rutile-TiO2 hollow hierarchical architecture modified by SnO2 toward efficient lithium storage

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0

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Anatase/rutile-TiO2 hollow hierarchical architecture modified by SnO2 toward efficient lithium storage Changchao Jia a, Xiao Zhang b, Ping Yang a,* a b

School of Material Science and Engineering, University of Jinan, Jinan, 250022, PR China School of Chemistry University of New South Wales, Sydney 2052, Australia

article info

abstract

Article history:

Hierarchical architecture of anatase/rutile-mixed phases TiO2 with hollow interior was

Received 20 September 2017

successfully fabricated via a Topotactic synthetic method, including the synthesis of

Received in revised form

CaTiO3 precursors and transforming them into TiO2 through ion-exchange process. The as-

20 November 2017

synthesized TiO2 hierarchical architectures as the anode materials were used as lithium-

Accepted 27 November 2017

ion batteries (LIBs). Compared with TiO2 samples, the TiO2@SnO2-5% shows the

Available online xxx

improved lithium storage capacity, cycling performance and rate properties. The impedance of the TiO2 electrode decreases evidently after adding few amount of SnO2. The

Keywords:

hollow hierarchical structure with different compositions provide much more active sites,

Synthetic methods

and well connect interface among anatase, rutile, and SnO2, facilitating the electron and

Hierarchical architecture

ion transport quickly and efficiently. Addition appropriate number of SnO2 not only well

Titanium

kept the hierarchical architecture but also enhanced the capacity and conductivity of the

Electrochemistry

TiO2 sample. As a result, TiO2@SnO2-5% exhibited excellent lithium storage performance.

Lithium storage

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Lithium-ion batteries (LIBs) have many advantages of high energy density, low cost and long cycle life, which is considered as one of the most promising energy storage devices [1e3]. The three most important performance indicators of LIB are the capacity, cyclic stability and rate capability [4e6]. It is imperative to seek an active electrode material with above excellent performance indicators. TiO2 materials have been intensively investigated due to their versatile and easy modification of nanostructures, as well as other advantages, such as low-cost, security and environmental friendliness [7e10]. However, the low ionic and electrical conductivity

severely limited the storage capacity, inhibiting its practical electrochemical application [11,12]. Thus, the study on TiO2 as anode material for lithium mass storage to meet the increasing demand still remains a great challenge and has a profound significance. In order to overcome the above problem of low ionic and electrical conductivity, lots of measures had been undertaken by previous researches [13e15]. Phase composition plays an important role in the physical and chemical properties of TiO2. Commonly, TiO2 has four crystalline polymorphs: anatase, rutile, brookite, and TiO2 (B) [16e19]. TiO2 nanomaterials with a mixed phase showed superior electrochemical properties in LIBs. Recently, TiO2 (B) and anatase heterostructures have been reported to exhibit superior electrochemical property,

* Corresponding author. E-mail address: [email protected] (P. Yang). https://doi.org/10.1016/j.ijhydene.2017.11.153 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jia C, et al., Anatase/rutile-TiO2 hollow hierarchical architecture modified by SnO2 toward efficient lithium storage, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.11.153

