Anatase TiO2: Better Anode Material Than Amorphous and Rutile

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Anatase TiO2: Better Anode Material than Amorphous and Rutile Phases of TiO2 for Na-ion Batteries Dawei Su, Shixue Dou, and Guoxiu Wang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 11, 2015

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Chemistry of Materials

Anatase TiO2: Better Anode Material than Amorphous and Rutile Phases of TiO2 for Na-ion Batteries Dawei Su,*† Shixue Dou, ‡ and Guoxiu Wang*†§ †

Centre for Clean Energy Technology, University of Technology Sydney, Broadway, Sydney, NSW 2007, Australia

‡

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia §

College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, P.R. China

ABSTRACT: Amorphous TiO2@C nanospheres were synthesized via a template approach. After being sintered under different conditions, two types of polyphase TiO2 hollow nanospheres were obtained. The electrochemical properties of the amorphous TiO2 nanospheres and the TiO2 hollow nanospheres with different phases were characterized as anodes for the Na-ion batteries. It was found that all the samples demonstrated excellent cyclability, which was sustainable for hundreds of cycles with tiny capacity fading, although the anatase TiO2 presented higher capability than the mixed anatase/rutile TiO2 and the amorphous TiO2@C. Through crystallographic analysis, it was revealed that the anatase TiO2 crystal structure supplies two-dimensional diffusion paths for Na ion intercalation and more accommodation sites. Density functional theory calculations indicated lower energy barriers for Na+ insertion into anatase TiO2. Therefore, anatase TiO2 hollow nanospheres show excellent high rate performance. Through ex-situ field emission scanning electron microscopy, it was revealed that the TiO2 hollow nanosphere architecture can be maintained for hundreds of cycles, which is the main reason for its superior cyclability.

1 Introduction Because sodium is substantially less expensive and more abundant than lithium, sodium ion batteries (Naion batteries) have been considered as a desirable alternative to lithium ion batteries (Li-ion batteries).1-3 The large sodium ion, however, with its high ionization potential, requires a more open framework to achieve acceptable mobility. Therefore, identifying new materials/microstructures for electrodes in Na-ion batteries is receiving extensive attention.4-6 Certain materials have been verified to be useful candidates as cathodes for Na-ion batteries.7-11 These materials present suitable structures for accommodating the bigger size Na ions, such as the layered structure, the olivine structure, and the sodium superionic conductor (NASICON) structure. For the anodes, hard carbonaceous materials were identified that were capable of reversibly inserting and extracting Na ions over 200 cycles, but with small capacity.12, 13 Later, researchers found metals, such as Sn, Sb, P, and Pb, which can reversibly alloy and de-alloy with Na, that demonstrated high storage capacity, e.g. 847 mA h g-1 for Na14Sn4 and 660 mA h g-1 for Na3Sb.3, 14-17 They suffer from poor cyclability, however. This is most likely due to the large volume changes and sluggish kinetics. There is still a limited choice of electrode materials that

are suitable hosts to accommodate Na ions and allow for reversible insertion–extraction reactions, especially for the anode side. Reversible insertion–extraction reactions could be of benefit for good cyclability. Therefore, it is useful to explore insertion electrodes based on transition metal oxides. Pre-sodiated Na2Ti3O7 and sodium-free anode materials, such as SnO2, SnO, and WS2, have been reported in this connection.18-22 Titanium dioxide (TiO2), an exceptionally stable, nontoxic, inexpensive, and abundant material, also has attracted much attention. Its good cyclablity and high rate capability have been reported for Li-ion batteries.23, 24 Recently, its possible application in Na-ion batteries has also inspired us to focus on this promising candidate. Johnson and Rajh et al. utilized amorphous electrochemically synthesized onedimensional (1D) TiO2 nanotubes as an anode for Na-ion batteries, which demonstrated reversible self-improving specific capacity of ∼150 mA h g-1.25 Mitlin et al. reported that crystalline mesoporous TiO2 electrodes exhibited a highly stable reversible charge storage capacity of ~ 150 mA h g-1 over 100 cycles. 26 It was also reported that reducing the particle size of TiO2 can lead to faster charge/discharge rates, because the faradaic reactions occurring at the surface of the material will make the main contribution to the capacity.27 Huang et al.28

