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Facile synthesis of Nb2O5 nanobelts assembled from nanorods and their applications in lithium ion batteries
Journal of Physics and Chemistry of Solids 111 (2017) 8–11
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Facile synthesis of Nb2O5 nanobelts assembled from nanorods and their applications in lithium ion batteries Xiaodi Liu a, b, Guangyin Liu b, *, Hao Chen b, Jianmin Ma a, **, Ruixue Zhang b a b
College of Physics and Electronics, Hunan University, Changsha 410022, China College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Electrochemical properties
Hierarchical 1D Nb2O5 nanobelts are successfully synthesized via a facile solvothermal method and following thermal treatment. The as-formed Nb2O5 nanobelts are characterized by XRD, FESEM, TEM, and BET, and the results indicate that they possess pseudohexagonal structure and are composed of ultranarrow nanorods with an average diameter of ca. 15 nm. When used as anodic materials for lithium ion batteries, the obtained Nb2O5 nanobelts can deliver initial discharge capacities of 209.3 mAh g1 at the current density of 0.5 C. In addition, the Nb2O5 nanobelts exhibit a reversible capacity of 95.8 mAh g1 after 200 cycles at relatively high current density of 5 C. The good electrochemical performance of the Nb2O5 nanobelts may be ascribed to their good monodispersity, high specific surface areas, and narrow rod-like building blocks. The Nb2O5 nanobelts can be developed as promising anodes for high-rate 2 V LIBs with good safety.
1. Introduction Lithium ion batteries (LIBs), as a fast-developing technology in electric energy storage, have made considerable contribution to several fields [1]. Most recently, with the development of micro/nanoelectronic devices, tremendous attention has been paid to the exploration of small, safe, and powerful LIBs with low operating voltage (2 V vs. Liþ/Li) [2]. Nb2O5 is considered as an appealing anode material for 2 V LIBs owing to its high valence state and good structural stability [3,4]. Moreover, similar to Li4Ti5O12, Nb2O5 possesses excellent safety advantages due to its appropriate operational voltage plateau (1.0–2.0 V vs. Liþ/Li), which can prevent the growth of lithium dendrites after long charge-discharge process and suppress the formation of SEI layers [5–7]. More importantly, compared with Li4Ti5O12 (175 mAh g1), Nb2O5 has a higher theoretical capacity of 200 mAh g1 [2]. So, Nb2O5 has attracted increasing attention in the fields of LIBs, especially in 2 V LIBs [8,9]. Recently, it has been demonstrated that the constructing of nanostructured anode materials can be used to reduce the diffusion length of Liþ ion, leading to improved electrochemical performance [10]. Furthermore, it is known that the electrochemical properties of nanoscale electrodes are closed related to their sizes and morphologies [11–13]. Thus, up to date, several Nb2O5 nanomaterials with various morphologies, including nanorods, hollow nanospheres, and nanosheets, have * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Liu), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.jpcs.2017.07.007 Received 10 April 2017; Received in revised form 3 July 2017; Accepted 10 July 2017 Available online 13 July 2017 0022-3697/© 2017 Elsevier Ltd. All rights reserved.
been successfully prepared to enhance their electrochemical properties [14–16]. Most especially, 1D nanostructured electrodes have achieved great interest because of their high surface areas, enhanced kinetic, and improved electrochemical properties [17–19]. In these regards, it is imperative to probe novel and effective methods of preparing 1D Nb2O5 nanomaterials with special morphologies and excellent properties. Herein, we report a novel and simple solvothermal route and following thermal treatment to synthesize 1D Nb2O5 nanobelts. The Nb2O5 nanobelts are assembled by “oriented attachment” of ultranarrow nanorods. The Li-ion storage performance of the as-formed Nb2O5 nanobelts is researched, and the results indicate that the Nb2O5 electrode possesses good electrochemical properties, including high reversible capacity and good rate performance. 2. Material and methods 2.1. Synthesis of Nb2O5 nanobelts In a typical synthesis, 1 mmol Nb(HC2O4)5 is added into 30 mL isopropanol, and then the mixture is stirred for 2 h and transferred into a Teflon-lined autoclave (50 mL). The autoclave is maintained at 180 C for 48 h. Subsequently, the powder is washed and dried in a vacuum oven at 80 C. Finally, the precursors are placed in a muffle furnace and calcined
X. Liu et al.
Journal of Physics and Chemistry of Solids 111 (2017) 8–11
Fig. 1. (a) XRD pattern and (b) Nitrogen adsorption-desorption isotherm of the Nb2O5 nanobelts.
at 600 C for 2 h to generate Nb2O5.
