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Rutile TiO2 Nanobundles on Reduced Graphene Oxides as Anode Materials for Li Ion Batteries
DOI: 10.1039/x0xx00000x www.rsc.org/
Mengmeng Zhen,a Xuejing Guo,a Guandao Gao,a Lu Liu,*a Zhen Zhou*b
A simple and steerable method was adopted to synthesize well-distributed rutile TiO2 nanobundles on reduced graphene oxides through two-step hydrothermal methods. The rutile TiO2/RGO composites was used as the anode of lithium ion batteries for investigation that had an original morphology and a reversible capacity of 300 mAh g-1 at 0.6 C and 200 mAh g-1 at 1.2 C after 500 cycles. Lithium ion batteries (LIBs) have gained tremendous interest for portable electronics, electric vehicles and smart grids because of their high capacity, long lifespan and environmental friendliness.1-3 Especially, nanostructured TiO2 has been attracting much attention as LIB anodes,4,5 and the reaction is as follows: TiO2+xLi++xe- ↔ LixTiO2 (0 ≤x ≤0.5), during the reaction, Li+ ions insert/extract in the TiO2 matrix, accompaning with electron transfer between Ti(III) and Ti(IV).6,7 The reversible process provides a considerable capacity of 167.5 mAh g-1 with only a small volume variation (< 4%), which is critical for the high rate capability and long-life cycling.8 However, TiO2 anode nanomaterials are severely hindered by their agglomeration and dissolution during charge/discharge cycles, thus leads to the decrease of electroactive sites and the fade of specific capacities.9 In order to solve this problem, the addition of conductive phases such as carbon, polymers, and metals is always necessary. Compared with conventional conductive additives,10 graphene, with ultrathin thickness, superior electronic conductivity, chemical stability, structural flexibility and high surface area, is expected to be an excellent substrate for anodes.9,11-15 As a typical example, Wang and co-workers prepared rutile TiO2 nanorodgraphene and anatase TiO2 spherical nanoparticle graphene hybrids, which displayed good capacity retention close to 180-160 mAh g-1 over 100 cycles at 1 C rate (167.5 mAh g-1).16 Other attempts mainly focused on hydrothermal/solvothermal processes; however, the asprepared TiO2/graphene composites demonstrated poor electrochemical performance for lithium storage owing to the unsatisfactory distribution of TiO2 nanocrystals on graphene nanosheets.17-19 Up to date, anatase TiO2/RGO composites have been widely investigated for Li storage. Rutile TiO2/RGO composites could also be used as anode materials. However, synthesis of highly crystalline rutile TiO2/RGO composites with high Li storage capacity and good stability has met with limited success so far. In this work, a simple and steerable method was adopted to synthesize well-distributed rutile TiO2 nanobundles growing on RGO through two-step hydrothermal methods. Compared with the reported hydrothermal/solvothermal routes, the current preparation
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was performed without high-temperature calcination and any surfactants addition. More significantly, the obtained rutile TiO2/RGO composites possessed highly crystalized bundle morphology, excellent high-rate performance and ultralong cycle life. This work is favorable to explore advanced TiO2-based composites as anodes for LIBs with high power density. /Insert Scheme 1/ The preparation of TiO2 nanobundles on RGO nanosheets can be schematically illustrated in Scheme 1. GO nanosheets were synthesized by a modified Hummers’ method first. Atomic force microscopy (AFM) displayed the 2D features of GO nanosheets with a thickness of 0.7-1.1 nm (Figure S1), corresponding to 2-3 layers GO nanosheets. These GO nanosheets were dispersed in the HCl/Li2SO4•H2O/tetrabutyl titanate (TBT) mixture as the ideal substrate for the growth of TiO2 nanobundles during the hydrothermal process (150 °C for 6 h). After the addition of ascorbic acid (AA) as an effective reduction reagent, the solution was kept at 150 °C for another 4 h, during which GO nanosheets were reduced to RGO. /Insert Figure 1/ The X-ray diffraction (XRD) patterns of the samples are shown in Figure 1. The characteristic peak (~10°) of GO can be obviously observed in the XRD patterns, corresponding to the (001) plane of GO. After the hydrothermal reduction, this peak entirely disappears and a broad peak at ~25° appears instead, indicating that –OH, C=O and –COOH groups have been removed from GO to a great extent.7 All the peaks of TiO2/RGO and pure TiO2 can be well assigned to the standard card (JCPDS 4-551) of rutile TiO2. All TiO2 crystal structures consist of TiO62- octahedral. Because rutile octahedra shows linear chains parallel to [001] while anatase octahedra arrang in zigzag chains along [221], the distribution of the third octahedron significantly determines the polymorphs.20 The importance of solution pH and supporting anions in the phase selective crystallization of TiO2 has been demonstrated.21,22 Cl- has a weak steric effect because of its small radius, which lead to the octahedron with Cl- and another octahedron would combine along the straight direction, and the orientation of the third octahedron promote the formation of a rutile nucleus. Meanwhile, the existence of a large amount of Cl- under highly acidic conditions generally promote the formation of rutile crystallites.21 In our preparation, the experimental system contained high hydrochloric acid concentration which led to the preferential formation rutile phase crystallization. The Figure S3 shows the obtained products have formed rutile phase TiO2 after 2h of solvothermal reaction, further confirming the experimental system can rapidly promote the formation of rutile phase TiO2.
