. It clearly reveals thatthe morphologies of the product remain the original shape aftercalcining, which is very important for keeping a large capacity andgood cycling performance. In our researches, it was found that thethickness of the carbon shell had great influence on the specificcapacity of V3O7@C composites. When the coated carbon is verythick, the specific capacity of the V3O7@C is very low. While thespecific capacity is increased with the thick becoming thin. Forinstance, these core-shell materials exhibit high specific capacitywhen the thickness of the carbon shell is less than 10 nm,which is inaccordance with the results of Li3V2(PO4)3@C [20], LiVPO4@C [21].Thus, in our experiment, we investigated the charge-discharge

e of capacitances of the as-obtained V3O7@C core-shell structured materials.

Y. Zhang et al. / Current Applied Physics 11 (2011) 1159e1163 1163

capabilityof thepost-annealing sampleobtainedbycalcining theas-prepared V3O7$H2O@C with the average thickness ca. 8.1 nm at400 �C for 2 h in flowing argon.

Fig. 6 respectively represents the initial galvanostatic discharge/charge curve and the relationship between the cycle performance ofspecific capacity and the cycle number for an electrode composedof V3O7@C core-shell structured materials in a voltage range of4.0e1.5 V and under a constant current density of 0.2 mA/cm2. Thecore-shellmaterials (V3O7@C) exhibit an initial discharge capacityofas high as 200 mAh/g, which is much higher than the value ofLi3V2(PO4)3@C [20]. V3O7@C with an initial high discharge capacitymight be attributed to the large surface area and short diffusiondistance resulting from the nanostructures [26,27]. The dischargecapacity of V3O7@C is 151.2mAh/g after 45 cycles, much higher thanpure V3O7$H2O (100 mAh/g after 30 cycles) [14], so the retentionrate in discharge capacities is 75.6% for V3O7@C after 50 cycles butonly 30% for pure V3O7$H2O nanoblets after 30 cycles, which mightbe due to the influence of amorphous carbon. Therefore, the V3O7@Ccomposites havebetter cyclic ability, achieving the aimof improvingthe cycle performance of V3O7$H2O nanoblets.

4. Conclusion

In conclusion, V3O7$H2O@C core-shell structured compositeshave been successfully synthesized using V3O7$H2O nanobelts asthe cores and glucose as the source of carbon in the presence of SDSvia an environmental hydrothermal method. The thickness of thecarbon shells can be controlled by the hydrothermal reaction timeand the quantity of glucose. When the synthetic conditions were2.00 g of glucose, 3 h and keeping other parameters constant, thethickness of amorphous carbon was very thindca. 8.1 nm. Whenwe change the synthetic conditions into 2.00 g of glucose and 3 h ofreaction time (keeping other parameters constant), the thickness ofamorphous carbonwas very thindca. 8.1 nm. It was found that thesurfactants had great influence on fabricating V3O7$H2O@C core-shell composites due to the surface of V3O7$H2O nanobelts con-taining some active function groups. V3O7@C composites weresubsequently obtained through thermal treatment withV3O7$H2O@C at 400 �C for 2 h in flowing argon. Furthermore, thedischarge capacity of V3O7@C is still 151.2 mAh/g after 45 cycles,much higher than that of pure V3O7$H2O (100 mAh/g after 30cycles), which indicates that the as-obtained V3O7@C compositeshave better cyclic ability.

Acknowledgement

This research work was partially supported by National Sci-ence Fund for Fostering Talents in Basic Science (J0730426), Key

Laboratory of Catalysis and Materials Science of Hubei Province(CHCL06003), Students’ Scientific Research Program of WuhanUniversity (2007138) and the Fourth Installment of Science andTechnology Development 2010 Program of Suzhou (SYG201001).

Appendix. Supplementary data

Supplementary data associated with this article can be found inonline version at doi:10.1016/j.cap.2011.02.010.

