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Facile fabrication of one-dimensional mesoporous titanium dioxide composed of nanocrystals for lithium storage
Electrochimica Acta 138 (2014) 155–162
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Facile fabrication of one-dimensional mesoporous titanium dioxide composed of nanocrystals for lithium storage Qinghua Tian a , Zhengxi Zhang a,1 , Li Yang a,∗ , Shin-ichi Hirano b a b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
a r t i c l e
i n f o
Article history: Received 15 April 2014 Received in revised form 20 May 2014 Accepted 9 June 2014 Available online 16 June 2014 Keywords: titanium dioxide one-dimension mesoporous anode materials lithium-ion batteries
a b s t r a c t Titanium dioxide (TiO2 ) has received increasing attention as promising anode for lithium ion batteries because it offers a distinct safety advantage in comparison to commercialized graphite anodes, whereas it also suffer from the drawbacks of low practical capacity and relatively low electronic conductivity. Herein, one-dimensional mesoporous anatase TiO2 composed of nanocrystals prepared by a facile procedure is reported for the first time. Such peculiar architecture and intrinsical mesoporous can effectively improve pseudocapacitance charge storage, increase contact interface between the active materials and electrolyte, and enhance the structure stability during cycling, therefore contributing to good lithium storage and excellent cycling stability. A reversible capacity of 202.9 mAhg−1 is obtained at 30 mAg−1 after 70 cycles. More importantly, 151 mAhg−1 can be obtained at 200 mAg−1 even after 500 cycles. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, it is highly desirable to develop safer electrodes and electrolytes due to several high-profile incidents that raised concerns about the hazards posed by lithium ion batteries (LIBs).[1] As is well known, the current LIBs predominantly use graphite as anode materials, which cannot meet the pressing need for safer power, since the graphite has a lower potential for lithium intercalation and easy presence of lithium platting issues during high current operation. Therefore, numerous efforts have been made to design and develop alternative anode materials to meet the need for safer next-generation LIBs. As the most promising candidates for anode materials, transition metal oxides with higher potential for lithium intercalation and higher capacities have stimulated great interest to replace the commercial graphite anode for LIBs in the past decade.[2,3] TiO2 is one of the most-studied anode materials owing to its better safety (the potential for lithium intercalation above 1.0 V), excellent structural stability, low cost and environmentally benign.[4] Despite the above inherent beneficial characteristics, there are still obstacles that hinder the practical use of TiO2 -based
∗ Corresponding author. Tel.: +86 21 54748917; fax: +86 21 54741297. E-mail addresses: [email protected] (Z. Zhang), [email protected] (L. Yang). 1 Tel.: +86 21 54748917; fax: +86 21 54741297. http://dx.doi.org/10.1016/j.electacta.2014.06.047 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
anode materials for safer power, especial the low practical capacity (ca. 170 mAhg−1 ).[5] Fortunately, there are increasing reports of nanostructured TiO2 , which exhibit better activity and higher lithium storage than bulk TiO2 ,[6–8] In the case of anatase TiO2 , especial for nanostructured anatase TiO2 , the discharge process can be divided into three domains. The first domain, corresponding to the formation of conductive Lix TiO2 in solid-solution stage, is marked by the monotonous potential decrease.[9] The second domain is the well-known two-phase switching occurs with phase equilibrium of the Li-poor Li0.01 TiO2 (tetragonal) phase and the Li-rich Li0.55 TiO2 (orthorhombic) phase, representing the process of Li+ insertion into and extraction from the interstitial octahedral site of TiO2 reversibly.[10] The third domain is a sloped potential, attributable to the lithium storage in the surface as well as the formation of Li1 TiO2 phase, which corresponds to an intercalation coefficient x = 1 (the maximum theoretical capacity of 335 mAhg−1 ).[11] The lithium storage on surface of the nanostructured TiO2 mainly attribute to pseudocapacitance. In other words, a major contribution to the charge accumulation in TiO2 electrodes with large specific surface areas comes from the pseudocapacitive behavior. Therefore, the pseudocapacitance becomes the dominant reversible charge storage mechanism for TiO2 at high charge/discharge current densities.[12] In addition, the pseudocapacitance significantly influenced by morphology and structure (surface structure in particular) of TiO2 electrodes. So it is expected that obtained high performance TiO2 electrodes by fabrication of peculiar architecture of TiO2 with large surface
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area and high surface-to-volume ratio (improve the pseudocapacitance, shorten lithium diffusion path and increase contact interface between active materials and electrolyte). So far, various TiO2 nanostructures have been designed and fabricated to use as potential anode materials for LIBs, such as nanoparticles,[13] nanofibers,[14] nanowires,[15] nanotubes,[16] nanoribbons,[17] and nanosheets[18] all exhibit interesting properties because the extensive interface between the active materials and the electrolyte can contribute to pseudocapacitance charge storage.[19] Nevertheless, these nanostructured electrodes also suffer from some serious drawbacks, such as low thermodynamic stability and difficulty of handling. To solve these problems, designing and fabricating nano/micro-hierarchical electrode materials is a promising choice.[20] The nano-/micro-hierarchical electrodes not only have the advantages of nanometer-sized building blocks, but also possess the merits of micro- or submicrometer-sized assemblies such as thermodynamic stability and high energy density.[21] In this regards, besides excellent lithium storage, they also guarantee less agglomeration and better strain relaxation upon cycling. For example, Chen et al. synthesized TiO2 hollow microspheres assembled by nanotubes exhibited higher lithium storage capacities and better cycling performance.[10] Liu et al. prepared nanosheet-constructed porous TiO2 -B with high rate capability (216mAhg−1 at 10 C).[18] Nguyen et al. fabricated self-assembled ultrathin anatase TiO2 nanosheets with highly enhanced reversible lithium storage.[22] Among variety of nano-/micro-hierarchical TiO2 potential anodes, hierarchical mesoporous TiO2 composed of nanocrystals has attracted much attention, because they not only have the advantages of nano-/micro-hierarchical, but also possess intrinsic advantages of high specific surface area, high pore volume which come from mesoporous structure. These merits bring their high chemical activity and improved pseudocapacitance charge storage, corresponding to excellent electrochemical properties. It is worth noting that meso-structure is a widely used strategy to insure high contact interface between the Li+ -hosting materials and the electrolyte, short diffusion path for Li+ and easy accommodation of strain during cycling, resulting in enhanced capacity, rate performance and cyclability.[23] Moreover, significant reduction in the crystallite size of TiO2 also beneficial to increase the extent of lithium intercalation.[24] Certainly, the hierarchical mesoporous TiO2 composed of nanocrystals possess the advantages of porous, nanocrystal and nano-/micro-hierarchical structure, is considered as a promising anode for LIBs. Herein, we report for the first time a facile method to prepare one-dimensional (1D) mesoporous TiO2 composed of nanocrystals. Besides the advantages of hierarchical mesoporous TiO2 composed of nanocrystals, our as-prepared product also has the merits of 1D nanostructure. As is well known, the 1D nanostructures can provide efficient electron pathways and short diffusion path for lithium ions.[25] Thus, this unique architecture of our product is beneficial to increase contact interface between the active materials and the electrolyte, enhance the structure stability and improve pseudocapacitance charge storage during cycling, as a result, excellent electrochemical performances are exhibited.