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/Insert Figure 2/ The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the TiO2/RGO composites are shown in Figures 2. The samples display bundle-like structure with uniform morphology and dimension, growing directly on RGO nanosheets. The high-resolution TEM (HRTEM) image further confirms the morphology and crystal characteristics of the rutile TiO2 crystals in the TiO2/RGO composites (Figure 2e and f).The crystalline lattice distance is 3.25 Å, corresponding to the (110) planes of rutile TiO2 which is consistent with the XRD results. /Insert Figure 3/ To probe the electrochemical performances of TiO2/RGO composites and pure TiO2, the samples were assembled as electrodes for LIBs. These electrodes showed the plateau at ~1.1 V of the first discharge curve and appeared as a sloping region at the second discharge curve, which is typical for rutile electrodes.23 The initial discharge/charge capacities of pure TiO2 were 247/171 mAh g-1 at 1.2 C and rapidly deceased to only 134 mAh g-1 after 100 cycles (Figure S4b). By sharp contrast, TiO2/RGO composites exhibited a higher discharge/charge capacity of 436/237 mAh g-1 at the initial cycle (Figure 3b). The irreversible capacity can be attributed to the formation of solid-electrolyte interface (SEI) films, which can be confirmed by electrochemical impedance spectroscopy (EIS) (see details below). More importantly, the TiO2/RGO composites exhibited a higher capacity of 200 mAh g-1 at 1.2 C even after 500 cycles. (Figure 3c and d). This stabilized capacity was much higher than that of pure TiO2 (Figure S4c and d). Take the thesis of Wang et al.24 as a typical example of recent progress, anatase TiO2 nanosheet/reduced graphene oxide composite was synthesized through hydrothermal reaction and the product exhibited relatively lower capacity and weaker cycling stability (180 mAh g-1 at 1 C after 200 cycles). Compared with previous work, our rutile TiO2/RGO exhibits a highly crystalline bundle morphology, higher rate performance and ultralong cycle life for lithium storage. /Insert Figure 4/ Profiting from the favorable features of the superior hybrid nanostructures, TiO2/RGO composites also exhibited excellent performances at high rates. When cycled at 0.6 C, 1.2 C, 3 C, 6 C and 12 C, the reversible capacities were 292, 190, 133, 101 and 78 mAh g-1, respectively (Figure 4a). More importantly, no obvious capacity loss happened when the rate reduced to 3 C and 0.6 C, indicating very good reversibility. These performances were superior to other TiO2-graphene composites prepared through the hydrothermal/solvothermal route.13,19,25 As a typical example, Lee et al. synthesized TiO2 nanorod arrays on RGO as LIB anodes, and the capacity was only 94 mAh g-1 at 5 C.19 To better understand the high-rate electrochemical behaviors of TiO2/RGO composites, cyclic voltammetry (CV) was performed between 0 and 3.0 V at different scanning rates from 0.5 to 5 mV s-1 (Figure S5). After the initial 4 cycles, a pair of redox peaks can be observed at 1.75 and 1.95 V at the low scanning rate of 0.5 mV s-1. Only one pair of distinct cathodic (insertion)/anodic (extraction) peaks can be observed at all scan rates.26 Contrastly, the voltage hysteresis became serious at higher scanning rates owing to the dramatically increasing polarization. The linear relationship between the peak current and scanning rate (the inset in Figure S5) suggests obvious surface lithium storage, i.e., the faradic pseudocapacitance effect, which occurs on the interface between active particles and electrolytes.8 To gain insight into the influence of RGO nanosheets, we conducted EIS on the cells comprising the TiO2/RGO composites and pure TiO2 as the working electrodes (Figure 4b). The samples show only one semicircle in the high-frequency range and an inclined line in the low-frequency range. The high frequency
2 | J. Name., 2014, 00, 1-3
Journal Name DOI: 10.1039/C4CC05480F semicircle is assigned to the charge-transfer impedance in the electrode/electrolyte interface, and the inclining line corresponds to the Li+ diffusion process. However, the EIS profile displays two semicircles after the first lithiation (iv); the high frequency semicircle is attributed to the formation of SEI film, which mainly grows in the initial discharge process.8 After 3 cycles, the EIS profile (v) of TiO2/RGO nanocomposites exhibit a smaller semicircle in the high-frequency range and an inclining line with a larger slope than that (ii) of pure TiO2 and the corresponding equivalent circuit mode is shown in the inset. In this equivalent circuit, Rs and Rct represent electrolyte resistance and charge-transfer resistance respectively,27 the two indexes were calculated by the equivalent circuit in Table 1. Rs values of two samples show little distinction, due to the usage of identical electrolyte system. Rct value of TiO2/RGO nanocomposites is only 43.09 Ω lower than that of the pure TiO2 (119.53 Ω). Moreover, the Li-ion diffusion behavior within the solid-state electrode materials can be represented by the linear Warburg regions at low frequency (from 0.2 to 0.01 Hz) in Nyquist plots.28 And the formula of Li+ diffusion coefficient is shown as followed (eq 1):