3. Results and discussion 3.1. Characterization of Nb2O5 nanobelts
2.2. Characterizations
The crystal structure of the as-synthesized Nb2O5 nanobelts is researched by XRD (Fig. 1a). The positions of all peaks are in good agreement with the reported data of Nb2O5 with a pseudohexagonal structure (JCPDS No. 07-0061, space group: P6/mmm). The strong peaks indicate the high crystalline nature of the sample; moreover, in comparison with the standard XRD pattern of Nb2O5, the (001) peak appears as the strongest one instead of the (100) peak, suggesting that the obtained Nb2O5 is oriented-growth [20]. Fig. 1b depicts the N2 adsorption and desorption isotherm of the sample, and it is found that as-formed Nb2O5 nanobelts have a BET surface area of ca. 28.1 m2 g1. The morphologies and nanostructures of the Nb2O5 nanobelts are observed by FESEM, TEM, and HRTEM. As shown in the FESEM image (Fig. 2a), the sample is mainly composed of belt-like nanostructures, which are 100–200 nm in width and 0.5–1.0 μm in length; moreover, the Nb2O5 nanobelts have rough surfaces and they are stacked by edge-byedge “oriented attachment” of nanorods. Similar to the FESEM result, the TEM image (Fig. 2b) indicates that the Nb2O5 nanobelts are constructed from parallel nanorods. The primary Nb2O5 nanorods have an average diameter of ~15 nm, which is in good agreement with the thickness of the Nb2O5 nanobelts (arrowed in Fig. 2a) and accordingly further proves the above “oriented attachment” mechanism. In the
The phase identification of the sample is carried out by X-ray diffractometer (XRD, Rigaku D/max-2500, Cu Kα). The morphology and nanostructure of the sample are performed with field-emission scanning electron microscopy (FESEM, SU8010), transmission electron microscopy (TEM, JEM-2100F), and high-resolution TEM (HRTEM, JEM2100F). The Brunauer-Emmett-Teller (BET) specific surface area of the sample is tested by measuring the N2 adsorption-desorption isotherm on a Quantachrome Autosorb-IQ gas adsorption analyzer.
2.3. Electrochemical measurements The Nb2O5 electrode is fabricated as follow. Nb2O5 nanobelts, acetylene black, and polyvinylidene fluoride (70:20:10 wt%) are dispersed into N-methyl-2-pyrrolidinone. Then, the mixture is pasted onto the Cu foils and the electrode is assembled into coin cell. Lithium is the counter and reference electrode. The electrolyte is 1 mol L1 LiPF6 dissolved into diethyl carbonate-ethylene carbonate (1:1 vol%). Cyclic voltammetry tests are conducted in an Electrochemical Workstation (CHI660D) at a potential window of 1.0–3.0 V. Galvanostatic tests are performed on a LAND-CT2001 system in the voltage range of 1.0–3.0 V (vs. Liþ/Li).
Fig. 2. (a) FESEM image and (b) TEM image of Nb2O5 the nanobelts, and the inset of b is the HRTEM lattice image of a typical Nb2O5 nanobelt originating from the blue square of b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Journal of Physics and Chemistry of Solids 111 (2017) 8–11
Fig. 3. Cyclic voltammogram curves of the Nb2O5 nanobelts at a scan rate of 0.1 mV s1. Fig. 6. Rate performance of the Nb2O5 nanobelts at 1–10 C rates.