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reported that the intimate integration of graphene with TiO2 can reduce the diffusion energy barrier, thus enhancing the Na ion intercalation pseudocapacitive process. TiO2 has polyphases, such as rutile, anatase, brookite, hollandite,29 and metastable monoclinic phase.30 Their crystal structures are formed by the stacking of distorted edge sharing TiO6 octahedra. These stacked TiO6 octahedra form tunnels, giving possible interstitial sites for Na-ion accommodation and suitable sized pathways for Na-ion diffusion. Because of the different tunnel structures of different phases of TiO2, they can present different electrochemical performance for Na ion storage. Here, we compare the electrochemical performances of amorphous TiO2, mixed anatase/rutile TiO2, and pure anatase TiO2. To obtain superior electrochemical performance, the TiO2 was synthesised in the form of hollow nanospheres, because this architecture can buffer volume changes, tolerate expansion and shrinkage during cycling, and increase the contact area between the electrode and electrolyte. When applied as anode material for Na-ion batteries, the amorphous TiO2@C nanospheres, and the crystalline anatase/rutile and pure anatase TiO2 hollow nanospheres all exhibited superior cycling performance. The anatase TiO2 presented the highest capacity compared with the anatase/rutile TiO2 and the amorphous TiO2@C, as the anatase TiO2 crystal structure supplies two-dimensional (2D) diffusion paths for Na ion intercalation and more accommodation sites. Furthermore, through density functional theory (DFT) calculations, it was found that anatase TiO2 shows lower energy barriers for Na+ insertion. Therefore, the anatase TiO2 hollow nanospheres also show well-defined high rate performance. Through ex-situ field emission scanning electron microscopy (FESEM) analysis, it was revealed that the outstanding cyclability and electrochemical performance of the TiO2 hollow nanospheres could be ascribed to the unique porous hollow nanosphere architecture, which can accommodate large strain without pulverization, and the very high stability of the TiO2 hollow nanosphere architecture, which can be maintained for hundreds of cycles. 2 Experimental Section 2.1 Synthesis and Method. Carbon nanospheres were first synthesized by the hydrothermal method. Glucose was dissolved in distilled water, and the solution was transferred to a Teflon autoclave. After heating at 180 ˚C for 4 h, the products were collected after being washed with distilled water and ethanol several times. The final products were dried in a vacuum oven overnight. The TiO2@C nanospheres were prepared by a reported method.31 In a typical synthesis, the as-prepared carbon nanospheres (100 mg) were dispersed in a mixture of hydroxypropyl cellulose (100 mg, average MW of ∼ 80 000, Sigma–Aldrich), absolute ethanol (20 mL), and distilled water (0.2 mL). After stirring, 0.3 mL tetrabutyl orthotitanate (TBOT) (≥ 97.0%,