(1) In the equation, Vm, F, A and σw represent the molar volume of rutile TiO2, Faraday constant, total contact surface area between the electrolyte and the electrode, and Warburg prefactor obtaining from the Warburg region of impedance response, respectively.28 The Li+ diffusion coefficient of TiO2/RGO composites (ca. 8.03×10-15 cm2·s1 ) is 2 times of pure TiO2 (ca. 3.65×10-15 cm2·s−1). Compared with pure TiO2, TiO2/RGO composites possess smaller Rct value and higher Li+ diffusion coefficient, indicating that the incorporation of TiO2 and RGO nanosheets can effectively improve the electron transport and Li+ diffusivity, and leads to higher discharge/charge capacities of TiO2/RGO composites than pure TiO2. Table 1. Physicochemical Properties Measured and Calculated from EIS Spectra (in Figure 4b) samples TiO2/RGO Pure TiO2
DLi (EIS) (cm2·s-1) 8.03×10-15 cm2·s-1 3.65×10-15 cm2·s-1
Rs (Ω) 3.34 4.15
Rct (Ω) 43.09 119.53
The excellent performance of TiO2/RGO composites can be attributed to several possible reasons. First, in nanometer-sized rutile TiO2, more than 0.5 Li+ per TiO2 can be inserted/extracted in the discharge/charge process and the electrochemical behavior of rutile TiO2 was greatly dependent on the particle size.29-32 It is commonly agreed that Li diffusion in rutile TiO2 is highly anisotropic, which proceeds through rapid diffusion along c-axis [001] channels. More importantly, HRTEM shows that the TiO2 nanocrystal growing on RGO was along the [110] plane (Figure 2f), and the building units (the width and length are smaller than 10 nm) of nanobundles shorten the Li+ ion diffusion length in the nanocrystals which could accelerate Li+ insertion and extraction.31 Second, some studies proposed that SEI films contain various inorganic/organic compounds,34,35 which can participate in the reaction to form various compounds under suitable potentials and contribute to much higher capacity than that of the counterpart oxides. In addition, graphene nanosheets have significant disorder/defects, this also contributes to the extra lithium storage capacity.36 Third, during the cycle, the crystallized TiO2 maybe change increasingly to be amorphous which makes more and more accessible active sites available for Li+ insertion.36 In addition, anchoring TiO2 nanobundles onto RGO nanosheets minimizes TiO2 agglomeration and RGO restacking,
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which helps keep high stability and large surface area during lithium insertion/extraction.37 In summary, we adopted two-step hydrothermal methods without using any surfactants and high-temperature calcination to fabricate rutile TiO2 nanobundles on RGO with desired dimension and morphology. Different from previous hydrothermal/solvothermal products, the obtained rutile TiO2/RGO composites possessed of a highly crystalline bundle of morphology, higher rate performance and ultralong cycle life for lithium storage. This work enlightens us on preparing nanobundles on graphene nanosheets for energy storage. This work was supported by NSFC (21271108), China US Center for Environmental Remediation and Sustainable Development, NSFC (21273118), Tianjin Municipal Science and Technology Commission (11JCZDJC24800) and MOE Innovation Team (IRT13022) in China.
Notes and references a
Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P.R. China. b Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, P.R. China. *Corresponding author. E-mail: [email protected] E-mail: [email protected] Electronic Supplementary Information (ESI) available: [Experimental details, AFM, SEM, XRD CV and discharge/charge profiles]. See DOI: 10.1039/c000000x/ 1. P. Meduri, E. Clark, J. H. Kim, E. Dayalan, G. U. Sumanasekera and M. K. Sunkara, Nano Lett., 2012, 12, 1784-1788. 2. B. Scrosati, Nature, 1995, 373, 557-558. 3. M. Ling, J. X. Qiu, S. Li, H. Zhao, G. Liu, and S. Q. Zhang, J. Mater. Chem. A, 2013, 1, 11543-11547. 4. G. Armstrong, A. R. Armstrong, J. Canales and P. G. Bruce, Chem. Commun., 2005, 41, 2454-2456. 5. J. S. Chen and X. W. Lou, J. Power Sources, 2010, 195, 2905-2908. 6. L. Kavan, M. Kalbac, M. Zukalová, I. Exnar, V. Lorenzen, R. Nesper and M. Graetzel, Chem. Mater., 2004, 16, 477-485. 7. J. S. Chen, H. Liu, S. Z. Qiao and X. W. Lou, J. Mater. Chem., 2011, 21, 5687-5692. 8. M. M. Zhen, L. W. Su, Z. H. Yuan, L. Liu and Z. Zhou, RSC Adv., 2013, 3, 13696-13701. 9. H. Kim, M. Kim, T. Shin, H. Shin and J. Cho, Electrochem. Commun., 2008, 10, 1669-1672. 10. P. M. Dziewonski and M. Grzeszczuk, Electrochim.Acta, 2010, 55, 3336-3347. 11. J. X. Qiu, P. Zhang, M. Ling, S. Li, P .R. Liu, H. J. Zhao and S. Q. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 3636-3642. 12. V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach, Energy Environ. Sci., 2011, 4, 3243-3262.