3.2. Electrochemical performance Generally, the Liþ insertion and extraction process for the Nb2O5 electrode can be expressed by Nb2O5 þ xLiþ þ xe →LixNb2O5, where x is the mole fraction of the inserted Liþ ions (0 < x < 2), and Nb2O5 has a maximum theoretically capacity of ~200 mAh g1 as x is 2 [22,23]. Fig. 3 shows the cyclic voltammetry (CV) curves of the Nb2O5 electrode in the potential range from 1.0 to 3.0 V (vs. Liþ/Li) at a scan rate of 0.1 mV s1. It is obvious that the CV curve possesses symmetric cathodic/ anodic peaks, suggesting a reversible lithiation and delithiation process; furthermore, the cathodic and anodic peaks are located at 1.60 and 1.85 V, respectively, which is consistent with other Nb2O5-based materials [4,24–26]. In the first and second cycles, the reduction/oxidation peaks almost remain unchanged, suggesting the good reversibility of Listorage properties. In addition, the CV curve has broad cathodic/anodic peaks, indicating that the electrochemical reactions have broad distribution of energy [24]. The Nb2O5 nanobelts are evaluated as anode materials for LIBs and their electrochemical properties are tested by a galvanostatic method. Fig. 4 displays the initial discharge-charge curves of the Nb2O5 electrode at different current densities. It is clearly that, similar to some other literature [19,23,24], the charge/discharge capacities are decreased with the increase of current density. Additionally, as the rate is increased, the charge curves are raised and the discharge curves are dropped, indicating that the polarization is increased. The first discharge capacity of the Nb2O5 electrode is 209.3 mAh g1 at 0.5 C, which is higher than the
Fig. 4. The initial discharge-charge curves of the Nb2O5 nanobelts at different rates from 0.5 to 10 C.
HRTEM lattice image recorded on the blue square (the inset of Fig. 2b), the spacing between the contiguous planes is 0.395 nm, corresponding to the (001) plane of Nb2O5, which suggests that the preferred growth direction of the rod-like Nb2O5 building blocks is the [001] direction [21].
Fig. 5. Cycling performance of the Nb2O5 nanobelts at (a) 0.5 C and (b) 5 C.
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Journal of Physics and Chemistry of Solids 111 (2017) 8–11
maximum theoretical value of Nb2O5 (200 mAh g1). The discharge capacities of the Nb2O5 electrode at various rates from 1 to 5 C are 155.4, 138.6 and 123.3 mAh g1. Especially, when the current density is increased to 10 C, the Nb2O5 electrode still remains a high discharge capacity of 104.2 mAh g1. Fig. 5a presents the cycling behavior of the Nb2O5 electrode in the voltage range of 1.0–3.0 V at a current density of 0.5 C. The electrode has a slow capacity fading and delivers a discharge capacity of 177.4 mAh g1 after 50 cycles. Furthermore, when cycled at a relatively high current density of 5 C, the final discharge capacity of the Nb2O5 electrode is 95.8 mAh g1 even over 200 cycles (Fig. 5b). The rate capability of the Nb2O5 electrode is shown in Fig. 6. The electrode is cycled at various current densities from 1 to 10 C and back to 5 and 2 C. It can be seen that a discharge capacity of 147.9 mAh g1 is achieved after 10 cycles at 1 C. Then, this value has a small decline with increasing C-rate. That is, when the rate is increased to 2, 5, and 10 C, the discharge capacity is slowly reduced to 139.0, 122.5, and 108.8 mAh g1, respectively. What is more, after cycled at 10 C, the discharge capacity of Nb2O5 can be increased to 123.0 and 134.2 mAh g1 as the rate is returned back to 5 and 2 C, respectively. Hence, the Nb2O5 electrode possesses good rate performance. The above electrochemical tests demonstrate that the Nb2O5 nanobelts electrode possesses good lithium storage performance (e.g., high reversible capacity and good rate performance), which can be attributed to the following reasons. The nanobelts have good dispersity and high surface areas, so they can supply plenty of active sites for Li-ion storage and large areas for the interaction between Nb2O5 and electrolyte, endowing them with high capacity [27]. On the other hand, the thin thickness of the 1D nanobelts provides a short Liþ ions diffusion path, which is beneficial for the fast transport of Liþ ions in the insertion and extraction process and leads to excellent rate capacity [28].
