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Template-free synthesis of V2O5 hierarchical nanosheet-assembled microspheres with excellent cycling stability
Journal of Power Sources 285 (2015) 538e542
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Template-free synthesis of V2O5 hierarchical nanosheet-assembled microspheres with excellent cycling stability Yujuan Dong, Huiying Wei, Wei Liu, Qianjin Liu, Wenjing Zhang, Yanzhao Yang* Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
h i g h l i g h t s We used the template-free solvothermal method to synthesize V2O5 microspheres. Microspheres was nanosheet-assembled. Nanosheet-assembled microspheres exhibited higher specific capacity than nanorods. Microspheres still kept a specific capacity of 200 mAh g1 even at a rate of 5 C after 500 cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2014 Received in revised form 7 March 2015 Accepted 12 March 2015 Available online 13 March 2015
V2O5 microspheres with hierarchical structure via a facile two-step strategy. The first is preparing vanadium glycolate precursor through polyol medium template-free process and the second is subsequent thermal annealing treatment of the well-prepared precursors at high temperature in air. However, just by one-plot calcination of vanadium(IV) acetylacetone, V2O5 nanorods were obtained. The gained metal oxide were characterized by different physical and electrochemical analytical techniques. Electrochemical testing results show that V2O5 microspheres displayed a high specific discharge capacity of 275 mAh g1 at 1 C which is higher than the obtained nanorods, and the microspheres still kept 243 mAh g1 after 200 cycles. Notably, the nanosheet-assembled microspheres as electrode materials still show 200 mAh g1 even at a rate of 5 C after 500 cycles. These results demonstrated that the wellprepared nanosheet-assembled microspheres are a fine cathode material for lithium ion battery with a high specific capacity and excellent cycle stability. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vandium pentoxide Nanosheet-assembled Microspheres Nanorods Electrochemical performance
1. Introduction Recently, V2O5 nano/micro materials have been extensively studied due to their advantageous features of high theoretical discharge capacity of 294 mAh g1 and facile lithium ion insertion as one type of promising materials for lithium ion battery [1e4]. However, the intrinsic low diffusion coefficient of Liþ (1014 to 1012 cm2 s1) [5,6] and poor electrical conductivities (102 to 103 S cm1) [7,8] limit the practical application of V2O5 to some extent. To improve the electrochemical performance, nanoscale electrode materials has been widely admitted for providing higher rate capability by means of enhancing electrode-electrolyte contacts and decreasing diffusion distances of electrons and Liþ [9e11].
* Corresponding author. E-mail address: [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.jpowsour.2015.03.078 0378-7753/© 2015 Elsevier B.V. All rights reserved.
It is regrettable that nanoscale structure is likely to appear structural collapse during lithium ion penetration procedure and particles agglomeration which not only generates the isolation between particles and conductive agents but also rises polar intensity and electron transfer resistance [12e16]. These easily leads to cycling instability. Therefore, an optimal structure for V2O5 as electrode materials with both high energy density and high rate capacity should be a microstructure consisted of primary nanostrucure units tightly compacted to form effectively channels for ion diffusion. In addition, hydrothermal and solvothermal methods during several synthetic methodologies are an effective to synthesize microstructure with complex interiors which can provide a chance to self-assembled oriented growth [3,12]. The polyol-mediated synthesis process with low cost, ease synthesis is considered as an ideal process, because polyol media may act as organic surfactants and polymers for controlling the shape and size of specific morphology. Ethylene glycol (EG) has
Y. Dong et al. / Journal of Power Sources 285 (2015) 538e542
previously used to prepare metal oxide nanowires as a ligand via refluxing or solvothermal process [17,18], Moreover, it also plays an important role in the formation of complex microspherical structure as a cross-linking reagent [3,19e21]. However, the preparation of complex architectures with sheets-assembled structure through facile synthesis procedures still remains a great challenge. Herein, we report a facile template-free and polyol-mediated solvothermal method to synthesize vanadium glycolate precursor, which can be readily transformed to V2O5 microspheres by thermal annealing treatment in air. Nanorods can be gained through one-plot calcination without solvothermal process from the materials. The well-synthesized hierarchical microspheres composed of nanosheets exhibits high-rate specific capacity and good cycling stability. 2. Experimental section Ethylene glycol (HOC2H4OH, absolute for analysis) and vanadium pentoxide (V2O5) are bought without further purity. 2.1. Synthesis of materials vanadium(IV) acetylacetone was synthesized according to the literature [22], 3.64 g vanadium pentoxide(V2O5) and 50 ml acetylacetone were added into single-neck flask through oil bath heating at 100 C for 24 h with stirring, and then the blue products were cooled to temperature naturally and collected by filtering. Finally, the obtained products were dried at 80 C for 12 h in air. 2.2. Synthesis of products In a typical synthesis process, 0.75 g (2.8 mmol) vanadium(IV) acetylacetone and 40 ml ethylene glycol were added into 50 ml stainless steel autoclave with a teflon lining under stirring until the color of the solution transferred into dark blue. Then, the autoclave was heated at 220 Cfor 17 h in an oven and naturally cooled to room temperature. The resulting green products were collected by centrifugation at 10,000 rpm for 3 min and washed with absolute ethanol three times. The well-collected solid were dried at 60 C for 12 h to produce the precursor and it was then calcined at 600 C for 2 h. Another sample was prepared by vanadium(IV) acetylacetone just through one-plot calcination at 600 C for 2 h.
