Iso-Oriented Anatase TiO2 Mesocages as a High Performance Anode Material for Sodium-Ion Storage

A major obstacle in realizing Na-ion batteries (NIBs) is the absence of suitable anode materials. Herein, we firstly report the anatase TiO2 mesocages constructed by crystallographically oriented nanoparticle subunits as a high performance anode for NIBs. The mesocages with tunable microstructures, high surface area (204 m2 g−1) and uniform mesoporous structure were firstly prepared by a general synthesis method under the assist of sodium dodecyl sulfate (SDS). It’s notable that the TiO2 mesocages exhibit a large reversible capacity and good rate capability. A stable capacity of 93 mAhg−1 can be retained after 500 cycles at 10 C in the range of 0.01–2.5 V, indicating high rate performance and good cycling stability. This could be due to the uniform architecture of iso-oriented mesocage structure with few grain boundaries and nanoporous nature, allowing fast electron and ion transport, and providing more active sites as well as freedom for volume change during Na-ion insertion. CV measurements demonstrate that the sodium-ion storage process of anatase mesocages is mainly controlled by pseudocapacitive behavior, which is different from the lithium-ion storage and further facilitates the high rate capability.

Scientific RepoRts | 5:11960 | DOi: 10.1038/srep11960 mechanism (CuO and MoS 2 ) 13,14 were studied, which showed high initial capacity but suffered from poor cycling performance owing to the large volume change or the sluggish kinetics. Classical insertion materials such as Na 2 Ti 3 O 7 was proposed as an alternative anode material operating at a low potential of around 0.3 V vs. Na/Na + , however, such a material showed a rather low capacity and poor cycling stability 15,16 . Therefore, it is still a challenge to develop appropriate anode materials with both high capacity and long cycling life.
Several types of TiO 2 polymorphs have been used as promising anode materials for LIBs due to its intrinsic advantages in safety, low cost and good cyclic stability [17][18][19][20] . However, only few reports were reported so far on the behavior of Na ions insertion into TiO 2 . Although Rajh et al. 21 firstly investigated the amorphous TiO 2 nanotube anode for rechargeable NIBs, they concluded that anatase TiO 2 is inactive in their NIBs. Most recently, it's found that the Na-ion storage performance of anatase TiO 2 nanostructures could be improved by carbon coating or graphene doping [22][23][24][25] . However, there are different opinions about the sodium storage process. Cha et al. 24 and Kim et al. 25 claimed the similar suggestion that sodium ions would be reversibly (de-)inserted into the anatase host observed from the lattice expansion and structure remaining during the intercalation and extraction process. On the contrary, Wu et al. 26 very recently reported that the anatase TiO 2 would be partly reduced to metallic titanium and amorphous sodium titanate phase which could reversibly store about 0.41 sodium per TiO 2 . These results encourage further study on developing an ideal nanostructure of this new promising anode material for NIBs and exploring their Na-ion storage mechanism.
Herein, we describe a general and facile synthesis route of iso-oriented TiO 2 mesocages with tunable microstructures and nanoporous nature and their application as high-performance anode in rechargeable Na-ion batteries. It's notable that the obtained TiO 2 mesocages are built by very tiny nanocrystals with a mutual orientation, holding a fine nanoporous structure and large surface area. The obtained iso-oriented TiO 2 mesocages exhibited large capacity and good cycling stability for NIBs. The sodium ions storage is mainly controlled by the pseudocapacitive process, which facilitates the high-rate capability.