[4] A.L. Viet, M.V. Reddy, R. Jose, B.V.R. Chowdari, S. Ramakrishna, Nanostructured Nb2O5 polymorphs by electrospinning for rechargeable lithium batteries, J. Phys. Chem. C 114 (2010) 664–671. [5] L. Yan, X. Rui, G. Chen, W. Xu, G. Zou, H. Luo, Recent advances in nanostructured Nb-based oxides for electrochemical energy storage, Nanoscale 8 (2016) 8443–8465. [6] T.F. Yi, S.Y. Yang, Y. Xie, Recent advances of Li4Ti5O12 as a promising next generation anode material for high power lithium-ion batteries, J. Mater. Chem. A 3 (2015) 5750–5777. [7] K.J. Griffith, A.C. Forse, J.M. Griffin, C.P. Grey, High-rate intercalation without nanostructuring in metastable Nb2O5 bronze phases, J. Am. Chem. Soc. 138 (2016) 8888–8899. [8] N. Kumagai, K. Tanno, T. Nakajima, N. Watanabe, Structural changes of Nb2O5 and V2O5 as rechargeable cathodes for lithium battery, Electrochim. Acta 28 (1983) 17–22. [9] K. Kim, M.S. Kim, P.R. Cha, S.H. Kang, J.H. Kim, Structural modification of selforganized nanoporous niobium oxide via hydrogen treatment, Chem. Mater 28 (2016) 1453–1461. [10] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682–2699. [11] X. Liu, G. Liu, L. Wan, Y. Li, Y. Ma, J. Ma, Morphology- and facet-controlled synthesis of CuO micro/nanomaterials and analysis of their lithium ion storage properties, J. Power Sources 312 (2016) 199–206. [12] H. Huang, Z. Yu, W. Zhu, Y. Gan, Y. Xia, X. Tao, W. Zhang, Hierarchically porous nanoflowers from TiO2-B nanosheets with ultrahigh surface area for advanced lithium-ion batteries, J. Phys. Chem. Solids 75 (2014) 619–623. [13] X.B. Zhong, B. Jin, Z.Z. Yang, C. Wang, H.Y. Wang, Facile shape design and fabrication of ZnFe2O4 as an anode material for Li-ion batteries, RSC Adv. 4 (2014) 55173–55178. [14] M. Liu, C. Yan, Y. Zhang, Fabrication of Nb2O5 nanosheets for high-rate lithium ion storage applications, Sci. Rep. 5 (2015) 8326. [15] H. Wen, Z. Liu, J. Wang, Q. Yang, Y. Li, J. Yu, Facile synthesis of Nb2O5 nanorod array films and their electrochemical properties, Appl. Surf. Sci. 257 (2011) 10084–10088. [16] M. Sasidharan, N. Gunawardhana, M. Yoshio, K. Nakashim, Nb2O5 hollow nanospheres as anode material for enhanced performance in lithium ion batteries, Mater. Res. Bull. 47 (2012) 2161–2164. [17] J. Wang, J. Liu, H. Xu, S. Ji, J. Wang, Y. Zhou, P. Hodgson, Y. Li, Gram-scale and template-free synthesis of ultralong tin disulfide nanobelts and their lithium ion storage performances, J. Mater. Chem. A 1 (2013) 1117–1122. [18] J. Liu, D. Xue, Nano structures via chemistry, Nanosci. Nanotechnol. Lett. 3 (2011) 337–364. [19] M. Wei, K. Wei, M. Ichihara, H. Zhou, Nb2O5 nanobelts: a lithium intercalation host with large capacity and high rate capability, Electrochem. Commun. 10 (2008) 980–983. [20] X. Liu, X. Duan, P. Peng, W. Zheng, Hydrothermal synthesis of copper selenides with controllable phases and morphologies from an ionic liquid precursor, Nanoscale 3 (2011) 5090–5095. [21] J. He, Y. Hu, Z. Wang, W. Lu, S. Yang, G. Wu, Y. Wang, S. Wang, H. Gu, J. Wang, Hydrothermal growth and optical properties of Nb2O5 nanorod arrays, J. Mater. Chem. C 2 (2014) 8185–8190. [22] S. Guo, X. Zhang, Z. Zhou, G. Gao, L. Liu, Facile preparation of hierarchical Nb2O5 microspheres with photocatalytic activities and electrochemical properties, J. Mater. Chem. A 2 (2014) 9236–9243. [23] M. Lübke, A. Sumboja, I.D. Johnson, D.J.L. Brett, P.R. Shearing, Z. Liu, J.A. Darr, High power nano-Nb2O5 negative electrodes for lithium-ion batteries, Electrochim. Acta 192 (2016) 363–369. [24] X. Wang, G. Li, R. Tjandra, X. Fan, X. Xiao, A. Yu, Fast lithium-ion storage of Nb2O5 nanocrystals in situ grown