539
2.3. Structural and phase characterization Phase measurements were performed by power X-ray diffraction (XRD, Philips X'Pert Pro Super diffractometer, Cu Ka radiation l ¼ 1.54178). The XRD patterns were recorded from 10 to 80 at a scanning speed of 2 min1. The morphology of the prepared products was characterized by using field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL). Surface analysis for the samples was performed with X-ray photoelectron spectroscopy (XPS, VGESCA-LABMK II spectrometer, using a twin-anode Al Ka (1486.6 eV) X-ray source). Fourier transform infrared spectroscopy (FTIR) investigation was conducted using the KBr method with Bruker EQUINOX55. Each FTIR spectrum was collected after 32 scans at a resolution of 4 cm1 from 400 to 4000 cm1. The dried samples (~1 mg) were mixed and grinded with about 0.1 g of KBr powder and prepared into sample slices for the FTIR measurement. 2.4. Electrochemical characterization Electrochemical analyses were performed using coin-type cells (CR2032). The diameter of the electrode is 12 mm, the thickness of the coating is 200 mm and the density of the active material is about 1 mg cm2. Working electrodes were prepared by active materials, carbon black and polyvinylidene fluoride (PVDF) at a weight ratio of 70:20:10 onto Al foil. Finally the well-coated foil were dried at 80 C for 12 h in air. The electrolyte was used 1 M LiPF6 mixed with inethylene carbonate (EC), dimethylcarbonate (DMC) and diethyl carbonate (DEC) in a ratio of 1:1:1 (v:v:v). The cells were assembled in an argon-filled M Braun glovebox model Unilab using airtight glass containers. The galvanostatic chargeedischarge cycling measurements were performed using CT2001A LAND Cell test system. The cyclic voltammetry (CV) was tested in the voltage range of 2e4 V by an electrochemical workstation (LK2005A). 3. Results and discussions We firstly synthesized V2O5 hierarchical sheet-assembled microspheres with an average diameter of about 22 mm through a polyol-mediated solvothermal process. FESEM images of V2O5 hierarchical microspheres are displayed in Fig. 1aec, which are close to 100% morphological yield. Especially, the FESEM images given in
Fig. 1. FESEM images of V2O5 hierarchical microspheres (aec), V2O5 nanorods (def).
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Y. Dong et al. / Journal of Power Sources 285 (2015) 538e542
selected area (Fig. 1b and c) obviously show the hierarchical structure consisted of self-assembled nanosheets. The growth process of nanosheet-assembled microsphere is simplely discussed, as showed in Fig. S1(see in supporting information). The morphology of the microspheres may evolve from small nanorods to nanorods-assembled hierarchical spheres, and the nanorods gradually becomes thinner, eventually to smaller free-standing nanosheets. When the reaction time is over 24 h, the morphology of the microspheres becomes nonuniform and the average size is a little smaller (see in Supporting information S2). However, the growth process and the plausible mechanism of nanosheetassembled microspheres should be further discussed. Without solvothermal treatment, we gained V2O5 nanorods which are displayed in Fig. 1dee with a range of about 200 nm in width, 100 nm in sickness and 1 mm in longth. The crystal structure of the microspheres precursor was investigated by XRD and FIIR analysis in Fig. 2a and b. A strong peaks of the micro-sphere precursor (Fig. 2a) around 10 is characterstic of metal glycolate, which corresponds to the previous report [21,23]. In Fig. 2b, we also observed that the appearance of CH2e at 2869 cm1, and CeC at 985 cm1 vibrational bands is from ethylene glycolate unit and the appearance of VeO at 647 cm1 and V]O at 1065 cm1 vibrational bands is from vanadium(IV) acetylacetone group. As shown in Fig. 2c, the XRD patterns of the two samples show that all of the peaks can be obvious indexed and assigned to the orthorhombic V2O5 phase (JCPDS card no. 41-1426, space group: Pmmn (56), a ¼ 11.516 Å, b ¼ 3.566 Å, c ¼ 3.777 Å) [24,25] without obvious impurity. In addition, the phase of the two samples reacting at 12 h and 24 h (see supporting information Fig. S2) in solvothermal system can be also assigned to the orthorhombic V2O5 (JCPDS card no.41e1426). The XPS of V2O5 microspheres (Fig. 2d) shows the V 2p3/2 band,V 2p1/2 band and O 1s band located at 517.8 eV, 525.3 eV and 530.6 eV, which corresponds to the V(5þ)-O stretch and agrees with previous reports [7,26,27]. Fig. 3 shows the typical dischargeecharge cyclic voltammogram
Fig. 3. First CV curves of V2O5 microspheres after 200 cycles in the voltage of 2e4 V vs. Li/Liþ at a scan of 0.5 mV s1.
