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Facile synthetic route towards shape-controlled anatase titania with tailored facets for lithium-ion batteries
Author’s Accepted Manuscript Facile synthetic route towards shape-controlled anatase titania with tailored facets for lithium-ion batteries Yun Jiang, Yu Xia, Minli Zhang, Weiwei Sun, Yumin Liu, Xing-Zhong Zhao www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30539-5 http://dx.doi.org/10.1016/j.matlet.2015.09.027 MLBLUE19543
To appear in: Materials Letters Received date: 29 July 2015 Accepted date: 7 September 2015 Cite this article as: Yun Jiang, Yu Xia, Minli Zhang, Weiwei Sun, Yumin Liu and Xing-Zhong Zhao, Facile synthetic route towards shape-controlled anatase titania with tailored facets for lithium-ion batteries, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.09.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Liu et al. Page 1
Facile synthetic route towards shape-controlled anatase titania with tailored facets for lithium-ion batteries Yun Jianga,b, Yu Xiaa,b, Minli Zhanga,b, Weiwei Sunc, Yumin Liua,b,*, Xing-Zhong Zhaoa,c a
b
Institute for Interdisciplinary Research (IIR), Jianghan University, Wuhan 430056, China. Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education,
Jianghan University, Wuhan 430056, China. c
Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, Wuhan University,
Wuhan 430072, China. *
Tel.: +86-27-84223798. E-mail: [email protected].
ABSTRACT We demonstrate a facile approach to synthesize shape-controlled anatase titania with tailored facets via the employment of 3-aminopropyltrimethoxysilane (APTMS) as shape-tunable and capping agent. The as-synthesized anatase TiO2 nanorods was highly crystalline and exposed with {001} and {100} facets. The LIBs based on these highly crystalline TiO2 nanorods with tailored facets reveals significant improvement in high-rate capacity and cycling performance. More importantly, this methodology enable the industrial application of anatase titania as anode materials for LIBs without high temperature annealing process.
Keywords: anatase titania; tailored facets; shape control; nanocrystalline materials; energy storage and conversion.
1. Introduction
Liu et al. Page 2 Lithium-ion batteries (LIBs) are among the most successful power sources that have been regarded as prime candidates for future plug-in hybrid electric and electric vehicles (P-HEVs and EVs) because of their high energy levels and power density[1,2]. In past decades, substantial efforts have been devoted to advanced anode materials for LIBs applications to overcome the safety concerns of graphite[3-7], such as the formation of lithium dendrite and thermal instability of solidelectrolyte interphase (SEI) film. Anatase titania has been considered as a great potential alternative materials to graphite based anodes because of its good stability, low volume expansion (3-4%), high operating voltage (1.5-1.8 V vs Li/Li+) and low cost[8]. However, anatase titania as an anode material for LIBs has its inhere drawbacks. The limited ionic and electric conductivity restricted the lithium insertion/extraction reaction and rate performances[9]. To address this issue, many strategies, such as developing novel anatase TiO2 nanostructures[10,11], coating with an conductive layer[12,13], have been explored to improve the anatase titania based LIBs performances. The synthesis of anatase TiO2 with tailored facets is a well-established method for improving the Li-ion storage properties and diffusion kinetics in LIBs[14-16]. It was found that Li-ion transport much faster across the high-energy facets (i.e. {001}) than low-energy facets (i.e. {101})[16]. Many studies have attempted to synthesize anatase TiO2 with exposed high-energy facets via introducing fluorine, especially HF, as a capping agent to stabilize {001} facets[17]. However, HF is highly corrosive and hazardous, it is necessary to develop new synthetic route to obtain anatase TiO2 with exposed highenergy facets for LIBs applications. Herein, we report a facile approach to synthesize highly crystalline anatase titania with tailored {001} and {100} facets via the employment of 3-aminopropyltrimethoxysilane (APTMS). Firstly, the precursor amorphous TiO2 nanospheres (PSP) was synthesized by a controlled hydrolysis process. Without the modification of APTMS, mesoporous TiO2 nanospheres (TSP) was obtained after a hydrothermal reaction at 160 ˚C for 12 h. Whereas highly crystalline TiO2 nanorods was evolved from the APTMS-modified PSP after the same hydrothermal process (Figure 1). The morphologies and structures of TSP and TNR were characterized by X-ray diffraction (XRD), field
Liu et al. Page 3 emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Inspired by the highly crystalline and tailored facets, TNR was used as the anode materials of LIBs without further annealing process, and corresponding electrochemical performances of the LIBs based on TNR were also reported.
