Hollow mesoporous frameworks without the annealing process for high-performance lithium–ion batteries: A case for anatase TiO2

Chemical Engineering Journal 228 (2013) 724–730

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Hollow mesoporous frameworks without the annealing process for high-performance lithium–ion batteries: A case for anatase TiO2 Junyao Shen a,b, Hai Wang a,b,⇑, Yu Zhou a,b, Naiqing Ye a, Yuanhao Wang c,⇑, Linjiang Wang a,b,⇑ a

State Key Laboratory Breeding Base of Nonferrous Metals and Specific Materials Processing, Guilin University of Technology, Guilin 541004, PR China College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, PR China c Department of Building Services Engineering, Faculty of Construction and Environment, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b

h i g h l i g h t s  The TiO2 anode with mesoporous shell was formed by one-step hydrothermal method.  The effects of hydrothermal temperature on the performance of LIBs were studied.  The TiO2 anode without annealing achieved superior electrochemical properties.  The synthesis method of anodes provided a new strategy for high-performance LIBs.

a r t i c l e

i n f o

Article history: Received 24 January 2013 Received in revised form 5 May 2013 Accepted 14 May 2013 Available online 22 May 2013 Keywords: Lithium–ion batteries Titanium dioxide Mesoporous structure Electrochemical properties Hydrothermal

a b s t r a c t Hierarchically porous hollow TiO2 microspheres composed of interconnect nanocrystals have been synthesized by a one-pot hydrothermal method without further thermal annealing. The as-prepared materials are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and electrochemical measurements. FESEM shows that the resultant hierarchical mesoporous TiO2 hollow microspheres (HTMs) comprised mesoporous shells with interconnected anatase nanocrystals. TEM reveals that the interconnected structure of hollow microspheres almost remained unchanged with increasing hydrothermal reaction time. The electrochemical measurements results showed that the initial Li insertion/extraction capacity of the electrodes obtained at 180 °C for 15 h is 158 mA h g1 and 151 mA h g1 at 5 C, respectively. Moreover, in the 100th cycle, the reversible capacity still remains about 131 mA h g1, indicating excellent cycling stability and good highrate performance. The electrochemical impedance spectroscopy measurements indicated that the surface reaction kinetics of TiO2 nanocrystals were improved significantly, which may be attributed to fast charge transfer in the interconnected TiO2 nanocrystals structure. With the advantages of low cost and ease of processing, these HTMs are not only promising LIBs anode materials but also provide a conceptual route of fabricating oxides electrodes in high-performance LIBs in the future. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction The ever-growing demand for the utilization of clean energy has prompted a number of studies to study a series of transition metal oxides for advanced rechargeable lithium–ion batteries (LIBs) due to their potential applications as power sources for micro- and nanodevices [1–3]. Among the developed anode materials, TiO2 is one of the most promising candidates due to its low cost, improved safety, and much larger reversible capacity compared to graphite [4–7]. On one hand, TiO2 is structurally stable, for example, anatase ⇑ Corresponding authors. Address: College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, PR China. Tel.: +86 773 5896 672; fax: +86 773 5896 671 (H. Wang, L. Wang). E-mail addresses: [email protected] (H. Wang), [email protected] (L. Wang).

TiO2 exhibits a special body-centered space group I41/1md crystal structure, and the TiO6 octahedra shares two adjacent edges with two other octahedral [8,9]. This unique three-dimensional architecture with large amounts of open channels facilitates Li/TiO2 electrochemical reaction during the charge–discharge process. On the other hand, TiO2 is itself not only a fast and low voltage insertion host for Li, but also an abundant, low-cost and environmentally benign electrode material. These features make TiO2 particularly attractive for LIBs. In recent years, among various types of shapes, hollow microspheres as the anode materials have attracted much attention in high-performance LIBs [10–15]. Numerous studies indicate that these microstructure materials composed of interconnected nanocrystals frameworks within mesoporous shells exhibited high specific surface area, shortened lithium ion diffusion pathways, and