on carbon nanotubes for high performance asymmetric supercapacitors, RSC Adv. 5 (2015) 41179–41185. [25] S. Li, Q. Xu, E. Uchaker, X. Cao, G. Cao, Comparison of amorphous, pseudohexagonal and orthorhombic Nb2O5 for high-rate lithium ion insertion, Cryst. Eng. Comm. 18 (2016) 2532–2540. [26] G. Liu, B. Jin, K. Bao, H. Xie, J. Guo, X. Ji, R. Zhang, Q. Jiang, Facile synthesis of porous Nb2O5 microspheres as anodes for lithium-ion batteries, Int. J. Hydrogen Energy 42 (2017) 6065–6071. [27] E. Lim, C. Jo, H. Kim, M.H. Kim, Y. Mun, J. Chun, Y. Ye, J. Hwang, K.S. Ha, K.C. Roh, K. Kang, S. Yoon, J. Lee, Facile synthesis of Nb2O5@carbon core-shell nanocrystals with controlled crystalline structure for high-power anodes in hybrid supercapacitors, ACS Nano 9 (2015) 7497–7505. [28] G. Liu, X. Liu, L. Wang, J. Ma, H. Xie, X. Ji, J. Guo, R. Zhang, Hierarchical Li4Ti5O12TiO2 microspheres assembled from nanoflakes with exposed Li4Ti5O12 (011) and anatase TiO2 (001) facets for high-performance lithium-ion batteries, Electrochim. Acta 222 (2016) 1103–1111.
4. Conclusions In conclusion, Nb2O5 nanobelts composed of nanorods have been synthesized by a facile, novel, and controllable solvothermal method. The Nb2O5 nanobelts have good monodispersity, large surface areas, and thin thickness, resulting in good electrochemical properties, including high reversible capacity and good rate performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21501101); the Program for Science & Technology Innovation Talents in University of Henan Province (No. 15HASTIT007); and the Natural Science Foundation of Henan Department of Education (No. 15A150019). References [1] T.F. Yi, L.J. Jiang, J. Shu, C.B. Yue, R.S. Zhu, H.B. Qiao, Recent development and application of Li4Ti5O12 as anode material of lithium ion battery, J. Phys. Chem. Solids 71 (2010) 1236–1242. [2] R. Kodama, Y. Terada, I. Nakai, S. Komaba, N. Kumagai, Electrochemical and in situ XAFS-XRD investigation of Nb2O5 for rechargeable lithium batteries, J. Electrochem. Soc. 153 (2006) A583–A588. [3] P. Arunkumar, A.G. Ashish, B. Babu, S. Sarang, A. Suresh, C.H. Sharma, M. Thalakulam, M.M. Shaijumon, Nb2O5/graphene nanocomposites for electrochemical energy storage, RSC Adv. 5 (2015) 59997–60004.
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Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Facile synthesis of Nb2O5 nanobelts assembled from nanorods and their applications in lithium ion batteries Xiaodi Liu a, b, Guangyin Liu b, *, Hao Chen b, Jianmin Ma a, **, Ruixue Zhang b a b
College of Physics and Electronics, Hunan University, Changsha 410022, China College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Electrochemical properties
Hierarchical 1D Nb2O5 nanobelts are successfully synthesized via a facile solvothermal method and following thermal treatment. The as-formed Nb2O5 nanobelts are characterized by XRD, FESEM, TEM, and BET, and the results indicate that they possess pseudohexagonal structure and are composed of ultranarrow nanorods with an average diameter of ca. 15 nm. When used as anodic materials for lithium ion batteries, the obtained Nb2O5 nanobelts can deliver initial discharge capacities of 209.3 mAh g1 at the current density of 0.5 C. In addition, the Nb2O5 nanobelts exhibit a reversible capacity of 95.8 mAh g1 after 200 cycles at relatively high current density of 5 C. The good electrochemical performance of the Nb2O5 nanobelts may be ascribed to their good monodispersity, high specific surface areas, and narrow rod-like building blocks. The Nb2O5 nanobelts can be developed as promising anodes for high-rate 2 V LIBs with good safety.