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which fully shows the advantage of mixed crystals [20]. Fewer researchers have studied on the electrochemical performance of anatase and rutile TiO2 mixed phase, which have attracted great interest in LIBs. The high anisotropic of Liþ diffusion in rutile, attracts researchers to create rutile nanorods growth along c-axis, which named the channels as a “highway” for Liþ transport [21e24]. The tetragonal rutile TiO2 (P42/mnm) consists of TiO6 octahedral, sharing corners in the ab plane and edges in the c-direction. The Liþ diffusion coefficient along the c-direction is approximately 106 cm2 s1, much faster than that in the ab-plane (1015 cm2 s1). Besides, small size anatase TiO2 nanoparticles show superior performance in lithium ion batteries [13]. Thus, to construct a structure, constituting of one dimensional rod-like rutile TiO2 combined with anatase TiO2, is expected. The special structural characteristics and the low volume expansion (<4%) during Liþ insertion-desertion process, endows the rutile and anatase TiO2 heterostructures good structural stability and long cycle life. Additionally, built-in electric field could be formed at the interface between rutile TiO2 nanorods and small size anatase TiO2, which induces much lower lithium-ion diffusion resistance and facilitates its transport in both insertion and extraction processes. Unfortunately, the electrical conductivity and capacity of TiO2 material is limited, which needs to be further improved. SnO2 has similar crystal structure with rutile TiO2 and high theoretical capacity than TiO2 material, however, it has disadvantage of the large specific volume changes through charge and discharge processes [25,26]. Thus, added appropriate number of SnO2 into the structure is the key to enhance the TiO2 material conductivity, capacity and well kept the structure. On the other hand, it is well known that the properties of materials are strongly dependent on their morphology [27e30]. Severe aggregation would happened due to the high surface energy of nanoparticles. To establish a high-order multi-dimensional structure composed by low-dimensional nanoparticles may offer opportunities to ‘‘tune-in’’ properties that is desirable for the intended application [31,32]. Researches have been reported to synthesize TiO2 hollow shells, aiming at enlarging the specific surface area for increasing the reaction active site. Other than increasing the electrolyte/ electrode contact area, the nanostructure also provides a shorter path for Liþ transport to reduce ionic conductivity. Hollow structured nanomaterials as electrode materials exhibit an excellent electrochemical performance, owing to the enhanced diffusion kinetics and structural stability [33,34]. Up to now, various approaches have been developed to controllably synthesize different hollow structured metal oxides [35e37]. To achieve uniform and well-defined hollow structured metal oxides, the hard-templating method has been considered as one of the most effective and straight forward strategies [38]. However, simple template method can hardly achieve the requirement to construct a special structure which not only creates hollow structure, but also makes one dimensional rod-like rutile TiO2 combined with anatase TiO2. Previous researches reported that one dimensional anatase/rutile TiO2 hierarchical nanorods were acted as anode electrode materials, and the lithium storage capacity need to

be improved [14,39]. Herein, we used Topotactic synthetic method to obtain the waxberry-like TiO2 hierarchical architecture. The method makes anatase and rutile phase of TiO2 connect closely with each other. Meanwhile, the special hollow hierarchical structure provides much more active sites, which is in favour of the improvement the electrochemical performance. When Sn source added in the initial solution, the hierarchical structure of TiO2@SnO2 samples were fabricated. TiO2@SnO2-5% with well connect interface among anatase, rutile, and SnO2 as well as c-channel formed inside stacked (001) planes in rutile TiO2 nanorods, which facilitates the electron and ion transport quickly and efficiently, showing an excellent performance in LIBs.

Experimental section Synthesis of CaTiO3 and CaTiO3-Sn precursor microcubes In a typical synthesis, 0.11 g of Calcium chloride dihydrate (CaCl2 2H2O) and a certain amount of SnCl4 5H2O (0, 0.0169 and 0.0338 g, respectively) were dissolved in mixed solution of 15 mL of ethanol and 5 mL of poly(ethylene glycol) (PEG-200, AR). After stirring for a while, 0.33 mL of Titanium n-butoxide (TBT, AR) was injected in the above mixture. Then 0.24 g of NaOH was added into the mixture. After mixing well, the feedstock was poured into a 50 mL Teflon-lined stainless-steel autoclave, and heated at 180  C for 15 h in an oven. The obtained samples were washed with deionized (DI, resistivity 18 MW cm1) water for 3 times, and then dried at 60  C for further characterization and application. The addition amounts of SnCl4$5H2O were 0, 0.0169 and 0.0338 g, which corresponds to the obtained samples named as CaTiO3, CaTiO3-Sn-5%, CaTiO3-Sn-10%, respectively. (CaTiO3-Sn sample indicates the composite structure of CaTiO3 and CaSnO3).