1385-8947/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.05.050

J. Shen et al. / Chemical Engineering Journal 228 (2013) 724–730

the extensive percolation networks of increased electrode–electrolyte interfaces. So far, a variety of chemical strategies have been developed for synthesizing hierarchical mesoporous hollow TiO2 microspheres, the reaction procedure often introduced either surfactants or metal–organic titanium precursor [16–20]. Such surfactants or residual organics, if they remain the surface of nanocrystals, may eventually affect the LIBs performance. Currently, the most commonly used method is to remove the surfactants or residual organics by annealing in air. Specifically, the TiO2 electrode materials in LIBs were generally prepared with an elevated-temperature heat treatment, with temperatures typically greater than 400 °C [6,12,16–18,20]. In addition to burn out the surface absorbed organic residues, another main purpose of such heat treatment is to increase mesoporous TiO2 nanocrystals crystallinity and the interconnection between the nanocrystals. It should be noted that the size of the as-prepared TiO2 nanocrystals via a thermal annealing process is usually too large to maintain smaller nanocrystals, resulting in low specific surface area. For LIBs applications, one cannot be ignored problem is that the decreases in the specific surface area will lead to poor electrochemical properties. In most cases, the relationship between low temperature and high specific surface area could be easily understood. Therefore, to prepare TiO2 electrodes with well-interconnected nanocrystals frameworks at low temperature, it is of both fundamental and practical importance to explore the effects of TiO2 electrode materials without the extra annealing process on the LIB performance. Until now, it is not entirely clear, however, whether the degree of crystallinity and interconnection of the TiO2 nanocrystals can be obtained simultaneously by the change of the hydrothermal reaction time. What’s more, the effects of hydrothermal reaction time on the LIBs properties had yet to be further revealed. The investigation of these issues would provide a conceptual route of fabricating other oxides electrodes in highperformance LIBs in the future. Herein we report a simple one-step hydrothermal synthesis of the hollow TiO2 microspheres (HTMs) composed of interconnected nanocrystals frameworks without the conventional heat treat process. This simple HTM structure offers two important features. (1) As-formed nanocrystalline structure can create three-dimensional interconnected electron transport pathways for efficient electron transport. (2) In the aspect of structural stability, the HTM is constructed from mesoporous networks, which ensures the structure stability against volume expansion, which is favorable for improved cycling stability. Further, we investigated the HTM-based LIBs performance via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) characterizations, with a focus on the effects of hydrothermal time on electrochemical properties. The as-prepared electrode materials have a reversible capacity of 151 mA h g1 at the first charge–discharge process and charge capacity is still up to 131 mA h g1 at 5 C (1 C = 170 mA g1) after 100 cycles, showing excellent cycling stability and superior rate capability.

2. Experimental 2.1. Preparation of TiO2 hierarchical hollow mesoporous microspheres TiO2 hierarchical hollow mesoporous microspheres were prepared via a simple hydrothermal method, using tetraisopropyl titanate as the Ti source, oxalic acid as a chelating agent and Cetyltrimethyl Ammonium Bromide (CTAB) as soft template. In a typical experiment, 2 g of oxalic acid and 2 g of CTAB were dissolved in 60 ml deionized water to form a transparent solution. Then 1 ml tetrabutyl titanate was added to the above solution under continuous stirring. Subsequently, the resulting suspension