Figure 1. Schematic illustration of the processes to synthesis shape-controlled anatase titania: TiO2 spheres (TSP) and TiO2 nanorods (TNR).
2. Experimental section 2.1 Synthesis of shape-controlled anatase titania The precursor amorphous TiO2 nanospheres (PSP) was synthesized by a controlled hydrolysis process according to the reported methods[18]. Titanium isopropoxide (48 mmol) was added in the solution of ethanol (480 mL) and acetonitrile (320 mL) with methylamine (8 mmol) and DI water (3.5 mL). The mixture was stirred for 30 min and kept static over night. Then the collected and washed PSP (0.1g) was dispersed into 20 mL ethanol. APTMS (1 mL) was added and refluxed at 80 ˚C for 4 h, followed by washing with ethanol several times. The as-synthesized PSP and APTMStreated PSP were transferred to Teflon-lined autoclave and heated at 160 ˚C for 12 h, respectively. The obtained TSP and TNR were washed with sufficient ethanol and dried in vacuum at 60 ˚C over night. 2.2 Characterization
Liu et al. Page 4 The morphologies of TSP and TNR were observed by field emission scanning electron microscopy (FESEM). TEM and HRTEM investigations were carried out using a FEI Tecnai G20. X-ray diffraction (XRD) patterns were performed on a PANalytical XRD system (X’Pert Powder) with Cu Kα radiation (λ=1.54056 Å). The lithium storage properties were performed using a 2032 coin-type half-cell with pure Li foil as the counter electrode and electrolyte of 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in volume) obtained from Shenzhen Kejing Instrument Co. Ltd. A polypropylene membrane (Celgard) was used as the separator. Electrochemical
impedance
spectroscopy
(EIS)
measurements
were
performed
on
an
electrochemical station (CHI660E). The galvanostatic charge-discharge testing with a voltage range of 1.0-3.0 V vs Li/Li+ was carried out on a Land test system at room temperature with different rates (1C was defined as 168 mA g-1 in out experiments).
3. Results and discussion
Figure 2. SEM images of (a-b) TSP and (c-d) TNR. (e) X-ray diffraction patterns (Cu Kα radiation) of the precursor spheres (PSP), TSP, and TNR. (f) TEM and selected HR-TEM images of TNR.
Liu et al. Page 5 The scanning electron microscopy (SEM) images of as-synthesized TSP and TNR are shown in Figure 2a-d. After a hydrothermal reaction at 160 ˚C for 12 h, the PSP was converted to mesoporous TSP composed with TiO2 nanocrystallines as shown in Figure 2a-b. Highly crystalline tetragonal TiO2 nanorods (Figure 2c-d) was evolved from APTMS-modified PSP after the same hydrothermal process. Figure 2e show the typical XRD patterns of as-synthesized precursor, which indicate PSP was amorphous. All observed peaks shown in XRD patterns of TSP and TNR can be indexed to a pure phase of anatase titania (JCPDS card No. 21-1272). Furthermore, the XRD peaks in sample TNR were broaden compared to that in TSP, which indicates the increase of crystalline size according to Scherrer formula: D=Kλ/βcosθ. The anatase TiO2 nanorods with tailored facets was further characterized by transmission electron microscopy (TEM), as shown in Figure 2f. The lattice fringes were clearly observed from high-resolution TEM images. The neighboring lattice fringes with distance of 0.47, 0.35 and 0.38 nm can be attributed corresponding to (001), (101) and(010) planes of the anatase phase, respectively. The tetragonal faceted TiO2 nanorod with four well lateral {100} facets grows along the [001] direction. The angle between [100] and [010] directions is 90˚, which was confirmed by top-view SEM image of single TiO2 nanorod (Figure 2d, inset). The electrochemical performances of LIBs based on TSP and TNR were investigated by cycling at room temperature using a 2032 coin-type half-cell with pure Li foil as the counter electrode. Representative charge-discharge profiles of LIBs based on TSP and TNR with 100 cycles at a rate of 1 C (168 mA g-1) in the potential range of 1.0-3.0 V are shown in Figure 3b-c. The electrode based on TNR exhibits a discharge capacity of about 248.9, 184.4, 173.3, 153.3, 135.6 and 122.2 mAh g-1 when the cycle number was 1, 2, 3, 10, 50 and 100, respectively. The increased specificcharging capacity of the initial cycle can be attributed to the formation of SEI on the electrode surface. After 100 cycles, a discharge capacity of 122.2 mAh g-1 was retained, but the TSP electrode only retained 34.5 mAh g-1. The cycling performances of LIBs based on TSP and TNR were tested at a rate of 1 C in the voltage range of 1.0–3.0 V (Figure 3a). A remarkable degradation of