References

[1] S. Koike, T. Fujieda, T. Sakai, S. Higuchi, J. Power Sources 82 (1999) 581e584.[2] C.V.S. Reddy, S.-i. Mho, R.R. Kalluru, Q.L. Williams, J. Power Sources 179 (2008)

854e857.[3] H. Qiao, X. Zhu, Z. Zheng, L. Liu, L. Zhang, Electrochem. Commun. 8 (2006)

21e26.[4] C.V.S. Reddy, E.H. Walker, S.A. Wicker, Q.L. Williams, R.R. Kalluru, Curr. Appl.

Phys. 9 (2009) 1195e1198.[5] F.B. Dejene, R.O. Ocaya, Curr. Appl. Phys. 10 (2010) 508e512.[6] F. Dong, S. Heinbuch, Y. Xie, J.J. Rocca, E.R. Bernstein, Z.-C. Wang, K. Deng,

S.-G. He, J. Am. Chem. Soc. 130 (2008) 1932e1943.[7] S. Feyel, D. Schroder, X. Rozansk, J. Sauer, H. Schwarz, Angew. Chem. Int. Edit.

45 (2006) 4677e4681.[8] C. Li, M. Isobe, H. Ueda, Y. Matsushita, Y. Ueda, J. Solid State Chem. 182 (2009)

3222e3225.[9] I. Hellmann, G.S. Zakharova, V.L. Volkov, C. Taschner, A. Leonhardt, B. Buchner,

R. Klingeler, J. Magn. Magn. Mater. 322 (2010) 878e881.[10] G.S. Zakharova, V.L. Volkov, C. Täschner, I. Hellmann, A. Leonhardt,

R. Klingeler, B. Büchner, Solid State Commun. 149 (2009) 814e817.[11] N. Rama, M.S.R. Rao, Solid State Commun. 150 (2010) 1041e1044.[12] S. Gao, Z. Chen, Mingdeng Wei, K. Wei, H. Zhou, Electrochim. Acta 54 (2009)

1115e1118.[13] S. Shi, M. Cao, X. He, H. Xie, Cryst. Growth Des. 7 (2007) 1893e1897.[14] Y. Zhang, X. Liu, G. Xie, L. Yu, S. Yi, M. Hu, C. Huang, Mater. Sci. Eng. B. 175

(2010) 164e171.[15] H.W. Ra, D.H. Choi, S.H. Kim, Y.H. Im, J. Phys. Chem. C 113 (2009) 3512e3516.[16] X. Sun, Y. Li, Angew. Chem. Int. Ed. 43 (2004) 597e601.[17] X.M. Sun, J.F. Liu, Y.D. Li, Chem. Mater. 18 (2006) 3486e3494.[18] A. Odani, V.G. Pol, S.V. Pol, M. Koltypin, A. Gedanken, D. Aurbach, Adv. Mater.

18 (2006) 1431e1436.[19] B. Zhao, Y. Jiang, H.J. Zhang, H.H. Tao, M.Y. Zhong, Z. Jiao, J. Power Sources 189

(2009) 462e466.[20] M.M. Ren, Z. Zhou, X.P. Gao, W.X. Peng, J.P. Wei, J. Phys. Chem. C 112 (2008)

5689e5693.[21] Y.G. Wang, Y.R. Wang, E.J. Hosono, K.X. Wang, H.S. Zhou, Angew. Chem. Int.

Edit. 47 (2008) 7461e7465.[22] Y. Oka, T. Yao, N. Yamamoto, J. Solid State Chem. 89 (1990) 372e377.[23] S.H. Xuan, L.Y. Hao, W.Q. Jiang, X.L. Gong, Y. Hu, Z.Y. Chen, Nanotechnology 18

(2007) 1e6.[24] A. llie, C. Durkan, W.I. Milne, M.E. Welland, Phys. Rev. B. 66 (2002) pp. 045412/

045411e045412/045413.[25] J.X. Wang, C.J. Curtis, D.L. Schulz, J.G. Zhang, J. Electrochem. Soc. 151 (2004)

A1eA7.[26] S.H. Ng, S.Y. Chew, J. Wang, D. Wexler, Y. Tournayre, K. Konstantinov, H.K. Liu,

J. Power Sources 174 (2007) 1032e1035.[27] Y. Wang, K. Takahashi, H. Shang, G. Cao, J. Phys. Chem. B. 109 (2005)

3085e3088.