725

was transferred into a Teflon-lined stainless steel autoclave (100 ml volume) for hydrothermal treatments at 180 °C for different time (5 h, 10 h and 15 h). Finally, the autoclave was cooled to ambient temperature naturally. Precipitates were filtered, and washed with deionized water and dried at 80 °C in air for 24 h. The samples were labeled as HTM-5 h, HTM-10 h and HTM-15 h, respectively, according to the reaction time. The obtained samples were subjected to electrochemical characterization. 2.2. Characterization of TiO2 hierarchical hollow mesoporous microspheres The crystal structure was performed by X-ray diffraction (XRD) analysis with a PANanalytic X’Pert spectrometer using Cu Ka radiation, k = 0.15405 nm. The surface morphologies of the samples were observed on field-emission scanning electron microscope (JEOL JSM-7000F, Japan). The high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM-2010F microscopy operating at 200 kV. The Nitrogen adsorption–desorption isotherm and Barret–Joiner–Halenda (BJH) poresize distribution was measured using a Micromeritics ASAP 2020 system. The sample was degassed at 180 °C and 106 Torr for 24 h at 77 K prior to the measurement. 2.3. Electrochemical measurements The electrochemical measurements were carried out using twoelectrode Swagelok cells with pure lithium metal as both the counter electrode and the reference electrode at room temperature. The working electrode consisted of active material, a conductive agent (carbon black, Super-P-Li), and a polymer binder polyvinylidene difluoride, PVDF in a 70:20:10 weight ratio. Celgard 3400 membrane was used as a separator and 1.0 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 volume, Novolyte Technologies, USA as the electrolyte). Cell assembly was performed in an Ar-filled glove box with concentrations of moisture and oxygen below 1.0 ppm. The charge–discharge cycling performance and cyclic voltammetry (1–3 V, 0.2 mV s1) was performed at room temperature using an electrochemical workstation (CHI 660 C) using a NEWARE battery tester at different current rates (1 C = 170 mA g1) with a voltage window of 1–3 V. The impedance measurements were performed at frequencies ranging from 102 to 105 Hz). 3. Results and discussion Scheme 1 shows the processes for fabricating the HTM. The amorphous TiO2 nanocrystals were initially prepared by chelation effect of tetrabutyl titanate and oxalic acid. They were then chemical deposition on the CTAB. It has been known that CTAB surfactant can form colloidal vesicles in polar media (i.e., water in our case), which act as a soft template for the self-assembly of TiO2 nanocrystals units. During the hydrothermal treatment, the hydrothermal reaction time would have a potential effect on the crystallinity of the samples. All the samples are obtained under the hydrothermal conditions at 180 °C. Fig. 1 shows the XRD patterns of the samples obtained after hydrothermal treatment at 5 h, 10 h and 15 h, respectively. All the XRD diffraction peaks of the samples in Fig. 1 can be indexed to tetragonal anatase TiO2 (JPCDS Card 84-1286, space group: I41/amd, a = 3.782 Å, c = 9.502 Å) and the crystallinity of the samples 1–3 is gradually improved, which could be confirmed by gradually enhanced diffractions. In the prolonged hydrothermal treatment, the sharper characteristic peaks of the anatase TiO2 can be observed, which reflected the continuous growth of the grains and more complete crystalline. According to

726

J. Shen et al. / Chemical Engineering Journal 228 (2013) 724–730

O HO

Ti O

OH

+

O O

O

chelation

oxalic acid

tetrabutyl titanate

Chemical deposition N+ Br -

Cetyltrimethyl Ammonium Bromide

hydrothermal

5h

10 h

15 h

Scheme 1. Schematic illustration of the fabrication of TiO2 hierarchical hollow mesoporous microspheres.

Fig. 1. XRD patterns of hollow mesoporous TiO2 microspheres at a constant reaction temperature of 180 °C for different time (5 h, 10 h and 15 h).

the Debye–Scherrer equation (D = kk/bcos h, k = 0.89), the average grain sizes were estimated 15.1 nm (5 h), 18.7 nm (10 h) and

20 nm (15 h), respectively. All these results demonstrated that the increased hydrothermal reaction time leads to higher crystallinity of the products. The specific surface area and porosity of the HTM-5, HTM-10 and HTM-15 were investigated by using the N2 adsorption–desorption isotherms, respectively, as shown in Fig. 2. Interestingly, the starting point of a hysteresis loop is gradually increased from 0.3 (HTM-5), 0.4 (HTM-10) and 0.6 (HTM-15) and all the three hysteresis loops with a stepwise adsorption and desorption branch are observed, all of which are similar in shape. According to the IUPAC classification [21], the three sample exhibits IV curves with a H3 hysteresis, indicating a three dimensional intersection. The pore size distribution of the three samples indicates that the HTM is hierarchically porous of mesopores and micropores. In fact, this type of pore structure has been widely applied in the photocatalytic field [22,23]. Such a hierarchical pore structure is essential to ensure a good electrochemical performance, since the mesoporous channels allow rapid electrolyte transport, while the microporous provide the HTM with higher surface areas and more surface active sites. The BET (Brunauer–Emmett–Teller) specific surface area, the pore volume and an average pore diameter of are shown in Table. 1. The measured pore diameter is in good agreement with the value determined by TEM and indicates that the nanocrystals of