(h). Afterward, 1.0 mL of tetrabutyl titanate (TBOT) was added dropwise in 5 min, and the reaction was allowed to proceed for ca. 3 h at 45 ◦ C under continuous mechanical stirring. The resultant products (CNTs@TiO2 ) were collected by centrifugation and washed with distilled water and ethanol thoroughly, and then dried in a vacuum oven at 60 ◦ C overnight, followed by calcination in a Muffle furnace at 500 ◦ C for 2 h under air at a ramping rate of 0.5 ◦ C/min. Finally, the one-dimensional mesoporous TiO2 composed of nanocrystals was obtained, and referred to as 1DM-TiO2 . In order to compared, we also calcined the CNTs@TiO2 in a tube furnace at 500 ◦ C for 2 h under Ar at a ramping rate of 0.5 ◦ C/min. And obtained product referred to as CNTs@TiO2 500Ar. Then we prepared a sample (TiO2 600) by CNTs@TiO2 calcination in a Muffle furnace at 600 ◦ C for 2 h under air at a ramping rate of 0.5 ◦ C/min. 2.2. Structure and electrochemical characterization The morphology and microstructure of the products were obtained using field emission scanning electron microscopy (FESEM, JEOL JSM-7401F), high resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) and transmission electron microscopy (TEM, JEOL JEM-2010) with a selected area electron diffraction pattern (SAED). The composition and crystal structure were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-Rbusing Cu K␣ radiation). The N2 adsorption/desorption isotherms were measured with Micromeritics ASAP 2010 instrument. Thermogravimetric analysis (TGA) of the as-prepared TiO2 was carried out with a thermogravimetric analysis instrument (TGA, SDT Q600 V8.2 Build 100). X-ray photoelectron spectroscopy (XPS) experiments were carried out on an AXIS ULTRA DLD instrument, using aluminum K␣ X-ray radiation during XPS analysis. Electrochemical measurements were performed using 2016type coin cells assembled in an argon-filled glove box (German, M. Braun Co., [O2 ] < 1 ppm, [H2 O] < 1 ppm). The working electrodes were composed of the active material (1DM-TiO2 ), conductive material (acetylene black, AB), and binder (poly-vinyldifluoride, PVDF) in a weight ratio of 1DM-TiO2 /AB/PVDF = 70:20:10 and pasted on Cu foil. Pure lithium foil was used as the counter electrode. A glass fiber (GF/A) from Whatman was used as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (EC + DMC) (1:1 in volume). The galvanostatic discharge/charge cycles were carried out on a CT2001a cell test instrument (LAND Electronic Co.) over a voltage range of 1.0 to 3.0 V at room temperature. Cyclic voltammetry (CV) was implemented by a CHI660D electrochemical workstation at a scan rate of 0.1 mVs−1 from 1.0 to 3.0 V. Electrochemical impedance spectrum (EIS) measurements were also performed by using CHI660D electrochemical workstation in the frequency range from 100 KHz to 0.001 Hz with an ac perturbation of 5 mV. For 1DM-TiO2 working electrode, all the specific capacities reported and current densities used were based on the weight of 1DM-TiO2 . 3. Results and discussion
2. Experimental Section 2.1. Preparation of samples Preparation of one-dimensional mesoporous TiO2 composed of nanocrystals (1DM-TiO2 ). A typical preparation process was as follows: 100 mg acid-treated carbon nanotubes (CNTs) were dispersed in absolute ethanol (134 mL) containing 400 mg of cetyl trimethyl ammonium bromide (CTAB), and mixed with concentrated ammonia solution (0.6 mL, 25-28 wt%) under ultrasound for 1 hour
One-dimensional mesoporous TiO2 composed of nanocrystals was facile prepared by using carbon nanotubes (CNTs) as templates. The fabrication process only included two simple steps shown in Fig. 1, TiO2 first coating CNTs by hydrolysis and condensation of tetrabutyl titanate, followed by a calcination process to remove the CNTs. The microstructure of the as-prepared 1DM-TiO2 was characterized by SEM and TEM, as shown in Fig. 2. Fig. 2a displays a SEM image of the 1DM-TiO2 . It is clearly seen from the image that the 1DM-TiO2 has a regular 1D shape with diameters of about 30 nm. The TEM image of the 1DM-TiO2 shown in Fig. 2b, further confirms
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Fig. 1. Schematic illustration of the fabrication process of 1DM-TiO2 .
that the 1DM-TiO2 has a 1D shape and possesses mesoporous characteristic (white zones with a average size smaller than 10 nm seen in 1DM-TiO2 ) upon the CNTs removed. Fig. 2c presents a magnified TEM image of 1DM-TiO2 , and indicates that the 1DM-TiO2 is formed by the aggregation of nanoparticles with a sizes of about 5-10 nm. SAED rings (inset of Fig. 2c) reveal that the 1DM-TiO2 consisting of polycrystalline anatase TiO2 , and the all rings can be indexed as anatase phase.[10] HRTEM image of a part of single 1DM-TiO2 (see in Fig. 2c) shown in Fig. 2d indicates that the nanoparticles are highly crystalline. The lattice fringe is found to be ca. 0.35 nm, corresponding to the d101-spacing in the XRD pattern.
For comparison, we prepared another two different samples in different preparation conditions which described in experimental section. One is CNTs@TiO2 500Ar, the other is TiO2 600. They are both shown in Fig. 3. Fig. 3(a, b) are the SEM and TEM images of CNTs@TiO2 , respectively, showing that the TiO2 nanoparticles coat CNTs uniformly and preserve the 1D shape of CNTs. Fig. 3(c, d) are the SEM and TEM images of TiO2 600, respectively, displaying that the TiO2 600 losses 1D shape and exhibits an almost irregular microstructure aggregation of nanoparticles with a sizes of about 10-20 nm. The average sizes much larger than the sizes of nanoparticles of 1DM-TiO2 due to the higher calcination temperature.
Fig. 2. SEM and TEM images of 1DM-TiO2 : (a) SEM image, (b) TEM image, (c) Magnified TEM image and SAED pattern (inset), (d) HRTEM image of the part of single 1DM-TiO2 .
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Fig. 3. SEM and TEM images of CNTs@TiO2 500Ar and TiO2 600: (a) SEM and (b) TEM images of CNTs@TiO2 500Ar, (c) SEM and (d) TEM images of TiO2 600.