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COMMUNICATION DOI: 10.1039/C4CC05480F 13. D. H. Wang, D. W. Choi, J. Li, Z. G. Yang, Z. M. Nie, R. Kou, D. H. Hu, C. M. Wang, L. V. Saraf, J. G. Zhang, I. A. Aksay and J. Liu, ACS Nano, 2009, 3, 907-914. 14. N. Li, G. Liu, C. Zhen, F. Li, L. Zhang and H. M. Cheng, Adv. Funct. Mater., 2011, 21, 1717-1722. 15. S. Li, Y. Z. Wang, C. Lai, J. X. Qiu, M. Ling, W. Martens, H. J. Zhao and S. Q. Zhang, J. Mater. Chem. A, 2014, 2, 10211–10217. 16. D. H. Wang, D. W. Choi, Z. G. Yang, V. V. Viswanathan, Z. M. Nie, C. M. Wang, Y. J. Song, J. G. Zhang and J. Liu, Chem. Mater., 2008, 20, 3435-3442. 17. H. Q. Cao, B. J. Li, J. X. Zhang, F. Lian, X. H. Kong, and M. Z. Qu, J. Mater. Chem., 2012, 22, 9759-9766. 18. S. J. Ding, J. S. Chen, D. Luan, F. Y. C. Boey, S. Madhavi and X. W. Lou, Chem. Commun., 2011, 47, 5780-5782. 19. L. F. He, R. G. Ma, N. Du, J. G. Ren, T. L. Wong, Y. Y. Li and S. T. Lee, J. Mater. Chem., 2012, 22, 19061-19066. 20. Y. Li, T. J. White and S. H. Lim, Journal of Solid State Chemistry, 2004, 177 1372-1381. 21. J. G. Li, T. Ishigaki and X. D. Sun, J. Phys. Chem. C, 2007, 111, 4969-4976. 22. M. C. Yan, F. Chen, J. L. Zhang and M. Anpo, J. Phys. Chem. B, 2005, 109, 8673-8678. 23. Z. S. Hong, Y. X. Xu, Y. B. Lui and M. D. Wei, Chem. Eur. J., 2012, 18, 10753-10760. 24. Z. Y. Wang, J. W. Sha, E. Z. Liu, C. N. He, C. S. Shi, J. J. Li and N. Q. Zhao, J. Mater. Chem. A, 2014, 2, 8893–8901. 25. Y. M. Ren, J. Zhang, Y. Y. Liu, H. B. Li, H. J. Wei, B. J. Li and X. Y. Wang, ACS Appl. Mater. Interface, 2012, 4, 4776-4780. 26. S. K. Das, S. Darmakolla and A. J. Bhattacharyya, J. Mater. Chem., 2010, 20, 1600-1606. 27. Y. Wang, H. Liu, K. Wang, H. Eiji, Y. Wang and H. Zhou, J. Mater. Chem., 2009, 19, 6789-6795. 28. J. X. Qiu, S. Li, E. Gray, H. W. Liu, Q. F. Gu, C. H. Sun, C. Lai, H. J. Zhao and S. Q. Zhang, J. Phys. Chem. C, 2014, 118, 8824-8830. 29. Y. S. Hu, L. Kienle, Y. G. Guo and J. Maier, Adv. Mater., 2006, 18, 1421-1426. 30. H. Qiao, Y. W. Wang, L. F. Xiao and L. Z. Zhang, Electrochem. Commun., 2008, 10, 1280-1283. 31. E. Baudrin, S. Cassaignon, M. Koelsch, J. P. Jolivet, L. Dupont and J. M. Tarascon, Electrochem. Commun., 2007, 9, 337-342. 32. M. A. Reddy, M. S. Kishore, V. Pralong, V. Caignaert, U. V. Varadaraju and B. Raveau, Electrochem. Commun., 2006, 8, 12991303. 33. H. L. Wang, Y. Yang, Y. Y. Liang, L. F. Cui, H. S. Casalongue, Y. G. Li, G. S. Hong, Y. Cui and H. J. Dai, Angew. Chem., 2011, 123, 7502-7506. 34. D. Aurbach, J. Power Sources, 2000, 89, 206-218. 35. L. W. Su, Y. R. Zhong and Z. Zhou, J. Mater. Chem. A, 2013, 1, 15158-15166. 36. C. X. Peng, B. D. Chen, Y. Qin, S. H. Yang, C. Z. Li, Y. H. Zuo, S. Y. Liu and J. H. Yang. ACS Nano, 2012, 6, 1074-1081. 37. V. Etacheri, J. E. Yourey and B. M. Bartlett, ACS Nano, 2014, 8, 1491-1499.