J. Shen et al. / Chemical Engineering Journal 228 (2013) 724–730

727

Fig. 2. Nitrogen adsorption–desorption isotherms and the corresponding pore size distribution plots (inset) of the HTM-5, HTM-10 and HTM-15.

Table 1 Specific surface area, pore volume and pore size of the HTM-5 h, HTM-10 h and HTM15 h. Samples

HTM-5 h

HTM-10 h

HTM-15 h

Specific surface area (m2 g1) Pore volume (cm3 g1) Average pore size (nm)

110.8 0.2240 5.8

112.1 0.2762 7.8

97.8 0.2822 8.9

microspheres are tightly adhered to each other. Such a HTM mesoporous microstructure with large surface area, high crystallinity and nanocrystalline aggregates is a favorable for high-performance

LIBs. Specifically, this mesoporous structure could have potential capability to accommodate strain and structure changes during high-rate charge–discharge process. The morphology and microstructure of the as-prepared samples were analyzed by electron microscopy characterization methods. Fig. 3 shows the effect of hydrothermal reaction time on the morphology change of HTM. The low (left) and high (middle) magnification FESEM images and TEM images (right) of the samples synthesized for 5 h (a), 10 h (b) and 15 h (c), respectively. The structural features of the series of samples, HTM-5 h, HTM-10 h and HTM-15 h, exhibited similar topological morphology under different hydrothermal time. In the TEM images, a strong contrast

Fig. 3. Time-dependent HTM: low (left) and high (middle) magnification FESEM and TEM image (right) of the samples synthesized at 180 °C for different reaction times: 5 h (a), 10 h (b) and 15 h (c), respectively.

728

J. Shen et al. / Chemical Engineering Journal 228 (2013) 724–730

difference with a dark edge and relatively light inner center shows the features of hollow structure of samples and mesoporous structure. The synthesized hollow microspheres had a diameter of ca. 1 lm and consisted of nanocrystals with an average of 20 nm in diameter (Fig. 3). From the TEM images (Fig. 3a–c), it can be clearly observed that these nanocrystals are primary building units and constituent components of the final hollow mesoporous structure. This indicates that the hydrothermal reaction leads to the aggregation of single nanocrystals within the entire mesoporous shell, which is likely associated with interfacial tension and van der Waals attractive forces. In our previous work, we have demonstrated that the aggregation of nanocrystals plays an important role in high-performance TiO2-based LIBs [24,25]. To investigate the effects of hydrothermal reaction time on the electrochemical stability of HTMs, cyclic voltammetry (CV) cycles of HTM-5 h, HTM-10 h and HTM-15 h for the 3rd cycle were compared, at a scan rate of 0.2 mV s1 in the voltage range 1–3 V, respectively, as shown in Fig. 4a. One cathodic peak and one anodic

peak are observed for the three electrodes. Based on the Li/TiO2 discharge þ electrochemical reaction equation TiO2 þ xLi þ xe Lix TiO2 charge [26–28]. The first reduction peak in the range of 1.5–1.8 V is ascribed to the delithiation process of LixTiO2, while the second oxide peak in the range of 2.0–2.5 V to the lithium intercalation process of TiO2. Compared HTM-5 h and HTM-10 h with HTM-15 h, it can be clearly seen that HTM-15 h with the longest hydrothermal reaction time have a higher electrochemical stability than that of the other two electrodes. The potential difference DEp between the anodic and cathodic peak potentials, which is a direct reflection of the electron transfer rate of electrodes. The narrower the gap between the two peaks, the faster the electron transfer rate. Compared with HTM-5 h and HTM-10 h electrode, the redox peaks observed at the HTM-15 h (Fig. 4a) were much narrower. It can be clearly seen from the comparison that the direct electron transfer between the HTM-15 h and electrolyte was greatly improved. Moreover, the change trends of the peak currents is much highest on the HTM-15 h electrode than that on the HTM-5 h and HTM-