Results
The iso-oriented TiO 2 mesocages were synthesized by a simple and low-temperature route. The samples obtained from HNO 3 and HCl solution were defined as TiO 2 -MN and TiO 2 -MC, respectivley. Figure 1a shows the related X-ray diffraction (XRD) patterns of as-prepared samples. All the diffraction peaks in Fig. 1 could be exclusively ascribed to tetragonal anatase TiO 2 (JCPDS 73-1764). The broadened diffraction peaks suggest a small crystallite size of the anatase samples. The average crystallite size of TiO 2 -MN was calculated to be approximately 11 nm (also 11 nm for TiO 2 -MC), using the Scherer equation, based on the (101) diffraction peak. N 2 adsorption-desorption isotherms measurements were adopted to reveal the Brunauer-Emmett-Teller (BET) surface area and pore size distribution, as shown in Fig. 1b,c. The BET surface area and the pore volume of TiO 2 -MN were determined to be 204 m 2 g −1 and 0.62 cm 3 g −1 , respectively. Morover, TiO 2 -MN exhibits rather uniform nanopores with an average diameter of 5.0 nm in Fig. 1b (inset). Compared to TiO 2 -MC (Fig. 1c), its BET surface area and pore volume were about 199 m 2 g −1 and 0.31 cm 3 g −1 , suggesting a lower pore volume and a smaller pore size mostly around 2.7 nm. Figure 2a,b show the low-magnification and high-magnification SEM image of the sample obtained from 2 M HNO 3 aqueous solution (TiO 2 -MN), indicating the large-scale formation of nanoparticles with a size of about 30-50 nm. The rough surface and porous nature of the nanoparticles could also be clearly observed (Fig. 2b). The TEM image shown in Fig. 2c further reveals the porous structure of TiO 2 -MN. Figure 2d presents a typical TEM image of a single nanoparticle, which confirms that the anatase TiO 2 nanoparticle was constructed from tiny nanoparticle subunits with diameter about 5 nm and has a cage-like morphology with nanoporous structure. As shown in the upper inset of Fig. 2d, the SAED pattern taken from the whole nanoparticle exhibited single-crystal-like diffraction, suggesting that the tiny nanoparticle subunits were highly ordered and oriented along the [100] direction. The HRTEM image (inset of Fig. 2d) further confirms that the primary nanocrystals were highly crystalline, the clear lattice fringe of 0.19 nm was assigned to the (200) spacing of anatase structure. It should be pointed that similar single-crystal-like TiO 2 mesocages were synthesized by Lu et al. 27 through a hard-template (SBA-15) method and demonstrated excellent photocatalytic activity. Herein, the iso-oriented TiO 2 mesocages with high surface area and uniform mesopores, constructed by highly oriented tiny nanocrystals, were successfully prepared by a facile and low-temperature route.
It's notable that the iso-oriented TiO 2 mesocages with tunable microstructures could be prepared by using different anion in this synthesis method. SEM and TEM images of TiO 2 -MC obtained from 2 M HCl aqueous solution are shown in Fig. 3. It is clearly shown that numerous nanoparticles with uniform size (40-60 nm) were formed. TiO 2 -MN possessed of a rough surface, actually, it was constructed by tiny nanoparticle subunits (about 3-5 nm). The corresponding SAED pattern in the inset of Fig. 3d for the whole nanoparticle with single-crystal-like diffractions indicates that the building of nanoparticle subunits were highly ordered, leading to the formation of a crystallographically oriented architecture along [001] direction. Such architecture was named mesocrystal by Cölfen et al. which is porous quasi-single crystal consisting of ordered assemblies of aligned nanoparticles 28 . Furthermore, the diffraction spots were slightly elongated (similar results were observed for TiO 2 -MN), indicating that there was a small mismatch between the boundaries of the nanoparticle subunits; this is usually found for the growth Scientific RepoRts | 5:11960 | DOi: 10.1038/srep11960 of many ordered nanoparticle assembles (mesocrystals) when they are assembled in the same orientation 28,29 .
To shed light on the formation mechanism of the TiO 2 mesocages, a series of samples were synthesized for different periods of time and the results are shown in Figure S1. It could be observed that some irregular particles were constructed by the tiny anatase TiO 2 nanocrystals, the boundaries among the nanocrystals were clear and they aggregated together by perfect or imperfect attachments. It's demonstrated that aggregation of nanocrystals with the same crystallographic orientations, as well as the presence of defects, are strong evidence of oriented attachment (OA) 30,31 . This growth mechanism is also a typical formation process for the ordered nanoparticle superstructures (mesocrystals) 28 . Besides the anion (NO 3 − and Cl − ), the amount of the SDS significantly affect the morphology of the final samples, more SEM images were shown in Figure S2. Therefore, the microstructures of the obtained TiO 2 mesocages depend on the cooperative effect of the surfactant and anion. It was pointed out that the organic additive could be in favor of the temporary stabilization of the primary nanocrystals, allowing their attachment and assembly into ordered aggregates 28,29 . The driving force for the oriented self-assembly could be assigned to the minimization of the total surface energy of disordered primary nanocrystals 28 .