of the electrode within a voltage range of 2.0e4.0 V after 200 cycles. As showed in Fig. 3, three cathodic peaks in discharge curve at 1 C are observed at around 3.3 V, 3.1 V, and 2.2 V (vs. Li/Liþ) which are ascribed to phase changes from a-V2O5 to ε-Li0.5V2O5, then to dLiV2O5, and finally to g-Li2V2O5 during Liþ ion intercalation [28,29]. Three anodic peaks in charge curves are at around 2.7, 3.4, and 3.7 V (vs. Li/Liþ) which are attributed to Liþ ion deintercalation and the corresponding reverse phase transformation of g-Li2V2O5, d-LiV2O5, ε-Li0.5V2O5 and a-V2O5, respectively. Mentionly, the anodic peak of phase change from d-LiV2O5 to ε-Li0.5V2O5 is misty. The result is consistent with voltage plateaus at a current density of 1500 mA g1, which just shows two excellent voltage plateaus. Possiblely because the phase change is not obvious at high rate or after cycling. In Fig. 4a, it is clearly that the electrode composed of 17 h-
Fig. 2. (a)XRD pattern of vanadium glycolate precursor; (b)FIIR pattern of vanadium glycolate precursors; (c)XRD pattern of orthorhombic V2O5 microspheres and nanorods; (d)XPS pattern of orthorhombic V2O5 microsphere.
Y. Dong et al. / Journal of Power Sources 285 (2015) 538e542
541
Fig. 4. (a) Dischargeecharge curves of V2O5 microspheres at different rates; (b) Dischargeecharge curves of V2O5 nanorods at different rates; (c) Cycling performance of V2O5 microspheres and nanorods at 1C; (d) Cycling performance of V2O5 microspheres at 5C.
treated microspheres has three representative voltage plateaus at a current density of 300 mA g1, however, the plateaus of nanorods becomes conspicuous at 300 mA g1. As showed in Fig. 4c, The initial charge and discharge specific capacity of nanosheetassembled microspheres are 274.4 and 275.7 mAh g1 at a rate of 1 C between 2.0 and 4.0 V, corresponding to a Coulombic Efficiency(CE) of 99.5%. While V2O5 nanorods firstly displayed a charge and discharge capacity of 266.7 mAh g1 and 264.9 mAh g1 at 1 C between 2.0 and 4.0 V. After 100 and 200 cycles, the discharge specific capacity of microsphere still remains 259.1 and 243.8 mAh g1 (about 93.98% and 88.43% retention), respectively, which is much higher than the results with the similar morphology. For example, Template-free synthesis of VO2 hollow micro-spheres with various interiors and their conversion into V2O5 for lithiumion batteries [4] shows a discharge capacity of about 225 mAh g1 at 1 C after 50 cycles. uniform V2O5 nanorodassembled hollow micro-flowers [12] gave a discharge specific capacity of 211 mAh g1 at 300 mAh g1 after 100 cycles. V2O5 hollow nano-spheres: A lithium intercalation host with good rate capability and capacity retention [4] displayed a discharge capacity of about 200 mAh g1 at 0.5 C after 50 cycles. The excellent electrochemical property of our samples originates from the unique hierarchical nanosheet-assembled architectures, which not only increase the contact between the electrode and the electrolyte to buffer the structure breakdown and but also effectively shorten Liþ diffusion distance. However, the specific capacity of microsphere obtained through solvothermal process for 24 h (Fig. S4) becomes low, possiblely because the morphology of the 24 h-treated microspheres is nonuniform which goes against Liþ transport to some certain extent. Mentionly, Fig. 4d image shows the discharge specific capacity of V2O5 sheet-assembled microspheres perfectly kept 200 mAh g1 after 500 cycles, Therefore, V2O5 hierarchical nanosheet-assembled microspheres as cathode materials shows high specific capacity and excellent high-rate cycling stability. 4. Conclusions A facile solvothermal method without any templates has been proposed to prepare vanadium glycolate precursor using