Then, we observed the compositional characterizations of different samples by X-ray diffraction measurement, the corresponding XRD patterns are shown in Fig. 4. It is clear from the patterns of XRD that all intensive peaks can be well indexed to anatase TiO2 (JCPDS ˚ withcard no. 21-1272, S.G.: I41/amd, ao = 3.7852 Å, co = 9.5139 A), out any other phases such as rutile, brookite, TiO2 (B), and residual CNTs.[26] Furthermore, the sharpening of Bragg peaks suggests the obtained TiO2 are well crystallized. Compared to CNTs@TiO2 and 1DM-TiO2 , the peaks intensities of the TiO2 600 increased owing
Fig. 4. XRD patterns of different samples.
to the higher calcination temperature. It is in good agreement with the HRTEM and SAED results. The nitrogen adsorption-desorption measurement was also measured to determine the pore-related information of the 1DMTiO2 . As described in Fig. 5a, a distinct hysteretic loop for P/Po ranges from 0.7 to 1.0 can be observed, indicating the presence of pores in the product. In addition, the N2 isotherms are identified as type IV, which is the characteristic isotherm of mesoporous materials. And based on the analysis of BJH method, the pore size is estimated to be about 2-12 nm. The BET surface area is measured to be 102.1 m2 g−1 based on N2 adsorption/desorption tests. For comparison, TiO2 600 and CNTs@TiO2 500Ar were studied, as shown in Fig. 5(b, c), respectively. They also exhibit mesoporous structure, and the BET surface areas are 31.9 and 81.5, respectively. The 4 nm pores in CNTs@TiO2 500Ar are attributed to CNTs. The 30 nm pores in TiO2 600 are interspaced pores among the larger nanoparticles. Compared with CNTs@TiO2 500Ar and TiO2 600, the 1DM-TiO2 has more appropriate pore sizes and larger BET surface area. The mesoporous structure of the 1DM-TiO2 facilitates better penetration of electrolyte and the possibility of the efficient transport of electrons and lithium ions. For further confirmed the templates were effectively removed by calcination, the thermogravimetric (TG) analysis of 1DM-TiO2 was carried out shown in Fig. 6, only ca. 2-3% weight loss was observed (mainly due to the evaporation of the weakly adsorbed water and trace carbon come from residual CNTs tiny fragments). And ca. 1% weight loss was observed between 200 and 600 ◦ C attributed to carbon [27]. It is worth noting that the trace carbon in 1DM-TiO2 is beneficial for improving the electronic conductivity of 1DM-TiO2 and bringing the 1DM-TiO2 good electrochemical
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Fig. 5. Nitrogen adsorption-desorption isotherms of 1DM-TiO2 , TiO2 600 and CNTs@TiO2 500Ar, the insets showing BJH pore-size distributions of these samples.
performance.[28,29] For comparison, the CNTs content of TiO2 600 and CNTs@TiO2 500Ar were also determined by TGA (Fig. 6). The CNTs content of CNTs@TiO2 500Ar was 67.5 wt.%. In TiO2 600, no weight loss was observed, indicating the CNTs were completely removed. A typical XPS spectrum for the 1DM-TiO2 is shown in Fig. 7a. Obvious C 1s, Ti 2p and O 1s are detected, and their high-resolution spectra are shown in Fig. 7(b-d), respectively. The C 1s spectrum (Fig. 7b) comprises two peaks with binding energies (BEs) of 284.6 eV and 288.3 eV, which are well attributable to C in aromatic rings (sp2, C = C/C-C) and carbonyl (C = O), respectively,[30]
Fig. 6. TGA analysis of different samples.
further indicating the presence of carbon in 1DM-TiO2 composite. The signal at 288.3 eV comes from carbonyl formed in the process of oxidizing acid treatment of CNTs, which in good agreement with TG analysis. The high-resolution spectrum of Ti 2p shows the binding energies for Ti 2p3/2 at 458.8 eV and Ti 2p1/2 at 464.6 eV (Fig. 7c) which are assigned to Ti4+ in TiO2 .[31] Then, in Fig. 6d, the O 1s spectrum comprises two peaks with binding energies (BEs) of 530.3 eV and 533 eV, which are well attributable to O2− in TiO2 and C-O and/or O = C-O groups in CNTs, respectively.[31,32] The analysis results from XPS further confirm that we have successfully fabricated a unique architecture 1DM-TiO2 consists of TiO2 nanocrystals. Undoubtedly, the electrochemical properties of the 1DM-TiO2 as potential anode material for lithium-ion batteries were investigated. Fig. 8a shows representative cyclic voltammogram of the 1DM-TiO2 at a scan rate of 0.1 mVs−1 between 1.0 and 3.0 V. The first scan shows two well-defined current peaks at 1.73 V (the cathodic sweep) and 2.0 V (the anodic sweep), which corresponds well to the previously reported values for anatase TiO2 .[33] The Li+ insertion and extraction processes occur during the cathodic and anodic sweeps, respectively.[34] The cathodic peak at 1.73 V can be attributed to the phase transition from tetragonal anatase to orthorhombic Li0.5 TiO2 , which corresponds to the insertion coefficient x≈ 0.5.[35] Obviously, a spontaneous phase separation into lithium-poor Li0.01 TiO2 and lithium-rich Li0.5 TiO2 take place at this stage. Two phases coexist at the equilibrium and Li+ flow continuously in between them during this process. Compare with first cycle, the following cycles show a large reduction in the net area under the curve indicating a large capacity loss. The serious capacity loss can be attributed to the adsorbed trace
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Fig. 7. XPS spectra for 1DM-TiO2 : (a) survey spectrum and high-resolution spectra for (b) C 1s, (c) Ti 2p, and (d) O 1s.
Fig. 8. Electrochemical characterizations of samples: (a) Representative CVs of 1DM-TiO2 at a scan rate of 0.1 mVs−1 between 1.0 V and 3.0 V; (b, c) Charge/discharge voltage profiles at 30 mAg−1 and 200 mAg−1 between 1.0 V and 3.0 V, respectively; (d) Cycling performance and Coulombic efficiency of 1DM-TiO2 at 30 mAg−1 between 1.0 V and 3.0 V. (e) Comparative cycling performance between different samples and Coulombic efficiency of 1DM-TiO2 at 200 mAg−1 between 1.0 V and 3.0 V; (f) Rate capabilities of 1DM-TiO2 .