1. Introduction Lithium ion batteries (LIBs), as a fast-developing technology in electric energy storage, have made considerable contribution to several fields [1]. Most recently, with the development of micro/nanoelectronic devices, tremendous attention has been paid to the exploration of small, safe, and powerful LIBs with low operating voltage (2 V vs. Liþ/Li) [2]. Nb2O5 is considered as an appealing anode material for 2 V LIBs owing to its high valence state and good structural stability [3,4]. Moreover, similar to Li4Ti5O12, Nb2O5 possesses excellent safety advantages due to its appropriate operational voltage plateau (1.0–2.0 V vs. Liþ/Li), which can prevent the growth of lithium dendrites after long charge-discharge process and suppress the formation of SEI layers [5–7]. More importantly, compared with Li4Ti5O12 (175 mAh g1), Nb2O5 has a higher theoretical capacity of 200 mAh g1 [2]. So, Nb2O5 has attracted increasing attention in the fields of LIBs, especially in 2 V LIBs [8,9]. Recently, it has been demonstrated that the constructing of nanostructured anode materials can be used to reduce the diffusion length of Liþ ion, leading to improved electrochemical performance [10]. Furthermore, it is known that the electrochemical properties of nanoscale electrodes are closed related to their sizes and morphologies [11–13]. Thus, up to date, several Nb2O5 nanomaterials with various morphologies, including nanorods, hollow nanospheres, and nanosheets, have * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Liu), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.jpcs.2017.07.007 Received 10 April 2017; Received in revised form 3 July 2017; Accepted 10 July 2017 Available online 13 July 2017 0022-3697/© 2017 Elsevier Ltd. All rights reserved.
been successfully prepared to enhance their electrochemical properties [14–16]. Most especially, 1D nanostructured electrodes have achieved great interest because of their high surface areas, enhanced kinetic, and improved electrochemical properties [17–19]. In these regards, it is imperative to probe novel and effective methods of preparing 1D Nb2O5 nanomaterials with special morphologies and excellent properties. Herein, we report a novel and simple solvothermal route and following thermal treatment to synthesize 1D Nb2O5 nanobelts. The Nb2O5 nanobelts are assembled by “oriented attachment” of ultranarrow nanorods. The Li-ion storage performance of the as-formed Nb2O5 nanobelts is researched, and the results indicate that the Nb2O5 electrode possesses good electrochemical properties, including high reversible capacity and good rate performance. 2. Material and methods 2.1. Synthesis of Nb2O5 nanobelts In a typical synthesis, 1 mmol Nb(HC2O4)5 is added into 30 mL isopropanol, and then the mixture is stirred for 2 h and transferred into a Teflon-lined autoclave (50 mL). The autoclave is maintained at 180 C for 48 h. Subsequently, the powder is washed and dried in a vacuum oven at 80 C. Finally, the precursors are placed in a muffle furnace and calcined
X. Liu et al.
Journal of Physics and Chemistry of Solids 111 (2017) 8–11
Fig. 1. (a) XRD pattern and (b) Nitrogen adsorption-desorption isotherm of the Nb2O5 nanobelts.
at 600 C for 2 h to generate Nb2O5.