Liu et al. Page 6
Figure 3. (a) Cycling performance of the batteries based on TSP and TNR. Charge-discharge galvanostatic curves for (b) TSP and (c)TNR cycled at room temperature obtained from a 2032 coin-type half cell using Li metal as negative at a rate of 1 C in the voltage range of 1.0-3.0 V. (d) Variation in charge and discharge capacities versus cycle number for TSP and TNR cycled at different current rates in the voltage range of 1.03.0 V. Rate capability of (e) TSP and (f) TNR with increasing rates from 1 C to 10 C between 1.0 and 3.0 V.
the discharge capacity was observed when using TSP electrode, whereas TNR showed an obviously lower fading rate. To further investigate the performance of TNR in high current rates, the battery was cycled at different rates ranging from 1 C to 10 C, as shown in Figure 3f. The TNR electrode delivers a discharge capacity of about 146.3, 126.3, 103.8 and 85.0 mAh g-1 at a rate of 1 C, 2 C, 5 C and 10 C, respectively. For comparison, the battery based on TSP was examined under the same conditions. The discharge capacity decreased drastically with increase in the current rate (Figure 3e). The cycling performance and rate capability of LIBs based on TSP could be remarkably improved via further high temperature annealing process after the solvothermal reaction[10,13]. Significantly, in this work, we achieved comparable lithium storage properties without high temperature annealing, which can be attributed to the highly crystalline and tailored high-energy facets of TNR.
Liu et al. Page 7
Figure 4. Nyquist plots of TSP and TNR based LIBs before galvanostatic discharging/charging and after 5 cycles. Inset shows the equivalent circuit for fitting the impedance data, which consists of the resistance of the electrolytes (Rs), the resistance for Li+ migration through the surface film (Ri), the charge transfer resistance (Rct), the surface film capacitance (Ci), the double-layer capacitance (Cdl), and the Warburg impedance (Zw).
The electrochemical impedance spectroscopy (EIS) measurements were undertaken to get further insight into the rate capability. Figure 4 illustrates the Nyquist plots of TSP and TNR based LIBs before galvanostatic discharging/charging and after 5 cycles at a current rate of 168 mA g-1 (1C). The Nyquist plots of these LIBs were fitted with an equivalent circuit (Figure 4, inset), which consists of the resistance of the electrolytes (Rs), the resistance for Li+ migration through the surface film (Ri), the charge transfer resistance (Rct), the surface film capacitance (Ci), the double-layer capacitance (Cdl), and the Warburg impedance (Zw). The semicircle in the medium-frequency range indicates the charge transfer resistance (Rct). Rct of the LIBs based on TNR were smaller than that of devices based on TSP before galvanostatic discharging/charging and after 5 cycles, which indicates the effective charge transport and facile Li+ diffusion between the TNR cathode materials and liquid electrolyte. Therefore, the significantly better rate performance of LIBs based on TNR can be attributed to the synergistic effect of enhanced electron transport and Li+ diffusion, resulting from the exposed {001} and {100} facets.
Liu et al. Page 8 4. Conclusions In summary, shape-controlled anatase titania with tailored facets was synthesized via the employment of 3-aminopropyltrimethoxysilane (APTMS) as shape-tunable and capping agent. The as-synthesized anatase TiO2 nanorods was highly crystalline and exposed with {001} and {100} facets without any high temperature annealing treatment. The LIBs based on the highly crystalline TiO2 nanorods reveals significant improvement in high-rate capacity and cycling performance, which can be explained by the enhance electron transport and Li+ diffusion resulting from the exposed {001} and {100} facets. This facile and low cost methodology enable the industrial application of anatase titania as anode materials for LIBs.