vanadium(IV) acetylacetone, which can be easily transformed into V2O5 hierarchical sheet-assembled microspheres. Without solvothermal treatment, V2O5 nanorods can be obtained through oneplot calcination from vanadium(IV) acetylacetone. High electrochemical performance was achieved by synthesizing hierarchical sheet-assembled microspheres, resulting in a specific discharge capacity of 243 mAh g1 at 1 C after 200 cycles. Moreover, V2O5 hierarchical microspheres as one kind of cathode materials for lithium ion battery still obtains the specific capacity of 200 mAh g1 even at a rate of 5 C after 500 cycles. Therefore, V2O5 hierarchical nanosheet-assembled microspheres will be a promising candidate as a cathode materials for lithium ion battery. Acknowledgments This work was supported by the Natural Science Foundation of China (grant nos 21276142). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.03.078. References [1] Manickam Sasidharan, Nanda Gunawardhana, Masaki Yoshio, Kenichi Nakashima, J. Electrochem. Soc. 159 (5) (2012) A618eA621. [2] S.Q. Wang, Z.D. Lu, D. Wang, C.G. Li, C.H. Chen, Y.D. Yin, J. Mater. Chem. 21 (2011) 6365e6369. [3] A.M. Cao, J.S. Hu, H.P. Liang, L.J. Wan, Angew. Chem. Int. Ed. 117 (2005) 4465e4469. [4] A.Q. Pan, H.B. Wu, L. Yu, X.W. (David) Lou, Angew. Chem. Int. Ed. 52 (2013) 2226e2230. [5] A.Q. Pan, J.G. Zhang, Z.M. Nie, G.Z. Cao, Bruce W. Arey, G.S. Li, S.Q. Liang, J. Liu, J. Mater. Chem. 20 (2010) 9193e9199. [6] T. Watanabe, Y. Ikeda, T. Ono, M. Hibino, M. Hosoda, K. Sakai, T. Kudo, Solid State Ion. 151 (2002) 313. [7] H. Yu, X.H. Rui, H.T. Tan, J. Chen, X. Huang, C. Xu, W.L. Liu, Y. Denis, W. Yu, Huey Hoon Hng, Harry E. Hoster, Q.Y. Yan, Nanoscale 5 (2013) 4937e4943. [8] J. Muster, G.T. Kim, V. Krstic, J.G. Park, Y.W. Park, S. Roth, M. Burghard, Adv. Mater. 12 (2000) 420e424. [9] Z.Y. Wang, L. Zhou, X.W. Lou, Adv. Mater. 24 (2012) 1903. [10] L. Zhou, D.Y. Zhao, X.W. Lou, Angew. Chem. Int. Ed. 51 (2012) 243. [11] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000)
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496. [12] A.Q. Pan, H.B. Wu, L. Zhang, X.W. (David) Lou, Energy Environ. Sci. 6 (2013) 1476e1479. [13] Y. Wang, G.Z. Cao, Adv. Mater. 20 (2008) 2252. [14] C.F. Zhang, Z.X. Chen, Z.P. Guo, X.W. Lou, Energy Environ. Sci. 6 (2013) 974e978. [15] H.F. Xiang, H. Wang, C.H. Chen, X.W. Ge, S. Guo, J.H. Sun, W.Q. Hu, J. Power Sources 191 (2009) 575. [16] Y.K. Sun, S.M. Oh, H.K. Park, B. Scrosati, Adv. Mater. 23 (2011) 5050. [17] X.C. Jiang, Y.L. Wang, Thurston Herricks, Y.N. Xia, J. Mater. Chem. 14 (2004) 695e703. [18] Y.L. Wang, X.C. Jiang, Y.N. Xia, J. Am. Chem. Soc. 125 (2003) 16176e16177. [19] Q.W. Li, Y.C. Zhu, Y. Yu, Y.T. Qian, Int. J. Electrochem. Sci. 7 (2012) 5557e5564. [20] D.W. Su, S.X. Dou, G.X. Wang, J. Mater. Chem. A 2 (2014) 11185e11194.