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Scheme 1 Schematic illustration of TiO2 nanobundles on RGO nanosheets. 39x33mm (300 x 300 DPI)
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Figure 1. XRD patterns of GO, RGO, pure TiO2 and TiO2/RGO composites. 39x27mm (300 x 300 DPI)
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Figure 2. SEM (a and b), TEM (c and d) and HRTEM (e and f) images of TiO2/RGO composites. The lattice distance in the black region is 3.25 Å. 39x40mm (300 x 300 DPI)
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Figure 3. Discharge/charge profiles of TiO2/RGO composites (a and b) and cycling performance of TiO2/RGO composites (c and d) at 0.6 C and 1.2 C. 39x29mm (300 x 300 DPI)
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Figure 4. Cycling performance (a) of TiO2/RGO composites, Nqyuist plots for the EIS (b) of pure TiO2 and TiO2/RGO composites at different states. (pure TiO2 i: before cycling, ii: after initial 3 cycles; TiO2/RGO iii: before cycling, iv: after 1st discharge, v: after initial 3 cycles). Inset: the corresponding equivalent circuit mode of two samples. 38x18mm (300 x 300 DPI)
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DOI: 10.1039/C4CC05480F
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. M. zhen, X. J. guo, G. Gao, L. Liu and Z. Zhou, Chem. Commun., 2014, DOI: 10.1039/C4CC05480F.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Journal Name
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Cite this: DOI: 10.1039/x0xx00000x
Rutile TiO2 Nanobundles on Reduced Graphene Oxides as Anode Materials for Li Ion Batteries
DOI: 10.1039/x0xx00000x www.rsc.org/
Mengmeng Zhen,a Xuejing Guo,a Guandao Gao,a Lu Liu,*a Zhen Zhou*b
A simple and steerable method was adopted to synthesize well-distributed rutile TiO2 nanobundles on reduced graphene oxides through two-step hydrothermal methods. The rutile TiO2/RGO composites was used as the anode of lithium ion batteries for investigation that had an original morphology and a reversible capacity of 300 mAh g-1 at 0.6 C and 200 mAh g-1 at 1.2 C after 500 cycles. Lithium ion batteries (LIBs) have gained tremendous interest for portable electronics, electric vehicles and smart grids because of their high capacity, long lifespan and environmental friendliness.1-3 Especially, nanostructured TiO2 has been attracting much attention as LIB anodes,4,5 and the reaction is as follows: TiO2+xLi++xe- ↔ LixTiO2 (0 ≤x ≤0.5), during the reaction, Li+ ions insert/extract in the TiO2 matrix, accompaning with electron transfer between Ti(III) and Ti(IV).6,7 The reversible process provides a considerable capacity of 167.5 mAh g-1 with only a small volume variation (< 4%), which is critical for the high rate capability and long-life cycling.8 However, TiO2 anode nanomaterials are severely hindered by their agglomeration and dissolution during charge/discharge cycles, thus leads to the decrease of electroactive sites and the fade of specific capacities.9 In order to solve this problem, the addition of conductive phases such as carbon, polymers, and metals is always necessary. Compared with conventional conductive additives,10 graphene, with ultrathin thickness, superior electronic conductivity, chemical stability, structural flexibility and high surface area, is expected to be an excellent substrate for anodes.9,11-15 As a typical example, Wang and co-workers prepared rutile TiO2 nanorodgraphene and anatase TiO2 spherical nanoparticle graphene hybrids, which displayed good capacity retention close to 180-160 mAh g-1 over 100 cycles at 1 C rate (167.5 mAh g-1).16 Other attempts mainly focused on hydrothermal/solvothermal processes; however, the asprepared TiO2/graphene composites demonstrated poor electrochemical performance for lithium storage owing to the unsatisfactory distribution of TiO2 nanocrystals on graphene nanosheets.17-19 Up to date, anatase TiO2/RGO composites have been widely investigated for Li storage. Rutile TiO2/RGO composites could also be used as anode materials. However, synthesis of highly crystalline rutile TiO2/RGO composites with high Li storage capacity and good stability has met with limited success so far. In this work, a simple and steerable method was adopted to synthesize well-distributed rutile TiO2 nanobundles growing on RGO through two-step hydrothermal methods. Compared with the reported hydrothermal/solvothermal routes, the current preparation
This journal is © The Royal Society of Chemistry 2014
was performed without high-temperature calcination and any surfactants addition. More significantly, the obtained rutile TiO2/RGO composites possessed highly crystalized bundle morphology, excellent high-rate performance and ultralong cycle life. This work is favorable to explore advanced TiO2-based composites as anodes for LIBs with high power density. /Insert Scheme 1/ The preparation of TiO2 nanobundles on RGO nanosheets can be schematically illustrated in Scheme 1. GO nanosheets were synthesized by a modified Hummers’ method first. Atomic force microscopy (AFM) displayed the 2D features of GO nanosheets with a thickness of 0.7-1.1 nm (Figure S1), corresponding to 2-3 layers GO nanosheets. These GO nanosheets were dispersed in the HCl/Li2SO4•H2O/tetrabutyl titanate (TBT) mixture as the ideal substrate for the growth of TiO2 nanobundles during the hydrothermal process (150 °C for 6 h). After the addition of ascorbic acid (AA) as an effective reduction reagent, the solution was kept at 150 °C for another 4 h, during which GO nanosheets were reduced to RGO. /Insert Figure 1/ The X-ray diffraction (XRD) patterns of the samples are shown in Figure 1. The characteristic peak (~10°) of GO can be obviously observed in the XRD patterns, corresponding to the (001) plane of GO. After the hydrothermal reduction, this peak entirely disappears and a broad peak at ~25° appears instead, indicating that –OH, C=O and –COOH groups have been removed from GO to a great extent.7 All the peaks of TiO2/RGO and pure TiO2 can be well assigned to the standard card (JCPDS 4-551) of rutile TiO2. All TiO2 crystal structures consist of TiO62- octahedral. Because rutile octahedra shows linear chains parallel to [001] while anatase octahedra arrang in zigzag chains along [221], the distribution of the third octahedron significantly determines the polymorphs.20 The importance of solution pH and supporting anions in the phase selective crystallization of TiO2 has been demonstrated.21,22 Cl- has a weak steric effect because of its small radius, which lead to the octahedron with Cl- and another octahedron would combine along the straight direction, and the orientation of the third octahedron promote the formation of a rutile nucleus. Meanwhile, the existence of a large amount of Cl- under highly acidic conditions generally promote the formation of rutile crystallites.21 In our preparation, the experimental system contained high hydrochloric acid concentration which led to the preferential formation rutile phase crystallization. The Figure S3 shows the obtained products have formed rutile phase TiO2 after 2h of solvothermal reaction, further confirming the experimental system can rapidly promote the formation of rutile phase TiO2.