Acknowledgment This project was supported by the National Natural Science Foundation of China (Grant No. 61404060) and the National Basic Research Program of China (Grant No. 2011CB933300). We acknowledge the financial support from PhD research foundation of Jianghan University.
References [1] Y. Tang, Y. Zhang, J. Deng, J. Wei, H. L. Tam, B. K. Chandran, Z. Dong, Z. Chen, X. Chen, Adv. Mater., 2014, 26, 6111-6118. [2] Y. Yang, B. Qiao, X. Yang, L. Fang, C. Pan, W. Song, H. Hou, X. Ji, Adv. Funct. Mater., 2014, 24, 4349-4356. [3] L. F. Shen, Q. Che, H. S. Li, X. G. Zhang, Adv. Funct. Mater., 2014, 24, 2630-2637. [4] N, Liu, Z. D. Lu, J. Zhao, M. T. McDowell, H. W. Lee, W. T. Zhao, Y. Cui, Nat. Nanotechnol., 2014, 9, 187-192. [5] C. Z. Yuan, H. B. Wu, Y. Xie, X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 1488-1504. [6] G. M. Zhou, F. Li, H. M. Cheng, Energy Environ. Sci., 2014, 7, 1307-1338. [7] J. X. Zhu, D. Yang, Z. Y. Yin, Q. Y. Yan, H. Zhang, Small, 2014, 10, 3480-3498.
Liu et al. Page 9 [8] W. Li, F. Wang, Y. P. Liu, J. X. Wang, J. P. Yang, L. J. Zhang, A. A. Elzatahry, D. Al-Dahyan, Y. Y. Xia, D. Y. Zhao, Nano Lett., 2015, 15, 2186-2193. [9] 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, J. Liu, ACS Nano, 2009, 3, 907-914. [10] Y. Y. Zhou, S. Yoon, Electrochem. Commun., 2014, 40, 54-57. [11] G. Kim, C. Jo, W. Kim, J. Chun, S. Yoon, J. Lee, W. Choi, Energy Environ. Sci., 2013, 6, 29322938. [12] X. Yan, Y. J. Li, F. Du, K. Zhu, Y. Q. Zhang, A. Y. Su, G. Chen, Y. J. Wei, Nanoscale, 2014, 6, 4108-4116. [13] N. T. H. Trang, Z. Ali, D. J. Kang, ACS Appl. Mater. Interfaces, 2015, 7, 3676-3683. [14] J. S. Chen, Y. L. Tan, C. M. Li, Y. L. Cheah, D. Luan, S. Madhavi, F. Y. C. Boey, L. A. Archer, X. W. Lou, J. Am. Chem. Soc., 2010, 132, 6124-6130. [15] J. S. Chen, X. W. Lou, Electrochem. Commun., 2009, 11, 2332-2335. [16] C. H. Sun, X. H. Yang, J. S. Chen, Z. Li, X. W. Lou, C. Li, S. C. Smith, G. Q. Lu, H. G. Yang, Chem. Commun., 2010, 46, 6129-6131. [17] G. Liu, H. G. Yang, J. Pan, Y. Q. Yang, G. Q. Lu, H. M. Cheng, Chem. Rev., 2014, 114, 95599612. [18] Y. J. Kim, M. H. Lee, H. J. Kim, G. Lim, Y. S. Choi, N. G. Park, K. Kim and W. I. Lee, Adv. Mater., 2009, 21, 3668-3673.
Highlights 1. APTMS was employed as shape-tunable and capping agent to synthesize anatase titania anode materials.
Liu et al. Page 10
2. The as-synthesized anatase TiO2 nanorods was highly crystalline and exposed with {001} and {100} facets. 3. The LIBs have achieved significant improvement in high-rate capacity and cycling performance.