Fig. 4. Electrochemical performance of HTM electrodes cycled between 1.0 and 3.0 V vs. Li/Li+: (a) Typical cyclic voltammetric curves for the samples (5 h, 10 h and 15 h) with a scan rate of 0.2 mV s1 in the voltage range 1–3 V. Electrolyte solution: 1.0MLiPF6 + EC/DEC (1:1, volume); (b) Charge–discharge voltage profiles of HTM-15 h at a current density of 5 C from 1st to 100th; (c) cycling performance of HTM electrodes (5 h, 10 h and 15 h) at 5 C from 1st to 100th; (d) galvanostatic insertion and desertion curves of HTM electrodes measured at 5 C after 1th at 1–3 V vs. Li/Li+.

J. Shen et al. / Chemical Engineering Journal 228 (2013) 724–730

10 h electrode, that is, HTM-15 h > HTM-10 h > HTM-5 h, confirming that hydrothermal reaction time played an important role in facilitating the electron transfer. The reason that the prolonged hydrothermal reaction time could facilitate the electron transfer rate could be elucidated as follows. First, the interconnected TiO2 nanocrystals may probably act as a continuous conductive framework to favor the electron transfer. It should be noted that the peaks intensity in all CV increase gradually with the increasing the reaction time from 5 to 10 h, indicating HTM-15 h has high crystallinity after a longer reaction time. This is also confirmed by the XRD results in Fig. 1. Second, the mesoporous networks ensure the structure stability against volume expansion, which is favorable for improved electrochemical stability. Similar trends have been observed by other research groups, however, the experimental results revealed herein for the three electrodes is quite different from the previously reported ones, in which an extra annealing process was required [9,16,18,28]. The galvanostatic charge–discharge curves recorded during the first 100 cycles at a current density of 5 C (850 mA h g1) in the voltage range of 1.0–3.0 V vs. Li/Li+ for HTM-15 h are displayed in Fig. 4b. Generally, whole lithium–ion storage mechanism for anatase TiO2 during the discharge procedure can be divided into three regions, which consist of solid solution for Li and TiO2, biphasic changes to Li-rich orthorhombic Li0.5TiO2 and new extra Li storage in the nano-sized material surface to break the maximum value of insertion coefficient of 0.5 for bulk anatase. For the first discharge curve, the first region from 1.6 to 3 V was less 25 mA h g1 and subsequent region reached a long and flat plateau, which are related to the phase transition between the tetragonal and orthorhombic phases with Li insertion into anatase TiO2. The initial charge and discharge capacity of the HTM-15 h were 151 and 158 mA h g1, respectively, indicating a reversible efficiency as high as 95.6% for lithium ion insertion and extraction, and the stabilized capacity was as high as 131 mA h g1 after 100 cycles at 5 C. Quite encouragingly, for the first time, the superior electrochemical performance was achieved by one-pot hydrothermal method without the extra annealing process. With further optimization of the reaction conditions including choosing surfactants, proper pore size and volume, and hydrothermal reaction time, higher performance might be achievable. Fig. 4c displays the capacity-cycle number curves from the first cycle to the 100th cycle for the HTM-5 h, HTM-10 h, and HTM-15 h electrodes at the current density of 5 C. The HTM-15 h electrode exhibits good cycling performance and a high reversible specific capacity of approximately 131 mA h g1 up to 100 cycles, with a high coulombic efficiency of nearly 100%. The phenomenon may be attributed to several reasons. It is highly believed that the main reason is that the degree of the interconnectivity between nanocrystals is increased due to the nanocrystals aggregation, which may provide an effective electron transfer pathway. Interestingly, the HTM electrodes can effectively create three-dimensional mesoporous frameworks only by adjusting the hydrothermal reaction time. In addition, the crystalline degree of the HTM may provide the contribution to improve the cycling stability. The crystalline is often considered as an effective way to increase the cycling stability of electrode materials because of the fact that it can stabilize the nanostructure and improve the conductivity of host materials, which is confirmed by the results observed in the electro-chemical impedance spectroscopy (EIS) analysis. It should be noted that the battery performance was achieved in our first attempt under nonoptimized conditions. The value is comparable to the previously reported literature [9,16,18,28]. In a previous study, the TiO2 anodes prepared using a similar hydrothermal method including further annealing process at 400 °C for 3 h. Considering the hierarchical porous and hollow structure are two beneficial factors for high-performance LIBs, therefore, we conclude that the anodes