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/Insert Figure 2/ The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the TiO2/RGO composites are shown in Figures 2. The samples display bundle-like structure with uniform morphology and dimension, growing directly on RGO nanosheets. The high-resolution TEM (HRTEM) image further confirms the morphology and crystal characteristics of the rutile TiO2 crystals in the TiO2/RGO composites (Figure 2e and f).The crystalline lattice distance is 3.25 Å, corresponding to the (110) planes of rutile TiO2 which is consistent with the XRD results. /Insert Figure 3/ To probe the electrochemical performances of TiO2/RGO composites and pure TiO2, the samples were assembled as electrodes for LIBs. These electrodes showed the plateau at ~1.1 V of the first discharge curve and appeared as a sloping region at the second discharge curve, which is typical for rutile electrodes.23 The initial discharge/charge capacities of pure TiO2 were 247/171 mAh g-1 at 1.2 C and rapidly deceased to only 134 mAh g-1 after 100 cycles (Figure S4b). By sharp contrast, TiO2/RGO composites exhibited a higher discharge/charge capacity of 436/237 mAh g-1 at the initial cycle (Figure 3b). The irreversible capacity can be attributed to the formation of solid-electrolyte interface (SEI) films, which can be confirmed by electrochemical impedance spectroscopy (EIS) (see details below). More importantly, the TiO2/RGO composites exhibited a higher capacity of 200 mAh g-1 at 1.2 C even after 500 cycles. (Figure 3c and d). This stabilized capacity was much higher than that of pure TiO2 (Figure S4c and d). Take the thesis of Wang et al.24 as a typical example of recent progress, anatase TiO2 nanosheet/reduced graphene oxide composite was synthesized through hydrothermal reaction and the product exhibited relatively lower capacity and weaker cycling stability (180 mAh g-1 at 1 C after 200 cycles). Compared with previous work, our rutile TiO2/RGO exhibits a highly crystalline bundle morphology, higher rate performance and ultralong cycle life for lithium storage. /Insert Figure 4/ Profiting from the favorable features of the superior hybrid nanostructures, TiO2/RGO composites also exhibited excellent performances at high rates. When cycled at 0.6 C, 1.2 C, 3 C, 6 C and 12 C, the reversible capacities were 292, 190, 133, 101 and 78 mAh g-1, respectively (Figure 4a). More importantly, no obvious capacity loss happened when the rate reduced to 3 C and 0.6 C, indicating very good reversibility. These performances were superior to other TiO2-graphene composites prepared through the hydrothermal/solvothermal route.13,19,25 As a typical example, Lee et al. synthesized TiO2 nanorod arrays on RGO as LIB anodes, and the capacity was only 94 mAh g-1 at 5 C.19 To better understand the high-rate electrochemical behaviors of TiO2/RGO composites, cyclic voltammetry (CV) was performed between 0 and 3.0 V at different scanning rates from 0.5 to 5 mV s-1 (Figure S5). After the initial 4 cycles, a pair of redox peaks can be observed at 1.75 and 1.95 V at the low scanning rate of 0.5 mV s-1. Only one pair of distinct cathodic (insertion)/anodic (extraction) peaks can be observed at all scan rates.26 Contrastly, the voltage hysteresis became serious at higher scanning rates owing to the dramatically increasing polarization. The linear relationship between the peak current and scanning rate (the inset in Figure S5) suggests obvious surface lithium storage, i.e., the faradic pseudocapacitance effect, which occurs on the interface between active particles and electrolytes.8 To gain insight into the influence of RGO nanosheets, we conducted EIS on the cells comprising the TiO2/RGO composites and pure TiO2 as the working electrodes (Figure 4b). The samples show only one semicircle in the high-frequency range and an inclined line in the low-frequency range. The high frequency
2 | J. Name., 2014, 00, 1-3
Journal Name DOI: 10.1039/C4CC05480F semicircle is assigned to the charge-transfer impedance in the electrode/electrolyte interface, and the inclining line corresponds to the Li+ diffusion process. However, the EIS profile displays two semicircles after the first lithiation (iv); the high frequency semicircle is attributed to the formation of SEI film, which mainly grows in the initial discharge process.8 After 3 cycles, the EIS profile (v) of TiO2/RGO nanocomposites exhibit a smaller semicircle in the high-frequency range and an inclining line with a larger slope than that (ii) of pure TiO2 and the corresponding equivalent circuit mode is shown in the inset. In this equivalent circuit, Rs and Rct represent electrolyte resistance and charge-transfer resistance respectively,27 the two indexes were calculated by the equivalent circuit in Table 1. Rs values of two samples show little distinction, due to the usage of identical electrolyte system. Rct value of TiO2/RGO nanocomposites is only 43.09 Ω lower than that of the pure TiO2 (119.53 Ω). Moreover, the Li-ion diffusion behavior within the solid-state electrode materials can be represented by the linear Warburg regions at low frequency (from 0.2 to 0.01 Hz) in Nyquist plots.28 And the formula of Li+ diffusion coefficient is shown as followed (eq 1):