3. Results and discussion 3.1. Characterization of Nb2O5 nanobelts
2.2. Characterizations
The crystal structure of the as-synthesized Nb2O5 nanobelts is researched by XRD (Fig. 1a). The positions of all peaks are in good agreement with the reported data of Nb2O5 with a pseudohexagonal structure (JCPDS No. 07-0061, space group: P6/mmm). The strong peaks indicate the high crystalline nature of the sample; moreover, in comparison with the standard XRD pattern of Nb2O5, the (001) peak appears as the strongest one instead of the (100) peak, suggesting that the obtained Nb2O5 is oriented-growth [20]. Fig. 1b depicts the N2 adsorption and desorption isotherm of the sample, and it is found that as-formed Nb2O5 nanobelts have a BET surface area of ca. 28.1 m2 g1. The morphologies and nanostructures of the Nb2O5 nanobelts are observed by FESEM, TEM, and HRTEM. As shown in the FESEM image (Fig. 2a), the sample is mainly composed of belt-like nanostructures, which are 100–200 nm in width and 0.5–1.0 μm in length; moreover, the Nb2O5 nanobelts have rough surfaces and they are stacked by edge-byedge “oriented attachment” of nanorods. Similar to the FESEM result, the TEM image (Fig. 2b) indicates that the Nb2O5 nanobelts are constructed from parallel nanorods. The primary Nb2O5 nanorods have an average diameter of ~15 nm, which is in good agreement with the thickness of the Nb2O5 nanobelts (arrowed in Fig. 2a) and accordingly further proves the above “oriented attachment” mechanism. In the
The phase identification of the sample is carried out by X-ray diffractometer (XRD, Rigaku D/max-2500, Cu Kα). The morphology and nanostructure of the sample are performed with field-emission scanning electron microscopy (FESEM, SU8010), transmission electron microscopy (TEM, JEM-2100F), and high-resolution TEM (HRTEM, JEM2100F). The Brunauer-Emmett-Teller (BET) specific surface area of the sample is tested by measuring the N2 adsorption-desorption isotherm on a Quantachrome Autosorb-IQ gas adsorption analyzer.
2.3. Electrochemical measurements The Nb2O5 electrode is fabricated as follow. Nb2O5 nanobelts, acetylene black, and polyvinylidene fluoride (70:20:10 wt%) are dispersed into N-methyl-2-pyrrolidinone. Then, the mixture is pasted onto the Cu foils and the electrode is assembled into coin cell. Lithium is the counter and reference electrode. The electrolyte is 1 mol L1 LiPF6 dissolved into diethyl carbonate-ethylene carbonate (1:1 vol%). Cyclic voltammetry tests are conducted in an Electrochemical Workstation (CHI660D) at a potential window of 1.0–3.0 V. Galvanostatic tests are performed on a LAND-CT2001 system in the voltage range of 1.0–3.0 V (vs. Liþ/Li).
Fig. 2. (a) FESEM image and (b) TEM image of Nb2O5 the nanobelts, and the inset of b is the HRTEM lattice image of a typical Nb2O5 nanobelt originating from the blue square of b. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Journal of Physics and Chemistry of Solids 111 (2017) 8–11
Fig. 3. Cyclic voltammogram curves of the Nb2O5 nanobelts at a scan rate of 0.1 mV s1. Fig. 6. Rate performance of the Nb2O5 nanobelts at 1–10 C rates.
3.2. Electrochemical performance Generally, the Liþ insertion and extraction process for the Nb2O5 electrode can be expressed by Nb2O5 þ xLiþ þ xe →LixNb2O5, where x is the mole fraction of the inserted Liþ ions (0 < x < 2), and Nb2O5 has a maximum theoretically capacity of ~200 mAh g1 as x is 2 [22,23]. Fig. 3 shows the cyclic voltammetry (CV) curves of the Nb2O5 electrode in the potential range from 1.0 to 3.0 V (vs. Liþ/Li) at a scan rate of 0.1 mV s1. It is obvious that the CV curve possesses symmetric cathodic/ anodic peaks, suggesting a reversible lithiation and delithiation process; furthermore, the cathodic and anodic peaks are located at 1.60 and 1.85 V, respectively, which is consistent with other Nb2O5-based materials [4,24–26]. In the first and second cycles, the reduction/oxidation peaks almost remain unchanged, suggesting the good reversibility of Listorage properties. In addition, the CV curve has broad cathodic/anodic peaks, indicating that the electrochemical reactions have broad distribution of energy [24]. The Nb2O5 nanobelts are evaluated as anode materials for LIBs and their electrochemical properties are tested by a galvanostatic method. Fig. 4 displays the initial discharge-charge curves of the Nb2O5 electrode at different current densities. It is clearly that, similar to some other literature [19,23,24], the charge/discharge capacities are decreased with the increase of current density. Additionally, as the rate is increased, the charge curves are raised and the discharge curves are dropped, indicating that the polarization is increased. The first discharge capacity of the Nb2O5 electrode is 209.3 mAh g1 at 0.5 C, which is higher than the
Fig. 4. The initial discharge-charge curves of the Nb2O5 nanobelts at different rates from 0.5 to 10 C.