PII: DOI: Reference:
S0167-577X(15)30539-5 http://dx.doi.org/10.1016/j.matlet.2015.09.027 MLBLUE19543
To appear in: Materials Letters Received date: 29 July 2015 Accepted date: 7 September 2015 Cite this article as: Yun Jiang, Yu Xia, Minli Zhang, Weiwei Sun, Yumin Liu and Xing-Zhong Zhao, Facile synthetic route towards shape-controlled anatase titania with tailored facets for lithium-ion batteries, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.09.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Liu et al. Page 1
Facile synthetic route towards shape-controlled anatase titania with tailored facets for lithium-ion batteries Yun Jianga,b, Yu Xiaa,b, Minli Zhanga,b, Weiwei Sunc, Yumin Liua,b,*, Xing-Zhong Zhaoa,c a
b
Institute for Interdisciplinary Research (IIR), Jianghan University, Wuhan 430056, China. Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education,
Jianghan University, Wuhan 430056, China. c
Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, Wuhan University,
Wuhan 430072, China. *
Tel.: +86-27-84223798. E-mail: [email protected].
ABSTRACT We demonstrate a facile approach to synthesize shape-controlled anatase titania with tailored facets via the employment of 3-aminopropyltrimethoxysilane (APTMS) as shape-tunable and capping agent. The as-synthesized anatase TiO2 nanorods was highly crystalline and exposed with {001} and {100} facets. The LIBs based on these highly crystalline TiO2 nanorods with tailored facets reveals significant improvement in high-rate capacity and cycling performance. More importantly, this methodology enable the industrial application of anatase titania as anode materials for LIBs without high temperature annealing process.
Keywords: anatase titania; tailored facets; shape control; nanocrystalline materials; energy storage and conversion.
1. Introduction
Liu et al. Page 2 Lithium-ion batteries (LIBs) are among the most successful power sources that have been regarded as prime candidates for future plug-in hybrid electric and electric vehicles (P-HEVs and EVs) because of their high energy levels and power density[1,2]. In past decades, substantial efforts have been devoted to advanced anode materials for LIBs applications to overcome the safety concerns of graphite[3-7], such as the formation of lithium dendrite and thermal instability of solidelectrolyte interphase (SEI) film. Anatase titania has been considered as a great potential alternative materials to graphite based anodes because of its good stability, low volume expansion (3-4%), high operating voltage (1.5-1.8 V vs Li/Li+) and low cost[8]. However, anatase titania as an anode material for LIBs has its inhere drawbacks. The limited ionic and electric conductivity restricted the lithium insertion/extraction reaction and rate performances[9]. To address this issue, many strategies, such as developing novel anatase TiO2 nanostructures[10,11], coating with an conductive layer[12,13], have been explored to improve the anatase titania based LIBs performances. The synthesis of anatase TiO2 with tailored facets is a well-established method for improving the Li-ion storage properties and diffusion kinetics in LIBs[14-16]. It was found that Li-ion transport much faster across the high-energy facets (i.e. {001}) than low-energy facets (i.e. {101})[16]. Many studies have attempted to synthesize anatase TiO2 with exposed high-energy facets via introducing fluorine, especially HF, as a capping agent to stabilize {001} facets[17]. However, HF is highly corrosive and hazardous, it is necessary to develop new synthetic route to obtain anatase TiO2 with exposed highenergy facets for LIBs applications. Herein, we report a facile approach to synthesize highly crystalline anatase titania with tailored {001} and {100} facets via the employment of 3-aminopropyltrimethoxysilane (APTMS). Firstly, the precursor amorphous TiO2 nanospheres (PSP) was synthesized by a controlled hydrolysis process. Without the modification of APTMS, mesoporous TiO2 nanospheres (TSP) was obtained after a hydrothermal reaction at 160 ˚C for 12 h. Whereas highly crystalline TiO2 nanorods was evolved from the APTMS-modified PSP after the same hydrothermal process (Figure 1). The morphologies and structures of TSP and TNR were characterized by X-ray diffraction (XRD), field
Liu et al. Page 3 emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Inspired by the highly crystalline and tailored facets, TNR was used as the anode materials of LIBs without further annealing process, and corresponding electrochemical performances of the LIBs based on TNR were also reported.
Figure 1. Schematic illustration of the processes to synthesis shape-controlled anatase titania: TiO2 spheres (TSP) and TiO2 nanorods (TNR).