729

without further annealing process would also achieve superior electrochemical performance. Experimental results elegantly support our preliminary ideas. The permeation transition occurred at 31 °C, which was close to the LCST of pure PNIPAAm in water and is consistent with the transition temperature determined by the contact angle measurements. These results demonstrate the hydration transition behavior of PNIPAAm was maintained in the hybrids. The electrochemical behavior of the HTM-5 h, HTM-10 h and HTM-15 h depending on the hydrothermal reaction temperature were investigated by EIS, respectively. The impedance spectra were obtained at the 30th of the 100% state of charging (SOC). Fig. 5 compares the Nyquist plots of the three electrodes and the fitting results using an equivalent circuit model are shown in the inset of Fig. 5. The high-frequency regions of these Nyquist plots are shown in the inset of Fig. 5 for clarity. The two semicircles (high and intermediate frequencies) and a low-frequency tail can easily be observed. In this equivalent circuit (inset), R0 and Rct are the total ohmic resistance of the separator, electrolyte, and contacts between the electrode materials and the current collector, and charge transfer resistance, respectively. Constant phase element CPE in series with the respective capacitances (Cs and Cct) are introduced to account for the depressed semicircles, Wd represents the Warburg impedance reflecting the diffusion of electrolyte ions into the active materials, which is associated with the inclined line at low frequencies, and Rf corresponds to the SEI layer resistance. For the three electrodes, the total resistance (R0) is similar. While the Rct is much smaller for the HTM-15 h electrode (Rct = 27.8 X) than for the HTM-5 h (Rct = 71 X) and HTM-10 h (Rct = 47 X) electrodes, which indicates that the interconnected nanocrystals could enable much easier charge transfer at the electrode–electrolyte interface and consequently decrease the overall LIBs internal resistance. The improved capacity and rate performance of the HTM15 h may also due to reduced solid electrolyte interphase (SEI) formation compared to HTM-5 h and HTM-10 h electrodes. It should be noted that the resistance of the SEI film for the three electrodes was also decreased from 54 (HTM-5 h), 22 (HTM-10 h) to 15 X (HTM-15 h) with the increase of the hydrothermal reaction time. High resistance of SEI layer likely causes a poor electrical contact between electrodes and the current collector and slower Li+ diffu-

Fig. 5. The Nyquist plots for AC impedance spectra of HTM electrodes at the 30th of the 100% state of charging (SOC). Dots represented the experimental data, and solid curves represented the fitted curve. The fitting results using the equivalent circuit shown in the inset.

730

J. Shen et al. / Chemical Engineering Journal 228 (2013) 724–730

Table 2 Fitted electrochemical parameters of LIBs using HTM as anodes as determined from the EIS analysis. Electrodes (X)