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water and irreversible Li+ insertion sites (i.e., surface defects such as surface vacancies or voids connected to structural distortion caused by Jahn-Teller effect), respectively.[10] In good agreement with the discharge/charge voltage profiles of 1DM-TiO2 shown in Fig. 8(b, c). From the following cycles, we can clear observation that no changes in the peak positions during subsequent anodic scans. Moreover, overlapping and mirror-like CV curves further demonstrating the excellent reversibility of 1DM-TiO2 during electrochemical Li+ insertion/extraction process. Fig. 8b shows the discharge/charge profiles of 1DM-TiO2 at a current density of 30 mAg−1 from 1.0 to 3.0 V. The distinct potential plateaus at ca. 1.77 and 1.89 V in the discharge/charge curves are observed, corresponding to Li+ insertion into and extraction from anatase lattice. The discharge curve can be divided into three domains. The first domain, corresponding to the formation of conductive Lix TiO2 in solid-solution stage, is marked by the monotonous potential decrease.[9] The second domain is the well-known two-phase switching occurs with phase equilibrium of the Li-poor Li0.01 TiO2 (tetragonal) phase and the Li-rich Li0.55 TiO2 (orthorhombic) phase, representing the process of Li+ insertion into and extraction from the interstitial octahedral site of TiO2 reversibly.[10] The third domain is a sloped potential, attributable to the lithium storage on the surface as well as the formation of Li1 TiO2 phase.[11] The discharge and charge capacities in the first cycle are 614.8 and 231.8 mAhg−1 , respectively, with a first cycle capacity loss of 62%. As we mentioned above, the high initial capacity loss can be attributed to the adsorbed trace water and irreversible lithium insertion sites such as surface defects and voids connected to structural distortion caused by Jahn-Teller effect. It is consistent with the peculiar architecture of 1DM-TiO2 , which possess larger specific surface area and abundant mesoporous. In the second cycle, 1DM-TiO2 delivers discharge and charge capacities of 266.6 and 214.9 mAhg−1 , respectively, and the coulombic efficiency is 80.6%. In the 10th cycle, the coulombic efficiency increases to 96.3%. Obviously, the discharge/charge curves after 10th cycles almost overlapped with the first one, showing a good cycling performance of the 1DM-TiO2 . Fig. 8c displays the discharge/charge profiles of 1DM-TiO2 at a high current density of 200 mAg−1 from 1.0 to 3.0 V. They exhibit the same behavior with the curves which discharge/charge at a current density of 30 mAg−1 . But, in the first cycle, the discharge and charge capacities are 426.6 and 211.5 mAhg−1 , respectively (the capacity loss much decreased). After 10th cycle, the discharge/charge curves also almost overlapped with the first one, further demonstrating a good cycling performance of the 1DM-TiO2 at high current density. Fig. 8d shows the cycling performance of 1DM-TiO2 at a current density of 30 mAg−1 . It delivers a discharge capacity of 202.9 mAhg−1 after 70 cycles, with a coulombic efficiency of 92.9%. Fig. 8e presents the comparison of cycling performances between 1DMTiO2 and other different samples at a current density of 200 mAg−1 (1 C rate corresponds to the current density of 167.5 mAg−1 )[36]. Compared with TiO2 600 and CNT@TiO2 500Ar, it can be clearly seen that the 1DM-TiO2 demonstrate great enhanced cyclic capacity retention. The 1DM-TiO2 delivers a high discharge capacity of 166.2 mAhg−1 after 100 cycles and even can remains at 151 mAhg−1 in 500th cycle with a coulombic efficiency of 99.3%, demonstrating its excellent cycling stability. More importantly, the coulombic efficiencyis very close to 100%, indicating the facile transportation of lithium ion and electron, provided by 1D structure and mesopores features. In sharp contrast, TiO2 600 and CNT@TiO2 500Ar deliver a discharge capacity of 83.5 and 55.5 mAh−1 after 500 cycles, respectively. Furthermore, we also compared the cycling performance of 1DM-TiO2 with that of similar anatase TiO2 electrodes which had been reported, such as TiO2 hollow microspheres assembled by nanotubes[10] (150 mAh−1 after 500 cycles at a rate of 1 C), self-assembled ultrathin anatase TiO2 nanosheets[22] (160 mAh−1
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Fig. 9. Nyquist plots of the 1DM-TiO2 , TiO2 600 and CNTs@TiO2 500Ar after 5 discharge/charge cycles, the inset showing magnified Nyquist plots at high frequency range.
after 150 cycles at a current density of 150 mAg−1 ) and the TiO2 nanosheets with discrete integrated nanocrystalline subunits[36] (150 mAhg−1 after 100 cycles at 2 C). Compared to them, our product not only enhanced lithium storage properties and cycling performance, but also exhibited a more facile fabrication process. To further demonstrated its advantage for high rate LIBs application, the rate performance of 1DM-TiO2 was further tested under different current densities from 30 to 3000 mAg−1 . As shown in Fig. 8f, the capacities of 1DM-TiO2 under all current densities are quite stable, showing excellent rate capability. Specifically, the last reversible capacities of 1DM-TiO2 are 210.9, 159.8, 116.2, 96.9, 91.7, and 83.4 mAhg−1 at the 10th cycles of 30, 200, 800, 1600, 2000, and 3000 mAg−1 , respectively, indicating the good rate performance and stability of the 1DM-TiO2 . Electrochemical impedance spectroscopy (EIS) measurements were also carried out to better demonstrate the effect of peculiar architecture on the electronic conductivity of the 1DM-TiO2 . Fig. 9 shows the Nyquist plots of the 1DM-TiO2 , TiO2 600 and CNTs@TiO2 500Ar electrodes after 5 discharge/charge cycles. Obviously, all electrodes show Nyquist plots consisting of a depressed semicircle at high frequency range and an angled line in the low frequency range. The diameter of the depressed semicircle is correlated with the electron transfer resistance on the electrode interface, and the angled line is related to a diffusion controlled process. Apparently, the 1DM-TiO2 exhibits a similar diameter of the high frequency semicircle with CNTs@TiO2 500Ar, but much smaller than the TiO2 600 electrode (may attributed to trace carbon in 1DM-TiO2 ). Moreover, the 1DMTiO2 shows an more slope angled line compared with TiO2 600 and CNTs@TiO2 500Ar electrodes, demonstrating enhanced electron and lithium ion transport.[37] These good results discussed above can attribute to the peculiar architecture of 1DM-TiO2 , which possess unique integration of one-dimensional mesoporous structure and nanocrystal units. Because of this structure with larger specific surface area and abundant mesoporous can effectively improve pseudocapacitance charge storage, shorten diffusion path of Li+ , increase contact interface between the active materials and electrolyte and enhance the structure stability during cycling, finally contributing to excellent cycling stability and high lithium storage. In short, the excellent electrochemical properties of the 1DM-TiO2 are derived from the intriguing hierarchical mesoporous one-dimensional morphology which not only shortening Li+ and electronic diffusion distance, but also providing a thermodynamically stable system. Moreover, the peculiar structure can not only ensure the sufficient contact between active materials and electrolyte, but also
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effectively suppress and relax the agglomeration and strain upon cycling. 4. Conclusion In summary, we have successfully prepared one-dimensional mesoporous TiO2 composed of nanocrystals. This unique and intriguing architecture not only has the advantages of nanometersized building blocks and porous structure, but also possess the advantages of submicrometer-sized assemblies. These advantages made the architecture beneficial for increasing contact interface between the active materials and the electrolyte, enhancing structure stability and improving pseudocapacitance charge storage during cycling. As a result, the as-prepared product delivers a reversible discharge capacity of 151 mAhg−1 even after 500 cycles at a current density of 200 mAg−1 . The good lithium storage capacity and excellent cycling performance mainly attributed to the unique integration of one-dimensional mesoporous structure and nanocrystal units. Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Grant no. 21103108 and 21173148) and the SJTU-UM collaborative research project. References [1] N.D. Petkovich, S.G. Rudisill, B.E. Wilson, A. Mukherjee, A. Stein, Inorg. Chem. 53 (2014) 1100. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. [3] F.Y. Cheng, J. Liang, Z.L. Tao, J. Chen, Adv. Mater. 23 (2011) 1695. [4] L. Shen, X. Zhang, H. Li, Y. Zhao, F. Bijarbooneh, V. Malgras, Y. Lee, Y.M. Kang, S.X. Dou, J. Am. Chem. Soc. 133 (2011) 19314. [5] H.L. jiang, X.L. yang, C. chen, Y.H. zhu, C.Z. Li, New J. Chem 37 (2013) 1578. [6] Y. Hu, L. Kienle, Y.G. Guo, J. Maier, Adv. Mater. 18 (2006) 1421. [7] C.J. Chen, X.L. Hu, Z.H. Wang, X.Q. Xiong, P. Hu, Y. Liu, Y.H. Huang, Carbon 69 (2014) 302. [8] T.B. Lan, Y.B. Liu, J. Dou, Z.S. Hong, M.D. Wei, J. Mater. Chem. A 2 (2014) 1102.