2. Experimental section 2.1 Synthesis of shape-controlled anatase titania The precursor amorphous TiO2 nanospheres (PSP) was synthesized by a controlled hydrolysis process according to the reported methods[18]. Titanium isopropoxide (48 mmol) was added in the solution of ethanol (480 mL) and acetonitrile (320 mL) with methylamine (8 mmol) and DI water (3.5 mL). The mixture was stirred for 30 min and kept static over night. Then the collected and washed PSP (0.1g) was dispersed into 20 mL ethanol. APTMS (1 mL) was added and refluxed at 80 ˚C for 4 h, followed by washing with ethanol several times. The as-synthesized PSP and APTMStreated PSP were transferred to Teflon-lined autoclave and heated at 160 ˚C for 12 h, respectively. The obtained TSP and TNR were washed with sufficient ethanol and dried in vacuum at 60 ˚C over night. 2.2 Characterization
Liu et al. Page 4 The morphologies of TSP and TNR were observed by field emission scanning electron microscopy (FESEM). TEM and HRTEM investigations were carried out using a FEI Tecnai G20. X-ray diffraction (XRD) patterns were performed on a PANalytical XRD system (X’Pert Powder) with Cu Kα radiation (λ=1.54056 Å). The lithium storage properties were performed using a 2032 coin-type half-cell with pure Li foil as the counter electrode and electrolyte of 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in volume) obtained from Shenzhen Kejing Instrument Co. Ltd. A polypropylene membrane (Celgard) was used as the separator. Electrochemical
impedance
spectroscopy
(EIS)
measurements
were
performed
on
an
electrochemical station (CHI660E). The galvanostatic charge-discharge testing with a voltage range of 1.0-3.0 V vs Li/Li+ was carried out on a Land test system at room temperature with different rates (1C was defined as 168 mA g-1 in out experiments).
3. Results and discussion
Figure 2. SEM images of (a-b) TSP and (c-d) TNR. (e) X-ray diffraction patterns (Cu Kα radiation) of the precursor spheres (PSP), TSP, and TNR. (f) TEM and selected HR-TEM images of TNR.
Liu et al. Page 5 The scanning electron microscopy (SEM) images of as-synthesized TSP and TNR are shown in Figure 2a-d. After a hydrothermal reaction at 160 ˚C for 12 h, the PSP was converted to mesoporous TSP composed with TiO2 nanocrystallines as shown in Figure 2a-b. Highly crystalline tetragonal TiO2 nanorods (Figure 2c-d) was evolved from APTMS-modified PSP after the same hydrothermal process. Figure 2e show the typical XRD patterns of as-synthesized precursor, which indicate PSP was amorphous. All observed peaks shown in XRD patterns of TSP and TNR can be indexed to a pure phase of anatase titania (JCPDS card No. 21-1272). Furthermore, the XRD peaks in sample TNR were broaden compared to that in TSP, which indicates the increase of crystalline size according to Scherrer formula: D=Kλ/βcosθ. The anatase TiO2 nanorods with tailored facets was further characterized by transmission electron microscopy (TEM), as shown in Figure 2f. The lattice fringes were clearly observed from high-resolution TEM images. The neighboring lattice fringes with distance of 0.47, 0.35 and 0.38 nm can be attributed corresponding to (001), (101) and(010) planes of the anatase phase, respectively. The tetragonal faceted TiO2 nanorod with four well lateral {100} facets grows along the [001] direction. The angle between [100] and [010] directions is 90˚, which was confirmed by top-view SEM image of single TiO2 nanorod (Figure 2d, inset). The electrochemical performances of LIBs based on TSP and TNR were investigated by cycling at room temperature using a 2032 coin-type half-cell with pure Li foil as the counter electrode. Representative charge-discharge profiles of LIBs based on TSP and TNR with 100 cycles at a rate of 1 C (168 mA g-1) in the potential range of 1.0-3.0 V are shown in Figure 3b-c. The electrode based on TNR exhibits a discharge capacity of about 248.9, 184.4, 173.3, 153.3, 135.6 and 122.2 mAh g-1 when the cycle number was 1, 2, 3, 10, 50 and 100, respectively. The increased specificcharging capacity of the initial cycle can be attributed to the formation of SEI on the electrode surface. After 100 cycles, a discharge capacity of 122.2 mAh g-1 was retained, but the TSP electrode only retained 34.5 mAh g-1. The cycling performances of LIBs based on TSP and TNR were tested at a rate of 1 C in the voltage range of 1.0–3.0 V (Figure 3a). A remarkable degradation of
Liu et al. Page 6
Figure 3. (a) Cycling performance of the batteries based on TSP and TNR. Charge-discharge galvanostatic curves for (b) TSP and (c)TNR cycled at room temperature obtained from a 2032 coin-type half cell using Li metal as negative at a rate of 1 C in the voltage range of 1.0-3.0 V. (d) Variation in charge and discharge capacities versus cycle number for TSP and TNR cycled at different current rates in the voltage range of 1.03.0 V. Rate capability of (e) TSP and (f) TNR with increasing rates from 1 C to 10 C between 1.0 and 3.0 V.