Finally, the sodium-ion storage properties of the iso-oriented TiO 2 mesocages with mesoporous structure were investigated. Cells made from commercial anatase TiO 2 nanoparticles (TiO 2 -NPs) were also fabricated as a reference sample. The XRD, SEM image and N 2 adsorption-desorption isotherms of TiO 2 -NPs were shown in Figure S3. The BET surface area and the pore volume of TiO 2 -NPs were determined to be 118 m 2 g −1 and 0.32 cm 3 g −1 , respectively. Figure 4a shows the rate capability of TiO 2 -MN, TiO 2 -MC and TiO 2 -NPs from 0.2 to 5 C (1C = 170 mAg −1 ). TiO 2 -MN exhibited good rate performance, with a stable capacity of 240 mAhg −1 at 0.5 C, 200 mAhg −1 at 1 C, 165 mAhg −1 at 2 C, and 137 mAhg −1 at 5 C, 120 mAhg −1 at 10 C. Furthermore, a reversible capacity of 180 mAhg −1 could be remained when the current rate returned to 1 C. TiO 2 -MC showed comparable performance at low current rates, but was not so good as TiO 2 -MN at high current rates which may be due to the lower pore volume and smaller pore size. However, both TiO 2 -MN and TiO 2 -MC exhibited superior rate performance compared to TiO 2 nanoparticles. It displays a stable discharge capacity of 160 mAhg −1 at 0.5 C, 125 mAhg −1 at 1 C, 97 mAhg −1 at 2 C, and 64 mAhg −1 at 5 C, 38 mAhg −1 at 10 C. Figure 4b shows typical charge-discharge profiles of the three samples at the second cycle, the slope profiles are similar to the previous reports of anatase TiO 2 as anode material for Na-ion batteries [22][23][24][25] . The character of the charge-discharge profiles was not apparently changed during cycling (Fig. 4c). Figure 4d presents the cycling performance of TiO 2 -MN and TiO 2 -MC, the capacity of 152 mAhg −1 and 128 mAhg −1 could be remained, respectively, after 60 cycles at 1 C. In order to test the long cyclic stability at high rates, a sodium-ion cell made from TiO 2 -MN were run at 10 C for 500 cycles, after aging at 0.5 C for 5 cycles. Figure 4e shows the capacity starts at 123 mAhg −1 and still maintains at 93 mAhg −1 after 500 cycles as well as high columbic efficiency, indicating a good cycling ability. Thus, anatase TiO 2 mesocages exhibit a larger capacity and better rate performance than that of layerd titanante nanotubes and nanorods 32,33 . It is suggested that (as the scheme in Fig. 4f) the crystallographically oriented anatase mesocages (composed of tiny nanocrystal subunits) were well connected with few grain boundaries compared with the irregularly oriented nanoparticles, which facilitated the fast Na + ion and electron transport. Moreover, the very large surface area and nanoporous nature in the mesocages could provide a high level of accessibility for the electrolyte and more active sites, and hence allow the efficient ion transport as well as the freedom for volume change.
Cyclic voltammetry (CV) experiments performed at various sweep rates (0.1-1 mV/s) were used to examine the redox processes occurring in the as-prepared iso-oriented TiO 2 mesocages. As shown in Fig. 5a, a couple of redox peaks are observed between 0.4 V and 0.9 V vs. Na/Na + , which could correspond to the insertion/extraction of Na-ion. The voltammetric response of electrode active material with respect to sweep rate can be calculated according to 34 : where i is the current density, v is the scan rate, and a and b are adjustable parameters. When the b-value approaches 1, the electrochemical process is mainly controlled by capacitance, and when the b value is close to 0.5, the insertion behavior (battery behavior) dominates. The pseudocapacitive behavior of electrode material arises from the surface faradic redox reactions, and the insertion behavior is controlled by bulk diffusion process. However, both of them displays obvious redox peaks on CV measurements in a specific voltage region. Figure 5b shows the relationship between peak current and sweep rate on NIBs. As the b-value approaches 1 (0.89 calculated from Fig. 5b), the sodium-ion storage of TiO 2 mesocages is mainly controlled by the pseudocapacitive process, leading to a fast Na + insertion/extraction and extended cycling life. Rajh et al. 21 also found the pseudocapacitive characteristic of the amorphous TiO 2 nanotube anode for rechargeable NIBs. However, the lithium-ion storage of the amorphous TiO 2 nanotube anode is controlled by capacitive process as the discharge voltage was below 1.8 V. In this study, a couple of typical redox peaks are observed from Fig. 5c, corresponding to the insertion/extraction of Li-ion. The b-value approaches 0.5 (0.58 calculated from Fig. 5d), demonstrating the typical insertion behavior of lithium storage in TiO 2 mesocages. Thus, the Na + and Li + ion storage performance of TiO 2 mesocages is mainly controlled by pseudocapacitive behavior and insertion behavior, respectively.
Recently, it's reported that anatase TiO 2 nanocrystals displayed a large irreversible capacity at the first cycle especially when the discharge voltage was below 0. 3 V 35,26 . This large irreversible capacity could be due to occurrence of side reactions with electrolyte, a solid− electrolyte interface (SEI) formation and some irreversible electrochemical reaction of the active materials 25,26,35 . This phenomenon was also found in the Na 2 Ti 3 O 7 anode material for sodium-ion batteries 15,16 . Similar results were also found in this study, as shown in Fig. 6a  thus higher columbic efficiency of 46.1%. Moreover, the irreversible capacity (180 mAhg −1 ) under 0.3 V is also smaller than that of TiO 2 -NPs. This could be due to the crystallographically oriented mesocages with few grain boundaries compared with the irregularly oriented nanoparticles, leading to less irreversible reaction and fast electron transport.