[21] J. Bai, X.G. Li, G.Z. Liu, Y.T. Qian, S.L. Xiong, Adv. Funct. Mater. 24 (2014) 3012e3020. [22] L.J. Mao, C.Y. Liu, J. Li, J. Mater. Chem. 18 (2008) 1640e1643. [23] Y.L. Wang, X.C. Jiang, Y.N. Xia, J. Am. Chem. Soc. 125 (2003) 16176e16177. [24] A.Q. Pan, J.G. Zhang, Z.M. Nie, G.Z. Cao, Bruce W. Arey, G.S. Li, S.Q. Liang, J. Liu, J. Mater. Chem. 20 (2010) 9193e9199. [25] You Na Ko, Yun Chan Kang, Seung Bin Park, Nanoscale 5 (2013) 8899e8903. [26] Manas R. Parida, C. Vijayan, C.S. Rout, C.S. Suchand Sandeep, Reji Philip, P.C. Deshmukh, J. Phys. Chem. C 115 (2011) 112e117. [27] P. Ragupathy, S. Shivakumar, S.H. Vasan, N. Munichandraiah, J. Phys. Chem. C 112 (2008) 16700. [28] X.F. Zhang, K.X. Wang, X. Wei, J.S. Chen, Chem. Mater. 23 (2011) 5290e5292. [29] Y.S. Hu, X. Liu, D.S. Su, Angew. Chem. Int. Ed. 48 (2009) 210.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Template-free synthesis of V2O5 hierarchical nanosheet-assembled microspheres with excellent cycling stability Yujuan Dong, Huiying Wei, Wei Liu, Qianjin Liu, Wenjing Zhang, Yanzhao Yang* Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
h i g h l i g h t s We used the template-free solvothermal method to synthesize V2O5 microspheres. Microspheres was nanosheet-assembled. Nanosheet-assembled microspheres exhibited higher specific capacity than nanorods. Microspheres still kept a specific capacity of 200 mAh g1 even at a rate of 5 C after 500 cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 October 2014 Received in revised form 7 March 2015 Accepted 12 March 2015 Available online 13 March 2015
V2O5 microspheres with hierarchical structure via a facile two-step strategy. The first is preparing vanadium glycolate precursor through polyol medium template-free process and the second is subsequent thermal annealing treatment of the well-prepared precursors at high temperature in air. However, just by one-plot calcination of vanadium(IV) acetylacetone, V2O5 nanorods were obtained. The gained metal oxide were characterized by different physical and electrochemical analytical techniques. Electrochemical testing results show that V2O5 microspheres displayed a high specific discharge capacity of 275 mAh g1 at 1 C which is higher than the obtained nanorods, and the microspheres still kept 243 mAh g1 after 200 cycles. Notably, the nanosheet-assembled microspheres as electrode materials still show 200 mAh g1 even at a rate of 5 C after 500 cycles. These results demonstrated that the wellprepared nanosheet-assembled microspheres are a fine cathode material for lithium ion battery with a high specific capacity and excellent cycle stability. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vandium pentoxide Nanosheet-assembled Microspheres Nanorods Electrochemical performance
1. Introduction Recently, V2O5 nano/micro materials have been extensively studied due to their advantageous features of high theoretical discharge capacity of 294 mAh g1 and facile lithium ion insertion as one type of promising materials for lithium ion battery [1e4]. However, the intrinsic low diffusion coefficient of Liþ (1014 to 1012 cm2 s1) [5,6] and poor electrical conductivities (102 to 103 S cm1) [7,8] limit the practical application of V2O5 to some extent. To improve the electrochemical performance, nanoscale electrode materials has been widely admitted for providing higher rate capability by means of enhancing electrode-electrolyte contacts and decreasing diffusion distances of electrons and Liþ [9e11].
* Corresponding author. E-mail address: [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.jpowsour.2015.03.078 0378-7753/© 2015 Elsevier B.V. All rights reserved.