HRTEM lattice image recorded on the blue square (the inset of Fig. 2b), the spacing between the contiguous planes is 0.395 nm, corresponding to the (001) plane of Nb2O5, which suggests that the preferred growth direction of the rod-like Nb2O5 building blocks is the [001] direction [21].
Fig. 5. Cycling performance of the Nb2O5 nanobelts at (a) 0.5 C and (b) 5 C.
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maximum theoretical value of Nb2O5 (200 mAh g1). The discharge capacities of the Nb2O5 electrode at various rates from 1 to 5 C are 155.4, 138.6 and 123.3 mAh g1. Especially, when the current density is increased to 10 C, the Nb2O5 electrode still remains a high discharge capacity of 104.2 mAh g1. Fig. 5a presents the cycling behavior of the Nb2O5 electrode in the voltage range of 1.0–3.0 V at a current density of 0.5 C. The electrode has a slow capacity fading and delivers a discharge capacity of 177.4 mAh g1 after 50 cycles. Furthermore, when cycled at a relatively high current density of 5 C, the final discharge capacity of the Nb2O5 electrode is 95.8 mAh g1 even over 200 cycles (Fig. 5b). The rate capability of the Nb2O5 electrode is shown in Fig. 6. The electrode is cycled at various current densities from 1 to 10 C and back to 5 and 2 C. It can be seen that a discharge capacity of 147.9 mAh g1 is achieved after 10 cycles at 1 C. Then, this value has a small decline with increasing C-rate. That is, when the rate is increased to 2, 5, and 10 C, the discharge capacity is slowly reduced to 139.0, 122.5, and 108.8 mAh g1, respectively. What is more, after cycled at 10 C, the discharge capacity of Nb2O5 can be increased to 123.0 and 134.2 mAh g1 as the rate is returned back to 5 and 2 C, respectively. Hence, the Nb2O5 electrode possesses good rate performance. The above electrochemical tests demonstrate that the Nb2O5 nanobelts electrode possesses good lithium storage performance (e.g., high reversible capacity and good rate performance), which can be attributed to the following reasons. The nanobelts have good dispersity and high surface areas, so they can supply plenty of active sites for Li-ion storage and large areas for the interaction between Nb2O5 and electrolyte, endowing them with high capacity [27]. On the other hand, the thin thickness of the 1D nanobelts provides a short Liþ ions diffusion path, which is beneficial for the fast transport of Liþ ions in the insertion and extraction process and leads to excellent rate capacity [28].
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4. Conclusions In conclusion, Nb2O5 nanobelts composed of nanorods have been synthesized by a facile, novel, and controllable solvothermal method. The Nb2O5 nanobelts have good monodispersity, large surface areas, and thin thickness, resulting in good electrochemical properties, including high reversible capacity and good rate performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21501101); the Program for Science & Technology Innovation Talents in University of Henan Province (No. 15HASTIT007); and the Natural Science Foundation of Henan Department of Education (No. 15A150019). References [1] T.F. Yi, L.J. Jiang, J. Shu, C.B. Yue, R.S. Zhu, H.B. Qiao, Recent development and application of Li4Ti5O12 as anode material of lithium ion battery, J. Phys. Chem. Solids 71 (2010) 1236–1242. [2] R. Kodama, Y. Terada, I. Nakai, S. Komaba, N. Kumagai, Electrochemical and in situ XAFS-XRD investigation of Nb2O5 for rechargeable lithium batteries, J. Electrochem. Soc. 153 (2006) A583–A588. [3] P. Arunkumar, A.G. Ashish, B. Babu, S. Sarang, A. Suresh, C.H. Sharma, M. Thalakulam, M.M. Shaijumon, Nb2O5/graphene nanocomposites for electrochemical energy storage, RSC Adv. 5 (2015) 59997–60004.
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