Synthesis of hollow waxberry-like TiO2 and TiO2@SnO2 samples 75 mg of as-synthesized CaTiO3 or CaTiO3-Sn samples and 0.34 g of EDTA-2Na were dispersed in a mixed solution of ethylene glycol (EG, 10 mL) and DI water (30 mL). After mixing, the final mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 180  C for 12 h. The products were washed with water and dried at 60  C. The obtained powders were calcined at 400  C for 2 h with a temperature rising rate of 5  C min1. The as-obtained samples were denoted as TiO2, TiO2@SnO2-5%, and TiO2@SnO2-10%, respectively.

Characterization X-ray diffractometer (XRD, Germany Bruker D8-Advance) was used to characterize the crystal structure and phase composition of samples. High-resolution Raman spectrometer (LabRAM HR Evolution, HORIBA JOBIN YVON SAS) was carried out to analyse Raman spectra. Field-emission scanning electron microscope (SEM, QUANTA 250 FEG, FEI, USA) and the highresolution transmission electron microscopy (HRTEM, FEI

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Tecani F20 TEM) were applied for the morphology and structural characterization of the samples. Multifunction adsorption instrument (MFA-140, Builder Company, Beijing) works at 77 K to get the specific surface area and pore size distribution of the samples.

Electrochemical measurements The electrochemical properties of the prepared samples was carried out using CR2025 coin type cells with Li metal as the counter and reference electrodes at room temperature. The working electrode was fabricated by composition of the active materials, conductive material (carbon black), and binder (polyvinylidene fluoride, PVDF) at a weight ratio of 70:20:10 onto a copper foil collector. The cells were assembled in an Arfilled glovebox with the concentrations of moisture and oxygen below 1 ppm. Cyclic voltammograms (CV) were examined between 0.01 and 3.0 V (vs. Liþ/Li) at a scan rate of 0.2 mV s1 using a CHI 660E electrochemical workstation. The dischargecharge tests were obtained on a NEWARE battery tester. EIS (ZAHNER Elektrik) measurement was performed using a 10 mV AC (alternating current) amplitude under open circuit potential with 100 kHz-10 mHz frequency range.

Results and discussion Engineering hierarchical architecture of waxberry-like TiO2 with controllable phase of anatase and rutile was fabricated via a Topotactic synthetic method [40e42]. As shown in Scheme 1, the formation mechanism of hollow waxberry-like TiO2 hierarchical architecture mainly consists of two steps. Firstly, CaTiO3 microcubes were prepared in the mixture solvent of PEG-200 and ethanol. PEG-200 Polymer molecules absorbed on the surface of CaTiO3 tiny unit, which grown through the processes of orientated aggregation and Ostwald ripening. Finally nearly spherical cube-like structure of CaTiO3 was fabricated [43,44]. Secondly, EDTA-2Na as an effective chelating agent, making the ion-exchanging reaction happened in CaTiO3 structure [45].

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During the process, inner particles migrate to the cube surface to involve in ion-exchanging reaction, which results in the cube began to cavitation. Meanwhile, CaTiO3 was transformed into TiO2, accumulating on the cube surface. Through further cavitation and recrystallization process, well-designed waxberry-like TiO2 (Rutile-TiO2/anatase-TiO2) hierarchical structure was successfully obtained. Yin et al. had employed “silica-protected calcination” method which limits the growth of TiO2 during calcination by silica protection, leading to small anatase grains remained [46]. In order to control the phase ratio of anatase and rutile as well as the morphology of the waxberry-like TiO2 hierarchical architecture, Sn salts were added in the reaction system. CaSnO3 coated on the CaTiO3 surface, which may depressed the transformation of TiO2 from CaTiO3. The phase composition of the as-obtained samples will be discussed below. The XRD patterns of CaTiO3 and CaTiO3-Sn samples are shown in Fig. 1. The diffraction peaks of samples are well corresponding to bulk CaTiO3 (JCPDF No. 22-0153) and CaSnO3 (JCPDF No. 31-0312) phase. SEM images of CaTiO3 and CaTiO3Sn samples are shown in Fig. S1. Nearly spherical cubes of CaTiO3 samples were obtained as shown in Fig. S1a. When adding SnCl4 in the initial solution, CaTiO3 surface was coated with a thin CaSnO3 shell, which is clearly observed in Fig. S1c. TiO2 was derived from CaTiO3 through chelation reaction with EDTA-2Na. To improve the crystallinity of particles, the samples are treated by calcination under 400  C for 2 h. The obtained samples were characterized by XRD (Fig. 2a). The diffraction peaks of TiO2 samples are well corresponded to the phases of rutile TiO2 (JCPDS No. 21-1276) and anatase TiO2 (JCPD No. 21-2172). Some diffraction peaks of TiO2@SnO2 samples can match with SnO2 (JCPDS No. 41-1445). The peaks relative intensity of (101) in anatase and (110) in rutile presents upward tendency with increasing the adding amount of SnCl4. The phase ratio of rutile TiO2, anatase TiO2, and SnO2 were listed in Table 1. It indicates that phase ratio of anatase to rutile is controlled by the addition amount of SnCl4. The coexistence of anatase and rutile bi-phase in the TiO2 waxberry-like hierarchical architecture was further