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Facile fabrication of one-dimensional mesoporous titanium dioxide composed of nanocrystals for lithium storage Qinghua Tian a , Zhengxi Zhang a,1 , Li Yang a,∗ , Shin-ichi Hirano b a b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
a r t i c l e
i n f o
Article history: Received 15 April 2014 Received in revised form 20 May 2014 Accepted 9 June 2014 Available online 16 June 2014 Keywords: titanium dioxide one-dimension mesoporous anode materials lithium-ion batteries
a b s t r a c t Titanium dioxide (TiO2 ) has received increasing attention as promising anode for lithium ion batteries because it offers a distinct safety advantage in comparison to commercialized graphite anodes, whereas it also suffer from the drawbacks of low practical capacity and relatively low electronic conductivity. Herein, one-dimensional mesoporous anatase TiO2 composed of nanocrystals prepared by a facile procedure is reported for the first time. Such peculiar architecture and intrinsical mesoporous can effectively improve pseudocapacitance charge storage, increase contact interface between the active materials and electrolyte, and enhance the structure stability during cycling, therefore contributing to good lithium storage and excellent cycling stability. A reversible capacity of 202.9 mAhg−1 is obtained at 30 mAg−1 after 70 cycles. More importantly, 151 mAhg−1 can be obtained at 200 mAg−1 even after 500 cycles. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, it is highly desirable to develop safer electrodes and electrolytes due to several high-profile incidents that raised concerns about the hazards posed by lithium ion batteries (LIBs).[1] As is well known, the current LIBs predominantly use graphite as anode materials, which cannot meet the pressing need for safer power, since the graphite has a lower potential for lithium intercalation and easy presence of lithium platting issues during high current operation. Therefore, numerous efforts have been made to design and develop alternative anode materials to meet the need for safer next-generation LIBs. As the most promising candidates for anode materials, transition metal oxides with higher potential for lithium intercalation and higher capacities have stimulated great interest to replace the commercial graphite anode for LIBs in the past decade.[2,3] TiO2 is one of the most-studied anode materials owing to its better safety (the potential for lithium intercalation above 1.0 V), excellent structural stability, low cost and environmentally benign.[4] Despite the above inherent beneficial characteristics, there are still obstacles that hinder the practical use of TiO2 -based
∗ Corresponding author. Tel.: +86 21 54748917; fax: +86 21 54741297. E-mail addresses: [email protected] (Z. Zhang), [email protected] (L. Yang). 1 Tel.: +86 21 54748917; fax: +86 21 54741297. http://dx.doi.org/10.1016/j.electacta.2014.06.047 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
anode materials for safer power, especial the low practical capacity (ca. 170 mAhg−1 ).[5] Fortunately, there are increasing reports of nanostructured TiO2 , which exhibit better activity and higher lithium storage than bulk TiO2 ,[6–8] In the case of anatase TiO2 , especial for nanostructured anatase TiO2 , the discharge process can be divided into three domains. The first domain, corresponding to the formation of conductive Lix TiO2 in solid-solution stage, is marked by the monotonous potential decrease.[9] The second domain is the well-known two-phase switching occurs with phase equilibrium of the Li-poor Li0.01 TiO2 (tetragonal) phase and the Li-rich Li0.55 TiO2 (orthorhombic) phase, representing the process of Li+ insertion into and extraction from the interstitial octahedral site of TiO2 reversibly.[10] The third domain is a sloped potential, attributable to the lithium storage in the surface as well as the formation of Li1 TiO2 phase, which corresponds to an intercalation coefficient x = 1 (the maximum theoretical capacity of 335 mAhg−1 ).[11] The lithium storage on surface of the nanostructured TiO2 mainly attribute to pseudocapacitance. In other words, a major contribution to the charge accumulation in TiO2 electrodes with large specific surface areas comes from the pseudocapacitive behavior. Therefore, the pseudocapacitance becomes the dominant reversible charge storage mechanism for TiO2 at high charge/discharge current densities.[12] In addition, the pseudocapacitance significantly influenced by morphology and structure (surface structure in particular) of TiO2 electrodes. So it is expected that obtained high performance TiO2 electrodes by fabrication of peculiar architecture of TiO2 with large surface
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area and high surface-to-volume ratio (improve the pseudocapacitance, shorten lithium diffusion path and increase contact interface between active materials and electrolyte). So far, various TiO2 nanostructures have been designed and fabricated to use as potential anode materials for LIBs, such as nanoparticles,[13] nanofibers,[14] nanowires,[15] nanotubes,[16] nanoribbons,[17] and nanosheets[18] all exhibit interesting properties because the extensive interface between the active materials and the electrolyte can contribute to pseudocapacitance charge storage.[19] Nevertheless, these nanostructured electrodes also suffer from some serious drawbacks, such as low thermodynamic stability and difficulty of handling. To solve these problems, designing and fabricating nano/micro-hierarchical electrode materials is a promising choice.[20] The nano-/micro-hierarchical electrodes not only have the advantages of nanometer-sized building blocks, but also possess the merits of micro- or submicrometer-sized assemblies such as thermodynamic stability and high energy density.[21] In this regards, besides excellent lithium storage, they also guarantee less agglomeration and better strain relaxation upon cycling. For example, Chen et al. synthesized TiO2 hollow microspheres assembled by nanotubes exhibited higher lithium storage capacities and better cycling performance.[10] Liu et al. prepared nanosheet-constructed porous TiO2 -B with high rate capability (216mAhg−1 at 10 C).[18] Nguyen et al. fabricated self-assembled ultrathin anatase TiO2 nanosheets with highly enhanced reversible lithium storage.[22] Among variety of nano-/micro-hierarchical TiO2 potential anodes, hierarchical mesoporous TiO2 composed of nanocrystals has attracted much attention, because they not only have the advantages of nano-/micro-hierarchical, but also possess intrinsic advantages of high specific surface area, high pore volume which come from mesoporous structure. These merits bring their high chemical activity and improved pseudocapacitance charge storage, corresponding to excellent electrochemical properties. It is worth noting that meso-structure is a widely used strategy to insure high contact interface between the Li+ -hosting materials and the electrolyte, short diffusion path for Li+ and easy accommodation of strain during cycling, resulting in enhanced capacity, rate performance and cyclability.[23] Moreover, significant reduction in the crystallite size of TiO2 also beneficial to increase the extent of lithium intercalation.[24] Certainly, the hierarchical mesoporous TiO2 composed of nanocrystals possess the advantages of porous, nanocrystal and nano-/micro-hierarchical structure, is considered as a promising anode for LIBs. Herein, we report for the first time a facile method to prepare one-dimensional (1D) mesoporous TiO2 composed of nanocrystals. Besides the advantages of hierarchical mesoporous TiO2 composed of nanocrystals, our as-prepared product also has the merits of 1D nanostructure. As is well known, the 1D nanostructures can provide efficient electron pathways and short diffusion path for lithium ions.[25] Thus, this unique architecture of our product is beneficial to increase contact interface between the active materials and the electrolyte, enhance the structure stability and improve pseudocapacitance charge storage during cycling, as a result, excellent electrochemical performances are exhibited.