It's well known that Raman spectroscopy is an effective measurement to investigate the structure of titanium dioxide, and the results are shown in Fig. 6b. The curve of fresh electrode exhibits the typical . During the discharge and charge process, the Raman spectra of the electrode suggest that the crystallinity of the anatase TiO 2 continuously weakens, which is in agreement with the results reported by Wu et al. 26 However, the morphology of the TiO 2 mesocages remarkably remained unchanged even after different rate cycling, as displayed in Fig. 6c,d shows the HRTEM image and the related FFT pattern of TiO 2 mesocages after rate performance at full charge state, the whole particle was not well crystalline as before, which is in agreement with the Raman results. In spite of a relatively low crystallinity of the anatase TiO 2 mesocages, the lattice fringe of 0.21 nm could be clearly observed from the crystalline area, indicating a slight increase in the d-spacing distance during the repetition of Na + insertion and extraction. It's worth mentioning that the tiny nanoparticle subunits in TiO 2 mesocages still keep oriented along the [100] direction after Na + insertion and extraction. This result is different from the reports of Kim et al., which demonstrated reversible variation of the lattice of anatase TiO 2 nanorods even after 100 cycles 25 . It's suggested that Na + insertion into anatase single crystals is not easy, and a low crystallinity as well as an increase in lattice may provide a lower energy barrier for Na + insertion. Herein, the anatase mesocages composed of highly oriented tiny nanocrystal subunits with large surface area and porous nature of mesocages could provide more active reaction sites for Na + storage and a good freedom for volume change during Na + insertion.

Discussion
Iso-oriented TiO 2 mesocages with mesoporous nature and tunable microstructures were successfully fabricated via a facile synthesis route. The TiO 2 mesocages were used for the first time as anode materials in rechargeable Na-ion batteries, demonstrating a large reversible charge-discharge capacity, good rate capability and cycling performance. This could be attributed to the intrinsic characteristics of the mesocages constructed by crystallographically oriented nanoparticle subunits with few grain boundaries compared with the irregularly oriented nanoparticles, accompanied by a large surface area and uniform nanoporous nature. Such architecture facilitated fast electron and ion transport, and gave more active sites and freedom for volume change for Na-ion insertion. Moreover, the sodium-ion storage process of anatase mesocages was mainly controlled by pseudocapacitive behavior, leading to high rate performance and making them promising for applications in rechargeable Na-ion batteries.

Methods
Materials Synthesis. All chemicals were purchased from Aladdin without further purification. In a typical synthesis, 1.5 g sodium dodecyl sulfate (SDS) was first dissolved in 50 mL 2 M HNO 3 or HCl solution. After the solution was stirred for a few minutes, 1 mL (1.5 mL for HCl solution) of titanium (IV) isopropoxide (TIP) was added and kept at 70 °C (80 °C for HCl solution) for 48 h under stirring. The final products were obtained by centrifugation, washed thoroughly with distilled water and dried at 60 °C overnight, and then calcined at 400 °C for 30 min in air to remove the residual organics. The samples obtained from HNO 3 and HCl solution were denoted as TiO 2 -MN-and TiO 2 -MC, respectively.

Characterizations of the samples. Scanning electron microscopy (SEM, S8010 instrument) and
Transmission electron microscopy (TEM, FEI F20 S-TWIN instrument) were applied for the structural characterization of the resulting nanowires and mesocrystals. X-ray diffraction (XRD) patterns were recorded on a Rigaku X-ray diffractometer using Cu Kα (λ = 1.5418 Å) radiation. N 2 adsorption-desorption analysis was measured on a Micro-meritics TriStar II 3020 instrument (USA). The pore size distributions of the as-prepared samples were analyzed using the Non-Local-Density Functional Theory (NLDFT) methods. The Raman spectra were recorded on a LabRAM HR Evolution (HORIBA Jobin Yvon) with a 532 nm laser.
Electrochemical Measurements. For the electrochemical measurement of Na-ion intercalation, TiO 2 mesocages were admixed with polyvinylidene fluoride (PVDF) binder and acetylene black carbon additive in a weight ratio of 70:20:10. The mixture was spread and pressed on copper foil circular flakes as working electrodes (WE), and dried at 120 °C in vacuum for 12 h. Na-ion cells were assembled in coin-type cells (CR 2025) with a Na metal foil as the negative electrode, glass fiber separator (Whatman GF/F), and 1 M NaClO 4 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1/1 in volume) as the electrolyte. The cells were assembled in a glove box filled with highly pure argon gas (O 2 and H 2 O levels < 1 ppm), and charge/discharge tests were performed in the voltage range of 0.01 to 2.5 V (Na + / Na) on a Land automatic batteries tester (Land CT 2001A, Wuhan, China). The active material content in the electrode was around 1.2 mg cm −2 , and the amount of electrolyte was 170-200 μ L. The cells were tested under a constant temperature (25 °C). Cyclic voltammetry (CV) measurements were performed on Zennium (Zahner).