HTM-5 h

HTM-10 h

HTM-15 h

Rs Rct Rs

3.7 71 54

3.5 46.8 22

3.5 27.8 15

sion kinetics, which could eventually lead to capacity fading. Therefore, the better control of SEI formation is expected to further improve the cycle stability. The effects of hydrothermal time on the formation of SEI layer is seldom reported and rarely data were presented in previous studies. Further research would be carried out in the future. Table 2 lists the parameters of the equivalent circuit for HTM-5 h, HTM-10 h and HTM-15 h electrodes after fitting the diameter of the semicircular curve. This large capacity and cycling performance of the HTM-15 h electrode could be explained as follows: First, the hollow spheres are capable of accommodating large volume expansion without breaking. Second, the TiO2 nanocrystals are naturally interconnected, which acts as charge transfer frameworks that enable fast lithium diffusion and high rate capability. The combination of these factors indicates that this geometry is promising for use in LIBs. 4. Conclusions TiO2 hierarchical hollow mesoporous microspheres was synthesized by a simple hydrothermal process, and the effects of hydrothermal reaction time on its electrochemical characteristics were investigated in this study. The FESEM and TEM results show that as-synthesized hollow microspheres had a diameter of ca. 1 lm and consisted of nanocrystals with an average of 20 nm in diameter within the mesoporous frameworks. The electrochemical results clearly demonstrated that the electrode properties of the HTM-15 h were much better than those of the HTM-5 h and HTM-10 h electrodes. A large capacity of 131 mA h g1 can be kept after 100 cycles at 5 C. The excellent electrochemical performance of the HTM-15 h could be related to the combined effects of the interconnected nanostructure and mesoporous microstructure, which would contribute together to enhance structural stability and improve lithium diffusion kinetics. The synthesis process employed is very simple and convenient, and does not require an additional temperature treatment. The method presented here could also be adopted to other metal oxides in LIBs. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC-51064006), Department of Education, Guangxi Zhuang Autonomous Region of China (Grant Nos. 200103YB061 and 201010LX188) and the startup fund from Guilin University of Technology. References [1] M. Arm, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] P.G. Bruce, Energy storage beyond the horizon: rechargeable lithium batteries, Solid State Ionics 179 (2008) 752–760.