CaTiO3 þ H2O þ EDTA-2Na / TiO2 þ EDTA-Ca þ 2NaOH

Scheme 1 e Illustration of formation mechanism of hollow waxberry-like TiO2 hierarchical architecture.

Fig. 1 e XRD patterns of CaTiO3 and CaTiO3-Sn samples.

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Fig. 2 e (a) XRD patterns and (b) Raman spectra of TiO2 and TiO2@SnO2 samples. (i) TiO2, (ii) TiO2@SnO2-5%, (iii) TiO2@SnO210%. XPS spectra of TiO2@SnO2-5%: (c) Ti 2P, (d) Sn 3d.

characterized by Raman spectroscopy analysis, which was showed in Fig. 2b. The peaks centered at 143 cm1 (Eg), 397 cm1 (B1g), 516 cm1 (A1g), and 639 cm1 (Eg) are well matched with anatase TiO2 modes. While three Raman active modes of multi-proton process (237 cm1), Eg (442 cm1) and A1g (611 cm1) are observed for rutile TiO2 [46]. Both anatase and rutile structure are well preserved in the TiO2 and TiO2@SnO2 samples, and the relative intensity of anatase and rutile peaks increased with adding SnCl4, which is consist with XRD pattern. SnO2 peaks are not observed clearly in XRD and Raman spectra, due to the peaks overlapped with rutile TiO2 and little amount of SnO2 existed in the TiO2@SnO2 samples. The XPS spectra of Ti 2p and Sn 3d for TiO2@SnO2-5% are shown in Fig. 2c and d, respectively. Two obvious peaks at 458.4 and 464 eV in the Ti 2p spectrum are assigned to Ti 2p3/2 and Ti 2p1/2, respectively. Similarly, the peaks at 485.9 and 494.4 eV correspond to the binding energies of Sn 3d5/2 and Sn

Table 1 e Properties of TiO2 and TiO2@SnO2 samples. Sample TiO2 TiO2@SnO2-5% TiO2@SnO2-10%

Anatase (%)

Rutile (%)

SnO2 (%)

SBET [m2 g1]