(1) In the equation, Vm, F, A and σw represent the molar volume of rutile TiO2, Faraday constant, total contact surface area between the electrolyte and the electrode, and Warburg prefactor obtaining from the Warburg region of impedance response, respectively.28 The Li+ diffusion coefficient of TiO2/RGO composites (ca. 8.03×10-15 cm2·s1 ) is 2 times of pure TiO2 (ca. 3.65×10-15 cm2·s−1). Compared with pure TiO2, TiO2/RGO composites possess smaller Rct value and higher Li+ diffusion coefficient, indicating that the incorporation of TiO2 and RGO nanosheets can effectively improve the electron transport and Li+ diffusivity, and leads to higher discharge/charge capacities of TiO2/RGO composites than pure TiO2. Table 1. Physicochemical Properties Measured and Calculated from EIS Spectra (in Figure 4b) samples TiO2/RGO Pure TiO2
DLi (EIS) (cm2·s-1) 8.03×10-15 cm2·s-1 3.65×10-15 cm2·s-1
Rs (Ω) 3.34 4.15
Rct (Ω) 43.09 119.53
The excellent performance of TiO2/RGO composites can be attributed to several possible reasons. First, in nanometer-sized rutile TiO2, more than 0.5 Li+ per TiO2 can be inserted/extracted in the discharge/charge process and the electrochemical behavior of rutile TiO2 was greatly dependent on the particle size.29-32 It is commonly agreed that Li diffusion in rutile TiO2 is highly anisotropic, which proceeds through rapid diffusion along c-axis [001] channels. More importantly, HRTEM shows that the TiO2 nanocrystal growing on RGO was along the [110] plane (Figure 2f), and the building units (the width and length are smaller than 10 nm) of nanobundles shorten the Li+ ion diffusion length in the nanocrystals which could accelerate Li+ insertion and extraction.31 Second, some studies proposed that SEI films contain various inorganic/organic compounds,34,35 which can participate in the reaction to form various compounds under suitable potentials and contribute to much higher capacity than that of the counterpart oxides. In addition, graphene nanosheets have significant disorder/defects, this also contributes to the extra lithium storage capacity.36 Third, during the cycle, the crystallized TiO2 maybe change increasingly to be amorphous which makes more and more accessible active sites available for Li+ insertion.36 In addition, anchoring TiO2 nanobundles onto RGO nanosheets minimizes TiO2 agglomeration and RGO restacking,
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which helps keep high stability and large surface area during lithium insertion/extraction.37 In summary, we adopted two-step hydrothermal methods without using any surfactants and high-temperature calcination to fabricate rutile TiO2 nanobundles on RGO with desired dimension and morphology. Different from previous hydrothermal/solvothermal products, the obtained rutile TiO2/RGO composites possessed of a highly crystalline bundle of morphology, higher rate performance and ultralong cycle life for lithium storage. This work enlightens us on preparing nanobundles on graphene nanosheets for energy storage. This work was supported by NSFC (21271108), China US Center for Environmental Remediation and Sustainable Development, NSFC (21273118), Tianjin Municipal Science and Technology Commission (11JCZDJC24800) and MOE Innovation Team (IRT13022) in China.
Notes and references a
Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300071, P.R. China. b Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, P.R. China. *Corresponding author. E-mail: [email protected] E-mail: [email protected] Electronic Supplementary Information (ESI) available: [Experimental details, AFM, SEM, XRD CV and discharge/charge profiles]. See DOI: 10.1039/c000000x/ 1. P. Meduri, E. Clark, J. H. Kim, E. Dayalan, G. U. Sumanasekera and M. K. Sunkara, Nano Lett., 2012, 12, 1784-1788. 2. B. Scrosati, Nature, 1995, 373, 557-558. 3. M. Ling, J. X. Qiu, S. Li, H. Zhao, G. Liu, and S. Q. Zhang, J. Mater. Chem. A, 2013, 1, 11543-11547. 4. G. Armstrong, A. R. Armstrong, J. Canales and P. G. Bruce, Chem. Commun., 2005, 41, 2454-2456. 5. J. S. Chen and X. W. Lou, J. Power Sources, 2010, 195, 2905-2908. 6. L. Kavan, M. Kalbac, M. Zukalová, I. Exnar, V. Lorenzen, R. Nesper and M. Graetzel, Chem. Mater., 2004, 16, 477-485. 7. J. S. Chen, H. Liu, S. Z. Qiao and X. W. Lou, J. Mater. Chem., 2011, 21, 5687-5692. 8. M. M. Zhen, L. W. Su, Z. H. Yuan, L. Liu and Z. Zhou, RSC Adv., 2013, 3, 13696-13701. 9. H. Kim, M. Kim, T. Shin, H. Shin and J. Cho, Electrochem. Commun., 2008, 10, 1669-1672. 10. P. M. Dziewonski and M. Grzeszczuk, Electrochim.Acta, 2010, 55, 3336-3347. 11. J. X. Qiu, P. Zhang, M. Ling, S. Li, P .R. Liu, H. J. Zhao and S. Q. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 3636-3642. 12. V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach, Energy Environ. Sci., 2011, 4, 3243-3262.