[3] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteries, Chem. Mater. 22 (2010) 587–603. [4] M. Li, Y. Liu, H. Wang, Q. Ye, H. Shen, Synthesis of TiO2 nanorings and nanorods on TCO substrate by potentiostatic anodization of titanium powder, Cryst. Res. Technol. 46 (2011) 413–416. [5] Y. Ren, Z. Liu, F. Pourpoint, A.R. Armstrong, C.P. Grey, P.G. Bruce, Nanoparticulate TiO2 (B): an anode for lithium–ion batteries, Angew. Chem. Int. Ed. 2 (2012) 1–5. [6] K. Saravanan, K. Ananthanarayanan, P. Balaya, Mesoporous TiO2 with high packing density for superior lithium storage, Energy Environ. Sci. 3 (2010) 939. [7] J.-Y. Shin, D. Samuelis, J. Maier, Sustained lithium-storage performance of hierarchical, nanoporous anatase TiO2 at high rates: emphasis on interfacial storage phenomena, Adv. Funct. Mater. 21 (2011) 3464–3472. [8] J.S. Chen, Y.L. Tan, C.M. Li, Y.L. Cheah, D.Y. Luan, S. Madhavi, F.Y.C. Boey, L.A. Archer, X.W. Lou, Constructing hierarchical spheres from large ultrathin anatase TiO2 nanosheets with nearly 100% exposed (0 0 1) facets for fast reversible lithium storage, J. Am. Chem. Soc. 132 (2010) 6124–6130. [9] J. Kim, J. Cho, Rate characteristics of anatase TiO2 nanotubes and nanorods for lithium battery anode materials at room temperature, J. Electrochem. Soc. 154 (2007) A542–A546. [10] J.S. Xu, Y.J. Zhu, Monodisperse Fe3O4 and gamma-Fe2O3 magnetic mesoporous microspheres as anode materials for lithium–ion batteries, ACS Appl. Mater. Int. 4 (2012) 4752–4757. [11] G.Q. Zhang, L. Yu, H.Bin Wu, H.E. Hoster, X.W. Lou, Formation of ZnMn2O4 ballin-ball hollow microspheres as a high-performance anode for lithium–ion batteries, Adv. Mater. 24 (2012) 4609–4613. [12] M. Sasidharan, N. Gunawardhana, M. Inoue, S. Yusa, M. YoshioK, Nakashima, La2O3 hollow nanospheres for high performance lithium–ion rechargeable batteries, Chem. Commun. 48 (2012) 3200–3202. [13] Z.Y. Wang, Z.C. Wang, S. Madhavi, X.W. Lou, One-step synthesis of SnO2 and TiO2 hollow nanostructures with various shapes and their enhanced lithium storage properties, Chem. Eur. J. 18 (2012) 7561–7567. [14] Y.L. Ding, X.B. Zhao, J. Xie, G.S. Cao, T.J. Zhu, H.M. Yu, C.Y. Sun, Double-shelled hollow microspheres of LiMn2O4 for high-performance lithium ion batteries, J. Mater. Chem. 21 (2011) 9475–9479. [15] X.F. Guan, L.P. Li, G.S. Li, Z.W. Fu, J. Zheng, T.J. Yan, Hierarchical CuO hollow microspheres: controlled synthesis for enhanced lithium storage performance, J. Alloys Compd. 509 (2011) 3367–3374. [16] S.W. Kim, T.H. Han, J. Kim, H. Gwon, H.S. Moon, S.W. Kang, S.O. Kim, K. Kang, Fabrication and electrochemical characterization of TiO2 threedimensional nanonetwork based on peptide assembly, ACS Nano 3 (2009) 1085–1090. [17] J.Z. Chen, L. Yang, Y.F. Tang, Electrochemical lithium storage of TiO2 hollow microspheres assembled by nanotubes, J. Power Sources 195 (2010) 6893– 6896. [18] J.P. Wang, Y. Bai, M.Y. Wu, J. Yin, W.F. Zhang, Preparation and electrochemical properties of TiO2 hollow spheres as an anode material for lithium–ion batteries, J. Power Sources 191 (2009) 614–618. [19] F. Zhang, Y. Zhang, S.Y. Song, H.J. Zhang, Superior electrode performance of mesoporous hollow TiO2 microspheres through efficient hierarchical nanostructures, J. Power Sources 196 (2011) 8618–8624. [20] J.S. Chen, Z.Y. Wang, X.C. Dong, P. Chen, X.W. Lou, Graphene-wrapped TiO2 hollow structures with enhanced lithium storage capabilities, Nanoscale 3 (2011) 2158–2161. [21] S.J. Gregg, K.S.W. Sing, H. Salzberg, Adsorption surface area and porosity, J. Electrochem. Soc. 114 (1967) 279C. [22] N. Yao, S.L. Cao, K.L. Yeung, Mesoporous TiO2–SiO2 aerogels with hierarchal pore structures, Microporous Mesoporous Mater 3 (2009) 570–579. [23] S.L. Cao, K.L. Yeung, P.L. Yue, Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase, photocatalytic oxidation of VOCs, Appl. Catal. B: Environ. (68) (2006) 99–108. [24] J.Y. Shen, H. Wang, Y. Zhou, N.Q. Ye, G.B. Li, L.J. Wang, Anatase/rutile TiO2 nanocomposite microspheres with hierarchically porous structures for highperformance lithium–ion batteries, RSC Adv 2 (2012) 9173–9178. [25] J.Y. Shen, H. Wang, Y. Zhou, N.Q. Ye, L.J. Wang, Continuous hollow TiO2 structures with three-dimensional interconnected single crystals and large pore mesoporous shells for high-performance lithium–ion batteries, Cryst. Eng. commun. 14 (2012) 6215–6220. [26] R. Cava, D. Murphy, S. Zahurak, A. Santoro, R. Roth, The crystal structures of the lithium-inserted metal oxides Li0.5TiO2 anatase, LiTi2O4 spinel, and Li2Ti2O4, J. Solid State Chem. 53 (1984) 64–75. [27] T. Ohzuku, T. Kodama, T. Hirai, Electrochemistry of anatase titanium dioxide in lithium nonaqueous cells, J. Power Sources 14 (1985) 153–166. [28] L. Kavan, J. Rathousky´, M. Grätzel, V. Shklover, A. Zukal, Surfactant-templated TiO2 (anatase): characteristic features of lithium insertion electrochemistry in organized nanostructures, J. Phys. Chem. B 104 (2000) 12012–12020.