the discharge capacity was observed when using TSP electrode, whereas TNR showed an obviously lower fading rate. To further investigate the performance of TNR in high current rates, the battery was cycled at different rates ranging from 1 C to 10 C, as shown in Figure 3f. The TNR electrode delivers a discharge capacity of about 146.3, 126.3, 103.8 and 85.0 mAh g-1 at a rate of 1 C, 2 C, 5 C and 10 C, respectively. For comparison, the battery based on TSP was examined under the same conditions. The discharge capacity decreased drastically with increase in the current rate (Figure 3e). The cycling performance and rate capability of LIBs based on TSP could be remarkably improved via further high temperature annealing process after the solvothermal reaction[10,13]. Significantly, in this work, we achieved comparable lithium storage properties without high temperature annealing, which can be attributed to the highly crystalline and tailored high-energy facets of TNR.
Liu et al. Page 7
Figure 4. Nyquist plots of TSP and TNR based LIBs before galvanostatic discharging/charging and after 5 cycles. Inset shows the equivalent circuit for fitting the impedance data, which consists of the resistance of the electrolytes (Rs), the resistance for Li+ migration through the surface film (Ri), the charge transfer resistance (Rct), the surface film capacitance (Ci), the double-layer capacitance (Cdl), and the Warburg impedance (Zw).
The electrochemical impedance spectroscopy (EIS) measurements were undertaken to get further insight into the rate capability. Figure 4 illustrates the Nyquist plots of TSP and TNR based LIBs before galvanostatic discharging/charging and after 5 cycles at a current rate of 168 mA g-1 (1C). The Nyquist plots of these LIBs were fitted with an equivalent circuit (Figure 4, inset), which consists of the resistance of the electrolytes (Rs), the resistance for Li+ migration through the surface film (Ri), the charge transfer resistance (Rct), the surface film capacitance (Ci), the double-layer capacitance (Cdl), and the Warburg impedance (Zw). The semicircle in the medium-frequency range indicates the charge transfer resistance (Rct). Rct of the LIBs based on TNR were smaller than that of devices based on TSP before galvanostatic discharging/charging and after 5 cycles, which indicates the effective charge transport and facile Li+ diffusion between the TNR cathode materials and liquid electrolyte. Therefore, the significantly better rate performance of LIBs based on TNR can be attributed to the synergistic effect of enhanced electron transport and Li+ diffusion, resulting from the exposed {001} and {100} facets.
Liu et al. Page 8 4. Conclusions In summary, shape-controlled anatase titania with tailored facets was synthesized via the employment of 3-aminopropyltrimethoxysilane (APTMS) as shape-tunable and capping agent. The as-synthesized anatase TiO2 nanorods was highly crystalline and exposed with {001} and {100} facets without any high temperature annealing treatment. The LIBs based on the highly crystalline TiO2 nanorods reveals significant improvement in high-rate capacity and cycling performance, which can be explained by the enhance electron transport and Li+ diffusion resulting from the exposed {001} and {100} facets. This facile and low cost methodology enable the industrial application of anatase titania as anode materials for LIBs.
Acknowledgment This project was supported by the National Natural Science Foundation of China (Grant No. 61404060) and the National Basic Research Program of China (Grant No. 2011CB933300). We acknowledge the financial support from PhD research foundation of Jianghan University.
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Highlights 1. APTMS was employed as shape-tunable and capping agent to synthesize anatase titania anode materials.
Liu et al. Page 10
2. The as-synthesized anatase TiO2 nanorods was highly crystalline and exposed with {001} and {100} facets. 3. The LIBs have achieved significant improvement in high-rate capacity and cycling performance.