It is regrettable that nanoscale structure is likely to appear structural collapse during lithium ion penetration procedure and particles agglomeration which not only generates the isolation between particles and conductive agents but also rises polar intensity and electron transfer resistance [12e16]. These easily leads to cycling instability. Therefore, an optimal structure for V2O5 as electrode materials with both high energy density and high rate capacity should be a microstructure consisted of primary nanostrucure units tightly compacted to form effectively channels for ion diffusion. In addition, hydrothermal and solvothermal methods during several synthetic methodologies are an effective to synthesize microstructure with complex interiors which can provide a chance to self-assembled oriented growth [3,12]. The polyol-mediated synthesis process with low cost, ease synthesis is considered as an ideal process, because polyol media may act as organic surfactants and polymers for controlling the shape and size of specific morphology. Ethylene glycol (EG) has
Y. Dong et al. / Journal of Power Sources 285 (2015) 538e542
previously used to prepare metal oxide nanowires as a ligand via refluxing or solvothermal process [17,18], Moreover, it also plays an important role in the formation of complex microspherical structure as a cross-linking reagent [3,19e21]. However, the preparation of complex architectures with sheets-assembled structure through facile synthesis procedures still remains a great challenge. Herein, we report a facile template-free and polyol-mediated solvothermal method to synthesize vanadium glycolate precursor, which can be readily transformed to V2O5 microspheres by thermal annealing treatment in air. Nanorods can be gained through one-plot calcination without solvothermal process from the materials. The well-synthesized hierarchical microspheres composed of nanosheets exhibits high-rate specific capacity and good cycling stability. 2. Experimental section Ethylene glycol (HOC2H4OH, absolute for analysis) and vanadium pentoxide (V2O5) are bought without further purity. 2.1. Synthesis of materials vanadium(IV) acetylacetone was synthesized according to the literature [22], 3.64 g vanadium pentoxide(V2O5) and 50 ml acetylacetone were added into single-neck flask through oil bath heating at 100 C for 24 h with stirring, and then the blue products were cooled to temperature naturally and collected by filtering. Finally, the obtained products were dried at 80 C for 12 h in air. 2.2. Synthesis of products In a typical synthesis process, 0.75 g (2.8 mmol) vanadium(IV) acetylacetone and 40 ml ethylene glycol were added into 50 ml stainless steel autoclave with a teflon lining under stirring until the color of the solution transferred into dark blue. Then, the autoclave was heated at 220 Cfor 17 h in an oven and naturally cooled to room temperature. The resulting green products were collected by centrifugation at 10,000 rpm for 3 min and washed with absolute ethanol three times. The well-collected solid were dried at 60 C for 12 h to produce the precursor and it was then calcined at 600 C for 2 h. Another sample was prepared by vanadium(IV) acetylacetone just through one-plot calcination at 600 C for 2 h.
539
2.3. Structural and phase characterization Phase measurements were performed by power X-ray diffraction (XRD, Philips X'Pert Pro Super diffractometer, Cu Ka radiation l ¼ 1.54178). The XRD patterns were recorded from 10 to 80 at a scanning speed of 2 min1. The morphology of the prepared products was characterized by using field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL). Surface analysis for the samples was performed with X-ray photoelectron spectroscopy (XPS, VGESCA-LABMK II spectrometer, using a twin-anode Al Ka (1486.6 eV) X-ray source). Fourier transform infrared spectroscopy (FTIR) investigation was conducted using the KBr method with Bruker EQUINOX55. Each FTIR spectrum was collected after 32 scans at a resolution of 4 cm1 from 400 to 4000 cm1. The dried samples (~1 mg) were mixed and grinded with about 0.1 g of KBr powder and prepared into sample slices for the FTIR measurement. 2.4. Electrochemical characterization Electrochemical analyses were performed using coin-type cells (CR2032). The diameter of the electrode is 12 mm, the thickness of the coating is 200 mm and the density of the active material is about 1 mg cm2. Working electrodes were prepared by active materials, carbon black and polyvinylidene fluoride (PVDF) at a weight ratio of 70:20:10 onto Al foil. Finally the well-coated foil were dried at 80 C for 12 h in air. The electrolyte was used 1 M LiPF6 mixed with inethylene carbonate (EC), dimethylcarbonate (DMC) and diethyl carbonate (DEC) in a ratio of 1:1:1 (v:v:v). The cells were assembled in an argon-filled M Braun glovebox model Unilab using airtight glass containers. The galvanostatic chargeedischarge cycling measurements were performed using CT2001A LAND Cell test system. The cyclic voltammetry (CV) was tested in the voltage range of 2e4 V by an electrochemical workstation (LK2005A). 3. Results and discussions We firstly synthesized V2O5 hierarchical sheet-assembled microspheres with an average diameter of about 22 mm through a polyol-mediated solvothermal process. FESEM images of V2O5 hierarchical microspheres are displayed in Fig. 1aec, which are close to 100% morphological yield. Especially, the FESEM images given in
Fig. 1. FESEM images of V2O5 hierarchical microspheres (aec), V2O5 nanorods (def).