9.6 22.8 90.1

90.4 72.7 4

e 4.5 5.9

47.20 39.27 54.56

3d3/2, respectively. It demonstrates that SnO2 exist in the TiO2@SnO2 samples. As shown in Fig. 3, waxberry-like hierarchical hollow structure of TiO2 was successfully obtained. Nearly spherical cube-like structure of CaTiO3 transformed into TiO2 hollow structure, while nanorods vertically arranged on the thin sphere surface. The hierarchical structure with hollow interior can well be maintained after calcination, as shown in Fig. 3c. From Fig. 3d we can clearly distinguish the hierarchical structure that nanorods nearly radial alignment grow on the external surface of hollow sphere, while small size of nanoparticles keep close connect with the bottom of nanorods. TiO2@SnO2-5% samples also exhibit hierarchical architecture (as shown in Fig. 4), but the number of nanorods decreases while nanoparticles increase in comparison with TiO2. Fig. 4c is the high magnification image of square frame in Fig. 4b. The lattice spacing of 0.32 nm corresponds to the (110) planes of rutile TiO2, which reveals that the single crystal rutile nanorod grows along [001] c-axis direction [24]. Fig. 4d is the high magnification image of circle in Fig. 4b. The lattice spacing of 0.35 nm in red cycle and 0.34 nm in white square frame are attributed to the (101) planes of anatase and (110) planes of SnO2, respectively. Anatase TiO2 and SnO2 are nanoparticles, consisting of the shell of the sphere, which are well connected with the bottom of rutile nanorods. When adding large amount of SnCl4 in the initial solution, hollow

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Fig. 3 e SEM images of TiO2 samples without calcination, (c) SEM and (d) TEM images of TiO2 samples after calcination under 400  C.

structure was not clearly observed in the Fig. S2. Much more nanoparticles instead of nanorods consist of microspheres. XRD patterns and Raman spectra results both displayed that the phase ratio of anatase and rutile increased. It indicates that CaSnO3 coated on the surface of CaTiO3, hindering them transformed to rutile TiO2 nanorods. N2 adsorption-desorption isotherms are used to measure the specific surface areas and the pore size distributions of TiO2 products, as shown in Fig. S3. The specific surface areas are calculated by Brunauer-Emmett-Teller (BET) methods. The pore size distributions are determined by using the BarrettJoyner-Halenda (BJH) model from the desorption branch of the isotherm. All the samples are showed a type III isotherm (BDDT classification) with a type H3 hysteresis loop. TiO2@SnO2-10% sample exhibits a large specific surface area of 54.56 m2 g1, which is higher than that of TiO2 (47.20 m2 g1) and TiO2@SnO2-5% (39.27 m2 g1). The pore sizes of the three samples are all mainly located at mesoporous region, as shown in Fig. S3b. The theoretical capacity of rutile is twice than that of anatase, but its stability is not good for the irreversible insert/ desert Li-ion reaction. Thus, to study mixed phase TiO2 applied in LIB has profound significance for enhancing the capacity and stability of the material. In order to evaluate the

electrochemical performance of hierarchical TiO2 and TiO2@SnO2 hollow structure with different phase compositions, the sample was investigated as an anode material in LIBs. The electrochemical Li insertion properties of the TiO2 and TiO2@SnO2 samples were investigated in a Li and TiO2 half-cell configuration by CV between 0.01 and 3 V at a slow scan rate of 0.2 mV s1, which shown in Fig. 5aec. The cathodic peaks corresponded to discharge process are located at 0.7, 1.0, 1.4, and 1.7 V, respectively. Among these peaks, 0.7, 1.0, and 1.4 V are associated with the Li insertion in the rutile lattice (TiO2 transformed to LiTiO2), while 1.7 V is belong to the reaction with anatase (TiO2 transformed to Li0$5TiO2). Obviously, the structural stability of TiO2 is degraded in the low voltage (under 1 V), owing to the rutile TiO2 happened Li insertion process at 0.7 V [29,47]. The anodic peak of charged process located at 2.07 V, whether anatase or rutile, is corresponding to the lithium extraction out TiO2 lattice. Because little amount of SnO2 exist in the TiO2@SnO2 samples, the charge-discharge peaks of SnO2 may not obviously characterized or located at the same position with that of TiO2. It is found that an obvious change in cathodic peak potential was observed after the first cycle, especially in high rutile content of TiO2 and TiO2@SnO2-5% samples. The higher phase ratio of rutile to anatase, the more obvious change was occurred when

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Fig. 4 e (a) SEM, (b) TEM, and (c, d) HRTEM images of TiO2@SnO2-5%.