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COMMUNICATION DOI: 10.1039/C4CC05480F 13. D. H. Wang, D. W. Choi, J. Li, Z. G. Yang, Z. M. Nie, R. Kou, D. H. Hu, C. M. Wang, L. V. Saraf, J. G. Zhang, I. A. Aksay and J. Liu, ACS Nano, 2009, 3, 907-914. 14. N. Li, G. Liu, C. Zhen, F. Li, L. Zhang and H. M. Cheng, Adv. Funct. Mater., 2011, 21, 1717-1722. 15. S. Li, Y. Z. Wang, C. Lai, J. X. Qiu, M. Ling, W. Martens, H. J. Zhao and S. Q. Zhang, J. Mater. Chem. A, 2014, 2, 10211–10217. 16. D. H. Wang, D. W. Choi, Z. G. Yang, V. V. Viswanathan, Z. M. Nie, C. M. Wang, Y. J. Song, J. G. Zhang and J. Liu, Chem. Mater., 2008, 20, 3435-3442. 17. H. Q. Cao, B. J. Li, J. X. Zhang, F. Lian, X. H. Kong, and M. Z. Qu, J. Mater. Chem., 2012, 22, 9759-9766. 18. S. J. Ding, J. S. Chen, D. Luan, F. Y. C. Boey, S. Madhavi and X. W. Lou, Chem. Commun., 2011, 47, 5780-5782. 19. L. F. He, R. G. Ma, N. Du, J. G. Ren, T. L. Wong, Y. Y. Li and S. T. Lee, J. Mater. Chem., 2012, 22, 19061-19066. 20. Y. Li, T. J. White and S. H. Lim, Journal of Solid State Chemistry, 2004, 177 1372-1381. 21. J. G. Li, T. Ishigaki and X. D. Sun, J. Phys. Chem. C, 2007, 111, 4969-4976. 22. M. C. Yan, F. Chen, J. L. Zhang and M. Anpo, J. Phys. Chem. B, 2005, 109, 8673-8678. 23. Z. S. Hong, Y. X. Xu, Y. B. Lui and M. D. Wei, Chem. Eur. J., 2012, 18, 10753-10760. 24. Z. Y. Wang, J. W. Sha, E. Z. Liu, C. N. He, C. S. Shi, J. J. Li and N. Q. Zhao, J. Mater. Chem. A, 2014, 2, 8893–8901. 25. Y. M. Ren, J. Zhang, Y. Y. Liu, H. B. Li, H. J. Wei, B. J. Li and X. Y. Wang, ACS Appl. Mater. Interface, 2012, 4, 4776-4780. 26. S. K. Das, S. Darmakolla and A. J. Bhattacharyya, J. Mater. Chem., 2010, 20, 1600-1606. 27. Y. Wang, H. Liu, K. Wang, H. Eiji, Y. Wang and H. Zhou, J. Mater. Chem., 2009, 19, 6789-6795. 28. J. X. Qiu, S. Li, E. Gray, H. W. Liu, Q. F. Gu, C. H. Sun, C. Lai, H. J. Zhao and S. Q. Zhang, J. Phys. Chem. C, 2014, 118, 8824-8830. 29. Y. S. Hu, L. Kienle, Y. G. Guo and J. Maier, Adv. Mater., 2006, 18, 1421-1426. 30. H. Qiao, Y. W. Wang, L. F. Xiao and L. Z. Zhang, Electrochem. Commun., 2008, 10, 1280-1283. 31. E. Baudrin, S. Cassaignon, M. Koelsch, J. P. Jolivet, L. Dupont and J. M. Tarascon, Electrochem. Commun., 2007, 9, 337-342. 32. M. A. Reddy, M. S. Kishore, V. Pralong, V. Caignaert, U. V. Varadaraju and B. Raveau, Electrochem. Commun., 2006, 8, 12991303. 33. H. L. Wang, Y. Yang, Y. Y. Liang, L. F. Cui, H. S. Casalongue, Y. G. Li, G. S. Hong, Y. Cui and H. J. Dai, Angew. Chem., 2011, 123, 7502-7506. 34. D. Aurbach, J. Power Sources, 2000, 89, 206-218. 35. L. W. Su, Y. R. Zhong and Z. Zhou, J. Mater. Chem. A, 2013, 1, 15158-15166. 36. C. X. Peng, B. D. Chen, Y. Qin, S. H. Yang, C. Z. Li, Y. H. Zuo, S. Y. Liu and J. H. Yang. ACS Nano, 2012, 6, 1074-1081. 37. V. Etacheri, J. E. Yourey and B. M. Bartlett, ACS Nano, 2014, 8, 1491-1499.
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Scheme 1 Schematic illustration of TiO2 nanobundles on RGO nanosheets. 39x33mm (300 x 300 DPI)
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Figure 1. XRD patterns of GO, RGO, pure TiO2 and TiO2/RGO composites. 39x27mm (300 x 300 DPI)
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Figure 2. SEM (a and b), TEM (c and d) and HRTEM (e and f) images of TiO2/RGO composites. The lattice distance in the black region is 3.25 Å. 39x40mm (300 x 300 DPI)
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Figure 3. Discharge/charge profiles of TiO2/RGO composites (a and b) and cycling performance of TiO2/RGO composites (c and d) at 0.6 C and 1.2 C. 39x29mm (300 x 300 DPI)
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Figure 4. Cycling performance (a) of TiO2/RGO composites, Nqyuist plots for the EIS (b) of pure TiO2 and TiO2/RGO composites at different states. (pure TiO2 i: before cycling, ii: after initial 3 cycles; TiO2/RGO iii: before cycling, iv: after 1st discharge, v: after initial 3 cycles). Inset: the corresponding equivalent circuit mode of two samples. 38x18mm (300 x 300 DPI)
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