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Y. Dong et al. / Journal of Power Sources 285 (2015) 538e542
selected area (Fig. 1b and c) obviously show the hierarchical structure consisted of self-assembled nanosheets. The growth process of nanosheet-assembled microsphere is simplely discussed, as showed in Fig. S1(see in supporting information). The morphology of the microspheres may evolve from small nanorods to nanorods-assembled hierarchical spheres, and the nanorods gradually becomes thinner, eventually to smaller free-standing nanosheets. When the reaction time is over 24 h, the morphology of the microspheres becomes nonuniform and the average size is a little smaller (see in Supporting information S2). However, the growth process and the plausible mechanism of nanosheetassembled microspheres should be further discussed. Without solvothermal treatment, we gained V2O5 nanorods which are displayed in Fig. 1dee with a range of about 200 nm in width, 100 nm in sickness and 1 mm in longth. The crystal structure of the microspheres precursor was investigated by XRD and FIIR analysis in Fig. 2a and b. A strong peaks of the micro-sphere precursor (Fig. 2a) around 10 is characterstic of metal glycolate, which corresponds to the previous report [21,23]. In Fig. 2b, we also observed that the appearance of CH2e at 2869 cm1, and CeC at 985 cm1 vibrational bands is from ethylene glycolate unit and the appearance of VeO at 647 cm1 and V]O at 1065 cm1 vibrational bands is from vanadium(IV) acetylacetone group. As shown in Fig. 2c, the XRD patterns of the two samples show that all of the peaks can be obvious indexed and assigned to the orthorhombic V2O5 phase (JCPDS card no. 41-1426, space group: Pmmn (56), a ¼ 11.516 Å, b ¼ 3.566 Å, c ¼ 3.777 Å) [24,25] without obvious impurity. In addition, the phase of the two samples reacting at 12 h and 24 h (see supporting information Fig. S2) in solvothermal system can be also assigned to the orthorhombic V2O5 (JCPDS card no.41e1426). The XPS of V2O5 microspheres (Fig. 2d) shows the V 2p3/2 band,V 2p1/2 band and O 1s band located at 517.8 eV, 525.3 eV and 530.6 eV, which corresponds to the V(5þ)-O stretch and agrees with previous reports [7,26,27]. Fig. 3 shows the typical dischargeecharge cyclic voltammogram
Fig. 3. First CV curves of V2O5 microspheres after 200 cycles in the voltage of 2e4 V vs. Li/Liþ at a scan of 0.5 mV s1.
of the electrode within a voltage range of 2.0e4.0 V after 200 cycles. As showed in Fig. 3, three cathodic peaks in discharge curve at 1 C are observed at around 3.3 V, 3.1 V, and 2.2 V (vs. Li/Liþ) which are ascribed to phase changes from a-V2O5 to ε-Li0.5V2O5, then to dLiV2O5, and finally to g-Li2V2O5 during Liþ ion intercalation [28,29]. Three anodic peaks in charge curves are at around 2.7, 3.4, and 3.7 V (vs. Li/Liþ) which are attributed to Liþ ion deintercalation and the corresponding reverse phase transformation of g-Li2V2O5, d-LiV2O5, ε-Li0.5V2O5 and a-V2O5, respectively. Mentionly, the anodic peak of phase change from d-LiV2O5 to ε-Li0.5V2O5 is misty. The result is consistent with voltage plateaus at a current density of 1500 mA g1, which just shows two excellent voltage plateaus. Possiblely because the phase change is not obvious at high rate or after cycling. In Fig. 4a, it is clearly that the electrode composed of 17 h-
Fig. 2. (a)XRD pattern of vanadium glycolate precursor; (b)FIIR pattern of vanadium glycolate precursors; (c)XRD pattern of orthorhombic V2O5 microspheres and nanorods; (d)XPS pattern of orthorhombic V2O5 microsphere.
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Fig. 4. (a) Dischargeecharge curves of V2O5 microspheres at different rates; (b) Dischargeecharge curves of V2O5 nanorods at different rates; (c) Cycling performance of V2O5 microspheres and nanorods at 1C; (d) Cycling performance of V2O5 microspheres at 5C.