(h). Afterward, 1.0 mL of tetrabutyl titanate (TBOT) was added dropwise in 5 min, and the reaction was allowed to proceed for ca. 3 h at 45 ◦ C under continuous mechanical stirring. The resultant products (CNTs@TiO2 ) were collected by centrifugation and washed with distilled water and ethanol thoroughly, and then dried in a vacuum oven at 60 ◦ C overnight, followed by calcination in a Muffle furnace at 500 ◦ C for 2 h under air at a ramping rate of 0.5 ◦ C/min. Finally, the one-dimensional mesoporous TiO2 composed of nanocrystals was obtained, and referred to as 1DM-TiO2 . In order to compared, we also calcined the CNTs@TiO2 in a tube furnace at 500 ◦ C for 2 h under Ar at a ramping rate of 0.5 ◦ C/min. And obtained product referred to as CNTs@TiO2 500Ar. Then we prepared a sample (TiO2 600) by CNTs@TiO2 calcination in a Muffle furnace at 600 ◦ C for 2 h under air at a ramping rate of 0.5 ◦ C/min. 2.2. Structure and electrochemical characterization The morphology and microstructure of the products were obtained using field emission scanning electron microscopy (FESEM, JEOL JSM-7401F), high resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) and transmission electron microscopy (TEM, JEOL JEM-2010) with a selected area electron diffraction pattern (SAED). The composition and crystal structure were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-Rbusing Cu K␣ radiation). The N2 adsorption/desorption isotherms were measured with Micromeritics ASAP 2010 instrument. Thermogravimetric analysis (TGA) of the as-prepared TiO2 was carried out with a thermogravimetric analysis instrument (TGA, SDT Q600 V8.2 Build 100). X-ray photoelectron spectroscopy (XPS) experiments were carried out on an AXIS ULTRA DLD instrument, using aluminum K␣ X-ray radiation during XPS analysis. Electrochemical measurements were performed using 2016type coin cells assembled in an argon-filled glove box (German, M. Braun Co., [O2 ] < 1 ppm, [H2 O] < 1 ppm). The working electrodes were composed of the active material (1DM-TiO2 ), conductive material (acetylene black, AB), and binder (poly-vinyldifluoride, PVDF) in a weight ratio of 1DM-TiO2 /AB/PVDF = 70:20:10 and pasted on Cu foil. Pure lithium foil was used as the counter electrode. A glass fiber (GF/A) from Whatman was used as the separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (EC + DMC) (1:1 in volume). The galvanostatic discharge/charge cycles were carried out on a CT2001a cell test instrument (LAND Electronic Co.) over a voltage range of 1.0 to 3.0 V at room temperature. Cyclic voltammetry (CV) was implemented by a CHI660D electrochemical workstation at a scan rate of 0.1 mVs−1 from 1.0 to 3.0 V. Electrochemical impedance spectrum (EIS) measurements were also performed by using CHI660D electrochemical workstation in the frequency range from 100 KHz to 0.001 Hz with an ac perturbation of 5 mV. For 1DM-TiO2 working electrode, all the specific capacities reported and current densities used were based on the weight of 1DM-TiO2 . 3. Results and discussion
2. Experimental Section 2.1. Preparation of samples Preparation of one-dimensional mesoporous TiO2 composed of nanocrystals (1DM-TiO2 ). A typical preparation process was as follows: 100 mg acid-treated carbon nanotubes (CNTs) were dispersed in absolute ethanol (134 mL) containing 400 mg of cetyl trimethyl ammonium bromide (CTAB), and mixed with concentrated ammonia solution (0.6 mL, 25-28 wt%) under ultrasound for 1 hour
One-dimensional mesoporous TiO2 composed of nanocrystals was facile prepared by using carbon nanotubes (CNTs) as templates. The fabrication process only included two simple steps shown in Fig. 1, TiO2 first coating CNTs by hydrolysis and condensation of tetrabutyl titanate, followed by a calcination process to remove the CNTs. The microstructure of the as-prepared 1DM-TiO2 was characterized by SEM and TEM, as shown in Fig. 2. Fig. 2a displays a SEM image of the 1DM-TiO2 . It is clearly seen from the image that the 1DM-TiO2 has a regular 1D shape with diameters of about 30 nm. The TEM image of the 1DM-TiO2 shown in Fig. 2b, further confirms
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Fig. 1. Schematic illustration of the fabrication process of 1DM-TiO2 .
that the 1DM-TiO2 has a 1D shape and possesses mesoporous characteristic (white zones with a average size smaller than 10 nm seen in 1DM-TiO2 ) upon the CNTs removed. Fig. 2c presents a magnified TEM image of 1DM-TiO2 , and indicates that the 1DM-TiO2 is formed by the aggregation of nanoparticles with a sizes of about 5-10 nm. SAED rings (inset of Fig. 2c) reveal that the 1DM-TiO2 consisting of polycrystalline anatase TiO2 , and the all rings can be indexed as anatase phase.[10] HRTEM image of a part of single 1DM-TiO2 (see in Fig. 2c) shown in Fig. 2d indicates that the nanoparticles are highly crystalline. The lattice fringe is found to be ca. 0.35 nm, corresponding to the d101-spacing in the XRD pattern.
For comparison, we prepared another two different samples in different preparation conditions which described in experimental section. One is CNTs@TiO2 500Ar, the other is TiO2 600. They are both shown in Fig. 3. Fig. 3(a, b) are the SEM and TEM images of CNTs@TiO2 , respectively, showing that the TiO2 nanoparticles coat CNTs uniformly and preserve the 1D shape of CNTs. Fig. 3(c, d) are the SEM and TEM images of TiO2 600, respectively, displaying that the TiO2 600 losses 1D shape and exhibits an almost irregular microstructure aggregation of nanoparticles with a sizes of about 10-20 nm. The average sizes much larger than the sizes of nanoparticles of 1DM-TiO2 due to the higher calcination temperature.
Fig. 2. SEM and TEM images of 1DM-TiO2 : (a) SEM image, (b) TEM image, (c) Magnified TEM image and SAED pattern (inset), (d) HRTEM image of the part of single 1DM-TiO2 .
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Fig. 3. SEM and TEM images of CNTs@TiO2 500Ar and TiO2 600: (a) SEM and (b) TEM images of CNTs@TiO2 500Ar, (c) SEM and (d) TEM images of TiO2 600.