compared with the three samples (Fig. 5aec). As previously reported, anatase TiO2 structure has a good reversibility in electrochemical reactions. The result demonstrated that the irreversibility of rutile TiO2 structure in electrochemical reactions at first cycle, however, it keeps the good reversibility in the electrode after the first cycle. Fig. 6 shows discharge-charge voltage profiles of hierarchical structure of TiO2 and TiO2@SnO2 at a current rate of 1 C within a cut-off window of 0.01e3 V. The first discharge curve shows four poorly defined plateau regions at about 1.7, 1.4, 1.0, 0.7 V vs. Li/Liþ, which consists with the CV results. It is an interesting finding that the initial discharge process leads to a very high initial capacity of 442 mAh g1 (Fig. 6b), which greatly exceeds the theoretical capacity of rutile TiO2 (335 mAh g1 for Liþ þ e þ TiO2 $ LiTiO2). This is probably related to the hierarchical hollow structure with multiplephase, which provides more active sites for lithium storage and other irreversible reactions (or forming solid electrolyte interface) [34]. However, a lower capacity of 213 mAh g1 is obtained in the following charge process, indicating the low initial efficiency of 48%. It might be that rutile TiO2 phase happened irreversible reactions and formed solid electrolyte interface at the initial stage [34]. Electrodes show unstable cyclability, which may be ascribed to an activation process of the material [18,29]. But the discharge capacity of the TiO2@SnO2-5% hierarchical structure remains about 213 mAh g1 after 100 cycles at a current rate of 1 C (shown in Fig. 5d, and the results listed in Table 2). The performance is

superior to the other two samples of TiO2 and TiO2-SnO2-10%, as well as most previous reported TiO2-based anode materials (shown in Table S1). The reason can be concluded that the hierarchical structure of rutile radial alignment grow on the external surface of hollow sphere composed by anatase TiO2 and small amount of SnO2 nanoparticles, which give the TiO2@SnO2-5% sample with superior performance in electrochemical performance. When compared to anatase phase electrode material with similar geometry, rutile and anatase TiO2 hollow hierarchical heterostructure shows superior performance [48]. It may be that built-in electric field is formed at the interface between rutile TiO2 nanorods and small size anatase TiO2, which induces much lower lithium-ion diffusion resistance and facilitates its transport in both insertion and extraction processes. The specific capacities for hierarchical structure of TiO2 and TiO2@SnO2 are detected at different current rates (shown in Fig. 6d). The average specific capacities of TiO2 are 225, 185, 127, 101, 52 mAh g1 at current rates of 0.5, 1, 5, 10, 50 C, respectively. When the current was cycled back to 0.5 C, a capacity of 228 mAh g1 was resumed, which indicated that outstanding high-rate cycling performance resulting from their good structural stability. Although TiO2 contains high content of rutile phase, resulting in irreversible reaction happened in the electrode, the sample had kept well stability at high current rate. It can be concluded that the mixed phase composition as well as the hierarchical structure improved the stability of the material. The hierarchical structure of

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Fig. 5 e (aec) CV curves of hierarchical TiO2 and TiO2@SnO2 hollow structure between 0.01 and 3 V with a scan rate of 0.2 mV.s-1, (a) TiO2, (b) TiO2@SnO2-5%, (c) TiO2@SnO2-10%. (d) Cycling performance of the samples at a current density of 1 C.

Fig. 6 e Chargeedischarge voltage profiles of (a) TiO2, (b) TiO2@SnO2-5%, and (c) TiO2@SnO2-10% for the 1st, 2nd, 10th, 50th, and 100th cycles at a current density of 1C, (d) Rate performance of TiO2 and TiO2@SnO2 samples at various current rates from 0.5 to 50 C. 1C ¼ 170 mA g¡1. Please cite this article in press as: Jia C, et al., Anatase/rutile-TiO2 hollow hierarchical architecture modified by SnO2 toward efficient lithium storage, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.11.153

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Table 2 e Electrochemical performance parameters of TiO2 and TiO2@SnO2 samples. Sample TiO2 TiO2@SnO2-5% TiO2@SnO2-10%

Initial coulombic efficiency (%)

initial discharge capacity (mAh g1)

100th discharge capacity (mAh g1)