treated microspheres has three representative voltage plateaus at a current density of 300 mA g1, however, the plateaus of nanorods becomes conspicuous at 300 mA g1. As showed in Fig. 4c, The initial charge and discharge specific capacity of nanosheetassembled microspheres are 274.4 and 275.7 mAh g1 at a rate of 1 C between 2.0 and 4.0 V, corresponding to a Coulombic Efficiency(CE) of 99.5%. While V2O5 nanorods firstly displayed a charge and discharge capacity of 266.7 mAh g1 and 264.9 mAh g1 at 1 C between 2.0 and 4.0 V. After 100 and 200 cycles, the discharge specific capacity of microsphere still remains 259.1 and 243.8 mAh g1 (about 93.98% and 88.43% retention), respectively, which is much higher than the results with the similar morphology. For example, Template-free synthesis of VO2 hollow micro-spheres with various interiors and their conversion into V2O5 for lithiumion batteries [4] shows a discharge capacity of about 225 mAh g1 at 1 C after 50 cycles. uniform V2O5 nanorodassembled hollow micro-flowers [12] gave a discharge specific capacity of 211 mAh g1 at 300 mAh g1 after 100 cycles. V2O5 hollow nano-spheres: A lithium intercalation host with good rate capability and capacity retention [4] displayed a discharge capacity of about 200 mAh g1 at 0.5 C after 50 cycles. The excellent electrochemical property of our samples originates from the unique hierarchical nanosheet-assembled architectures, which not only increase the contact between the electrode and the electrolyte to buffer the structure breakdown and but also effectively shorten Liþ diffusion distance. However, the specific capacity of microsphere obtained through solvothermal process for 24 h (Fig. S4) becomes low, possiblely because the morphology of the 24 h-treated microspheres is nonuniform which goes against Liþ transport to some certain extent. Mentionly, Fig. 4d image shows the discharge specific capacity of V2O5 sheet-assembled microspheres perfectly kept 200 mAh g1 after 500 cycles, Therefore, V2O5 hierarchical nanosheet-assembled microspheres as cathode materials shows high specific capacity and excellent high-rate cycling stability. 4. Conclusions A facile solvothermal method without any templates has been proposed to prepare vanadium glycolate precursor using
vanadium(IV) acetylacetone, which can be easily transformed into V2O5 hierarchical sheet-assembled microspheres. Without solvothermal treatment, V2O5 nanorods can be obtained through oneplot calcination from vanadium(IV) acetylacetone. High electrochemical performance was achieved by synthesizing hierarchical sheet-assembled microspheres, resulting in a specific discharge capacity of 243 mAh g1 at 1 C after 200 cycles. Moreover, V2O5 hierarchical microspheres as one kind of cathode materials for lithium ion battery still obtains the specific capacity of 200 mAh g1 even at a rate of 5 C after 500 cycles. Therefore, V2O5 hierarchical nanosheet-assembled microspheres will be a promising candidate as a cathode materials for lithium ion battery. Acknowledgments This work was supported by the Natural Science Foundation of China (grant nos 21276142). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.03.078. References [1] Manickam Sasidharan, Nanda Gunawardhana, Masaki Yoshio, Kenichi Nakashima, J. Electrochem. Soc. 159 (5) (2012) A618eA621. [2] S.Q. Wang, Z.D. Lu, D. Wang, C.G. Li, C.H. Chen, Y.D. Yin, J. Mater. Chem. 21 (2011) 6365e6369. [3] A.M. Cao, J.S. Hu, H.P. Liang, L.J. Wan, Angew. Chem. Int. Ed. 117 (2005) 4465e4469. [4] A.Q. Pan, H.B. Wu, L. Yu, X.W. (David) Lou, Angew. Chem. Int. Ed. 52 (2013) 2226e2230. [5] A.Q. Pan, J.G. Zhang, Z.M. Nie, G.Z. Cao, Bruce W. Arey, G.S. Li, S.Q. Liang, J. Liu, J. Mater. Chem. 20 (2010) 9193e9199. [6] T. Watanabe, Y. Ikeda, T. Ono, M. Hibino, M. Hosoda, K. Sakai, T. Kudo, Solid State Ion. 151 (2002) 313. [7] H. Yu, X.H. Rui, H.T. Tan, J. Chen, X. Huang, C. Xu, W.L. Liu, Y. Denis, W. Yu, Huey Hoon Hng, Harry E. Hoster, Q.Y. Yan, Nanoscale 5 (2013) 4937e4943. [8] J. Muster, G.T. Kim, V. Krstic, J.G. Park, Y.W. Park, S. Roth, M. Burghard, Adv. Mater. 12 (2000) 420e424. [9] Z.Y. Wang, L. Zhou, X.W. Lou, Adv. Mater. 24 (2012) 1903. [10] L. Zhou, D.Y. Zhao, X.W. Lou, Angew. Chem. Int. Ed. 51 (2012) 243. [11] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000)
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