Then, we observed the compositional characterizations of different samples by X-ray diffraction measurement, the corresponding XRD patterns are shown in Fig. 4. It is clear from the patterns of XRD that all intensive peaks can be well indexed to anatase TiO2 (JCPDS ˚ withcard no. 21-1272, S.G.: I41/amd, ao = 3.7852 Å, co = 9.5139 A), out any other phases such as rutile, brookite, TiO2 (B), and residual CNTs.[26] Furthermore, the sharpening of Bragg peaks suggests the obtained TiO2 are well crystallized. Compared to CNTs@TiO2 and 1DM-TiO2 , the peaks intensities of the TiO2 600 increased owing
Fig. 4. XRD patterns of different samples.
to the higher calcination temperature. It is in good agreement with the HRTEM and SAED results. The nitrogen adsorption-desorption measurement was also measured to determine the pore-related information of the 1DMTiO2 . As described in Fig. 5a, a distinct hysteretic loop for P/Po ranges from 0.7 to 1.0 can be observed, indicating the presence of pores in the product. In addition, the N2 isotherms are identified as type IV, which is the characteristic isotherm of mesoporous materials. And based on the analysis of BJH method, the pore size is estimated to be about 2-12 nm. The BET surface area is measured to be 102.1 m2 g−1 based on N2 adsorption/desorption tests. For comparison, TiO2 600 and CNTs@TiO2 500Ar were studied, as shown in Fig. 5(b, c), respectively. They also exhibit mesoporous structure, and the BET surface areas are 31.9 and 81.5, respectively. The 4 nm pores in CNTs@TiO2 500Ar are attributed to CNTs. The 30 nm pores in TiO2 600 are interspaced pores among the larger nanoparticles. Compared with CNTs@TiO2 500Ar and TiO2 600, the 1DM-TiO2 has more appropriate pore sizes and larger BET surface area. The mesoporous structure of the 1DM-TiO2 facilitates better penetration of electrolyte and the possibility of the efficient transport of electrons and lithium ions. For further confirmed the templates were effectively removed by calcination, the thermogravimetric (TG) analysis of 1DM-TiO2 was carried out shown in Fig. 6, only ca. 2-3% weight loss was observed (mainly due to the evaporation of the weakly adsorbed water and trace carbon come from residual CNTs tiny fragments). And ca. 1% weight loss was observed between 200 and 600 ◦ C attributed to carbon [27]. It is worth noting that the trace carbon in 1DM-TiO2 is beneficial for improving the electronic conductivity of 1DM-TiO2 and bringing the 1DM-TiO2 good electrochemical
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Fig. 5. Nitrogen adsorption-desorption isotherms of 1DM-TiO2 , TiO2 600 and CNTs@TiO2 500Ar, the insets showing BJH pore-size distributions of these samples.
performance.[28,29] For comparison, the CNTs content of TiO2 600 and CNTs@TiO2 500Ar were also determined by TGA (Fig. 6). The CNTs content of CNTs@TiO2 500Ar was 67.5 wt.%. In TiO2 600, no weight loss was observed, indicating the CNTs were completely removed. A typical XPS spectrum for the 1DM-TiO2 is shown in Fig. 7a. Obvious C 1s, Ti 2p and O 1s are detected, and their high-resolution spectra are shown in Fig. 7(b-d), respectively. The C 1s spectrum (Fig. 7b) comprises two peaks with binding energies (BEs) of 284.6 eV and 288.3 eV, which are well attributable to C in aromatic rings (sp2, C = C/C-C) and carbonyl (C = O), respectively,[30]
Fig. 6. TGA analysis of different samples.
further indicating the presence of carbon in 1DM-TiO2 composite. The signal at 288.3 eV comes from carbonyl formed in the process of oxidizing acid treatment of CNTs, which in good agreement with TG analysis. The high-resolution spectrum of Ti 2p shows the binding energies for Ti 2p3/2 at 458.8 eV and Ti 2p1/2 at 464.6 eV (Fig. 7c) which are assigned to Ti4+ in TiO2 .[31] Then, in Fig. 6d, the O 1s spectrum comprises two peaks with binding energies (BEs) of 530.3 eV and 533 eV, which are well attributable to O2− in TiO2 and C-O and/or O = C-O groups in CNTs, respectively.[31,32] The analysis results from XPS further confirm that we have successfully fabricated a unique architecture 1DM-TiO2 consists of TiO2 nanocrystals. Undoubtedly, the electrochemical properties of the 1DM-TiO2 as potential anode material for lithium-ion batteries were investigated. Fig. 8a shows representative cyclic voltammogram of the 1DM-TiO2 at a scan rate of 0.1 mVs−1 between 1.0 and 3.0 V. The first scan shows two well-defined current peaks at 1.73 V (the cathodic sweep) and 2.0 V (the anodic sweep), which corresponds well to the previously reported values for anatase TiO2 .[33] The Li+ insertion and extraction processes occur during the cathodic and anodic sweeps, respectively.[34] The cathodic peak at 1.73 V can be attributed to the phase transition from tetragonal anatase to orthorhombic Li0.5 TiO2 , which corresponds to the insertion coefficient x≈ 0.5.[35] Obviously, a spontaneous phase separation into lithium-poor Li0.01 TiO2 and lithium-rich Li0.5 TiO2 take place at this stage. Two phases coexist at the equilibrium and Li+ flow continuously in between them during this process. Compare with first cycle, the following cycles show a large reduction in the net area under the curve indicating a large capacity loss. The serious capacity loss can be attributed to the adsorbed trace
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Fig. 7. XPS spectra for 1DM-TiO2 : (a) survey spectrum and high-resolution spectra for (b) C 1s, (c) Ti 2p, and (d) O 1s.
Fig. 8. Electrochemical characterizations of samples: (a) Representative CVs of 1DM-TiO2 at a scan rate of 0.1 mVs−1 between 1.0 V and 3.0 V; (b, c) Charge/discharge voltage profiles at 30 mAg−1 and 200 mAg−1 between 1.0 V and 3.0 V, respectively; (d) Cycling performance and Coulombic efficiency of 1DM-TiO2 at 30 mAg−1 between 1.0 V and 3.0 V. (e) Comparative cycling performance between different samples and Coulombic efficiency of 1DM-TiO2 at 200 mAg−1 between 1.0 V and 3.0 V; (f) Rate capabilities of 1DM-TiO2 .