45 48 56

400 442 408

196 213 179

Fig. 7 e Nyquist plots obtained from electrodes composed of TiO2, TiO2@SnO2-5%, and TiO2@SnO2-10%.

rutile nanorods nearly radial alignment grow on the anatase hollow spherical surface is in favour of Liþ fast transport. Hollow structure provides large contact area between electrolyte and electrode, giving a shorter path for Liþ transport. At the same time, hierarchical structure of rutile, anatase and SnO2 may establish internal electric field facilitating electron and ion transferred directionally as well as the c-channel exist in the rutile TiO2 nanorods making a “highway” for Liþ transport [12,24,30,31]. To better understand the electrochemical kinetic properties of the samples, electrochemical impedance spectroscopy (EIS) is carried out and the Nyquist plots of the TiO2 and TiO2@SnO2 samples are shown in Fig. 7. They all show typical

characteristics of an LIB anode. An equivalent circuit model is shown in the insert of Fig. 7 for analyzing impedance spectra. Rs expresses the solution resistance which was obtained by the small intercept. Rct shows the charge transfer resistance, depicting a semicircle in the high to intermediate frequency range. Cdl is the constant phase-angle element which involves the double layer capacitance. W is the Warburg impedance of Li ion diffusion into the active materials, demonstrating a straight line at low frequency [49e51]. Rs of TiO2, TiO2@SnO25%, and TiO2@SnO2-10% electrodes are 5.04, 4.78, 4.27 U, which has little difference among the three samples. Rct of TiO2, TiO2@SnO2-5%, and TiO2@SnO2-10% electrodes are 665, 150, and 145 U, respectively. The impedance of the electrode decreases evidently after adding SnO2 into the TiO2 samples. And straight line of the TiO2@SnO2-5% sample angled at approximately at 45 to the X-axis. This suggests that the TiO2@SnO2-5% sample have the lower activation energy for the Liþ diffusion and undergo a fast faradaic reaction, exhibiting an increased high-rate performance. Thus, hierarchical architecture of waxberry-like TiO2@SnO2-5% sample exhibit superior performance of Liþ storage and rate capability. The mechanism of Liþ transport path in hierarchical architecture of waxberry-like TiO2 with cchannel rutile nanorods is shown in Scheme 2.

Conclusions Hierarchical architecture of waxberry-like TiO2 with c-channel rutile nanorods was fabricated via a Topotactic synthetic method. The phase composition of TiO2 is controlled through SnO2, and the ratio of anatase to rutile was increased with adding SnCl4. The electrochemical results demonstrate that the hollow hierarchical architectures exhibit superior lithium storage properties. The discharge capacity of TiO2@SnO2-5% remains about 213 mAh g1 after 100 cycles at a current rate of 1 C, and a stable capacity of 60 mAh g1 is maintained at high rate of 50 C. The excellent performance ascribes the special structure, which provides much more active sites, and well connect interface among anatase, rutile, and SnO2, facilitating the electron and ion transport quickly and efficiently. Meanwhile, c-channel exists in the rutile TiO2 nanorods making a “highway” for Liþ transport. Moreover, this work also provides a feasible route to design novel hierarchical hollow structure by an environmentally benign synthetic method.

Acknowledgements Scheme 2 e Schematic illustration of Liþ transport path in hierarchical architecture of waxberry-like TiO2@SnO2-5% sample with c-channel rutile nanorods.

This work was supported in part by the project from the National Basic Research Program of China (973 Program, 2013CB632401), the projects from the National Natural Science

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Foundation of China (51572109, 51501071, 51402123, and 51402124), and Graduate Innovation Foundation of University of Jinan, GIFUJN YCXB15002.

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2017.11.153.

[16]

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Please cite this article in press as: Jia C, et al., Anatase/rutile-TiO2 hollow hierarchical architecture modified by SnO2 toward efficient lithium storage, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.11.153

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Please cite this article in press as: Jia C, et al., Anatase/rutile-TiO2 hollow hierarchical architecture modified by SnO2 toward efficient lithium storage, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.11.153