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water and irreversible Li+ insertion sites (i.e., surface defects such as surface vacancies or voids connected to structural distortion caused by Jahn-Teller effect), respectively.[10] In good agreement with the discharge/charge voltage profiles of 1DM-TiO2 shown in Fig. 8(b, c). From the following cycles, we can clear observation that no changes in the peak positions during subsequent anodic scans. Moreover, overlapping and mirror-like CV curves further demonstrating the excellent reversibility of 1DM-TiO2 during electrochemical Li+ insertion/extraction process. Fig. 8b shows the discharge/charge profiles of 1DM-TiO2 at a current density of 30 mAg−1 from 1.0 to 3.0 V. The distinct potential plateaus at ca. 1.77 and 1.89 V in the discharge/charge curves are observed, corresponding to Li+ insertion into and extraction from anatase lattice. The discharge curve can be divided into three domains. The first domain, corresponding to the formation of conductive Lix TiO2 in solid-solution stage, is marked by the monotonous potential decrease.[9] The second domain is the well-known two-phase switching occurs with phase equilibrium of the Li-poor Li0.01 TiO2 (tetragonal) phase and the Li-rich Li0.55 TiO2 (orthorhombic) phase, representing the process of Li+ insertion into and extraction from the interstitial octahedral site of TiO2 reversibly.[10] The third domain is a sloped potential, attributable to the lithium storage on the surface as well as the formation of Li1 TiO2 phase.[11] The discharge and charge capacities in the first cycle are 614.8 and 231.8 mAhg−1 , respectively, with a first cycle capacity loss of 62%. As we mentioned above, the high initial capacity loss can be attributed to the adsorbed trace water and irreversible lithium insertion sites such as surface defects and voids connected to structural distortion caused by Jahn-Teller effect. It is consistent with the peculiar architecture of 1DM-TiO2 , which possess larger specific surface area and abundant mesoporous. In the second cycle, 1DM-TiO2 delivers discharge and charge capacities of 266.6 and 214.9 mAhg−1 , respectively, and the coulombic efficiency is 80.6%. In the 10th cycle, the coulombic efficiency increases to 96.3%. Obviously, the discharge/charge curves after 10th cycles almost overlapped with the first one, showing a good cycling performance of the 1DM-TiO2 . Fig. 8c displays the discharge/charge profiles of 1DM-TiO2 at a high current density of 200 mAg−1 from 1.0 to 3.0 V. They exhibit the same behavior with the curves which discharge/charge at a current density of 30 mAg−1 . But, in the first cycle, the discharge and charge capacities are 426.6 and 211.5 mAhg−1 , respectively (the capacity loss much decreased). After 10th cycle, the discharge/charge curves also almost overlapped with the first one, further demonstrating a good cycling performance of the 1DM-TiO2 at high current density. Fig. 8d shows the cycling performance of 1DM-TiO2 at a current density of 30 mAg−1 . It delivers a discharge capacity of 202.9 mAhg−1 after 70 cycles, with a coulombic efficiency of 92.9%. Fig. 8e presents the comparison of cycling performances between 1DMTiO2 and other different samples at a current density of 200 mAg−1 (1 C rate corresponds to the current density of 167.5 mAg−1 )[36]. Compared with TiO2 600 and CNT@TiO2 500Ar, it can be clearly seen that the 1DM-TiO2 demonstrate great enhanced cyclic capacity retention. The 1DM-TiO2 delivers a high discharge capacity of 166.2 mAhg−1 after 100 cycles and even can remains at 151 mAhg−1 in 500th cycle with a coulombic efficiency of 99.3%, demonstrating its excellent cycling stability. More importantly, the coulombic efficiencyis very close to 100%, indicating the facile transportation of lithium ion and electron, provided by 1D structure and mesopores features. In sharp contrast, TiO2 600 and CNT@TiO2 500Ar deliver a discharge capacity of 83.5 and 55.5 mAh−1 after 500 cycles, respectively. Furthermore, we also compared the cycling performance of 1DM-TiO2 with that of similar anatase TiO2 electrodes which had been reported, such as TiO2 hollow microspheres assembled by nanotubes[10] (150 mAh−1 after 500 cycles at a rate of 1 C), self-assembled ultrathin anatase TiO2 nanosheets[22] (160 mAh−1
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Fig. 9. Nyquist plots of the 1DM-TiO2 , TiO2 600 and CNTs@TiO2 500Ar after 5 discharge/charge cycles, the inset showing magnified Nyquist plots at high frequency range.
after 150 cycles at a current density of 150 mAg−1 ) and the TiO2 nanosheets with discrete integrated nanocrystalline subunits[36] (150 mAhg−1 after 100 cycles at 2 C). Compared to them, our product not only enhanced lithium storage properties and cycling performance, but also exhibited a more facile fabrication process. To further demonstrated its advantage for high rate LIBs application, the rate performance of 1DM-TiO2 was further tested under different current densities from 30 to 3000 mAg−1 . As shown in Fig. 8f, the capacities of 1DM-TiO2 under all current densities are quite stable, showing excellent rate capability. Specifically, the last reversible capacities of 1DM-TiO2 are 210.9, 159.8, 116.2, 96.9, 91.7, and 83.4 mAhg−1 at the 10th cycles of 30, 200, 800, 1600, 2000, and 3000 mAg−1 , respectively, indicating the good rate performance and stability of the 1DM-TiO2 . Electrochemical impedance spectroscopy (EIS) measurements were also carried out to better demonstrate the effect of peculiar architecture on the electronic conductivity of the 1DM-TiO2 . Fig. 9 shows the Nyquist plots of the 1DM-TiO2 , TiO2 600 and CNTs@TiO2 500Ar electrodes after 5 discharge/charge cycles. Obviously, all electrodes show Nyquist plots consisting of a depressed semicircle at high frequency range and an angled line in the low frequency range. The diameter of the depressed semicircle is correlated with the electron transfer resistance on the electrode interface, and the angled line is related to a diffusion controlled process. Apparently, the 1DM-TiO2 exhibits a similar diameter of the high frequency semicircle with CNTs@TiO2 500Ar, but much smaller than the TiO2 600 electrode (may attributed to trace carbon in 1DM-TiO2 ). Moreover, the 1DMTiO2 shows an more slope angled line compared with TiO2 600 and CNTs@TiO2 500Ar electrodes, demonstrating enhanced electron and lithium ion transport.[37] These good results discussed above can attribute to the peculiar architecture of 1DM-TiO2 , which possess unique integration of one-dimensional mesoporous structure and nanocrystal units. Because of this structure with larger specific surface area and abundant mesoporous can effectively improve pseudocapacitance charge storage, shorten diffusion path of Li+ , increase contact interface between the active materials and electrolyte and enhance the structure stability during cycling, finally contributing to excellent cycling stability and high lithium storage. In short, the excellent electrochemical properties of the 1DM-TiO2 are derived from the intriguing hierarchical mesoporous one-dimensional morphology which not only shortening Li+ and electronic diffusion distance, but also providing a thermodynamically stable system. Moreover, the peculiar structure can not only ensure the sufficient contact between active materials and electrolyte, but also
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effectively suppress and relax the agglomeration and strain upon cycling. 4. Conclusion In summary, we have successfully prepared one-dimensional mesoporous TiO2 composed of nanocrystals. This unique and intriguing architecture not only has the advantages of nanometersized building blocks and porous structure, but also possess the advantages of submicrometer-sized assemblies. These advantages made the architecture beneficial for increasing contact interface between the active materials and the electrolyte, enhancing structure stability and improving pseudocapacitance charge storage during cycling. As a result, the as-prepared product delivers a reversible discharge capacity of 151 mAhg−1 even after 500 cycles at a current density of 200 mAg−1 . The good lithium storage capacity and excellent cycling performance mainly attributed to the unique integration of one-dimensional mesoporous structure and nanocrystal units. Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Grant no. 21103108 and 21173148) and the SJTU-UM collaborative research project. References [1] N.D. Petkovich, S.G. Rudisill, B.E. Wilson, A. Mukherjee, A. Stein, Inorg. Chem. 53 (2014) 1100. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496. [3] F.Y. Cheng, J. Liang, Z.L. Tao, J. Chen, Adv. Mater. 23 (2011) 1695. [4] L. Shen, X. Zhang, H. Li, Y. Zhao, F. Bijarbooneh, V. Malgras, Y. Lee, Y.M. Kang, S.X. Dou, J. Am. Chem. Soc. 133 (2011) 19314. [5] H.L. jiang, X.L. yang, C. chen, Y.H. zhu, C.Z. Li, New J. Chem 37 (2013) 1578. [6] Y. Hu, L. Kienle, Y.G. Guo, J. Maier, Adv. Mater. 18 (2006) 1421. [7] C.J. Chen, X.L. Hu, Z.H. Wang, X.Q. Xiong, P. Hu, Y. Liu, Y.H. Huang, Carbon 69 (2014) 302. [8] T.B. Lan, Y.B. Liu, J. Dou, Z.S. Hong, M.D. Wei, J. Mater. Chem. A 2 (2014) 1102.
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