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Sigma–Aldrich) in ethanol (1.7 mL) was slowly injected into the mixture. Then, the solution was refluxed at 80 °C for 100 min. The precipitate was collected by filtering and washing with ethanol. After being dried at 80 ˚C in a vacuum oven overnight, the TiO2@C was obtained. The crystalline TiO2 hollow nanospheres were synthesized by sintering the TiO2@C at 400 ˚C for 2 h with a 2 ˚C min-1 heating rate in a tube furnace in air atmosphere. Pure anatase TiO2 was obtained by further sintering at 800 ˚C for 2 h under the protection of 5 % H2/Ar gas flow in the tube furnace. 2.2 Structural and physical characterization. The crystal structure and phase of the as-prepared materials were characterized by X-ray diffraction (XRD, Siemens D5000) using Cu Kα radiation with a very slow scanning step of 0.01° s-1. The morphology was examined by field emission scanning electron microscopy (FESEM, Zeiss Supra 55VP). The details of the structure were further characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL JEM-2011). Selected area electron diffraction (SAED) patterns were recorded by a Gatan charge-coupled device (CCD) camera in a digital format. Thermogravimetric/differential thermal analysis (TG/DTA) was performed at a heating rate of 5 °C min-1 under air flow from room temperature to 800 ºC with a 2960 SDT system. 2.3 Electrochemical testing. The electrodes were prepared by dispersing 80 wt. % as-prepared TiO2, 10 wt. % acetylene carbon black, and 10 wt. % poly (vinylidene fluoride) binder (PVDF, (CH2CF2)n, Sigma-Aldrich) in N-methyl-2-pyrrolidone (NMP, C5H9NO, Sigma-Aldrich, 99.5 %) on copper foil. After being dried in a vacuum oven for 12 h, the electrodes were pressed at 200 kg cm-2, with the loading of each electrode kept at ~ 1.2 mg cm-2. CR2032 coin cells were assembled for electrochemical testing. Na metal (Sigma-Aldrich, 99.95%) was used as reference and counter electrode, glass nanofiber (Whatman) was used as the separator, and the electrolyte was 1 M sodium perchlorate (NaClO4, Sigma-Aldrich, ≥ 99 %) dissolved in a mixture of ethylene carbonate (EC, C3H4O3, Sigma-Aldrich, 99 %) and propylene carbonate (PC, C4H6O3, Sigma-Aldrich, 99.7 %) in a volume ratio of 1:1. The cells were assembled in an argon-filled glove box (UniLab, Mbraun, Germany). Electrochemical measurements were conducted by galvanostatic charge-discharge testing at ambient temperature under different current densities in the voltage range from 0.01 to 3 V. In order to investigate the morphological changes in the as-prepared TiO2 hollow nanospheres, Swagelok-type cells were assembled. After cycling, the active materials were removed from the electrodes and washed with PC before being used for exsitu scanning electron microscope (SEM) analysis. 2.4 Computational methods. DFT calculations were performed based on the generalized gradient approximation (GGA), employing ultra-soft pseudopotential (USPP) formalism for the


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exchange-correlation energy function with energy cut-off of 300 eV.32 We used a 6×6×7 Monkhorst-Pack k-point grid. The maximum self-consistent field convergence tolerance was less than 2 × 10–6 eV atom-1. All calculations were performed in reciprocal space. For the density of states (DOS) and partial density of states (PDOS) calculations, the bond energy tolerance was 1 × 10-5 eV. The energy barriers for Na+ insertion were calculated from the difference between the energy of the bulk rutile TiO2 or anatase TiO2, ETiO2, the energy of Na, ENa, and the energy of sodiated rutile TiO2 or anatase TiO2, Esodiated TiO2: Eenergy barriers = Esodiated TiO2 - (ETiO2 + ENa)

(1)

3 Results and discussion 3.1 Phase, morphology and structure characterization Figure 1(a) illustrates the synthesis process from amorphous TiO2@C to crystalline TiO2 hollow nanospheres. Their morphologies are shown in Figure 1(b, c, and d), respectively. It can be seen that all of them present a uniform nanosphere structure (insets). The FESEM image of the amorphous TiO2@C nanosphere (Figure 1(b) inset) clearly shows that its diameter is around 150 – 220 nm. The carbon nanosphere templates are shown in Figure S1 (Supplementary information, SI), which shows the uniform size distribution. The TEM image (Figure 1(e)) of the free-standing amorphous TiO2@C nanospheres confirms their diameter, and it also shows that the as-prepared TiO2@C nanospheres are solid spheres, without any obvious contrast change across the whole sphere diameter (inset of Figure 1(e)). From the corresponding blurry SAED pattern (Figure 1(h)), it can be identified that the TiO2@C nanospheres are amorphous. Furthermore, there is no crystalline lattice in the HRTEM image of the TiO2@C nanosphere that was examined (Figure 1(k)), confirming the amorphous nature of this material. The C content was measured to be 75 wt. % in the amorphous TiO2@C nanosphere composite, as revealed by TG/DTA analysis (Figure S2, SI). When the amorphous TiO2@C nanospheres were sintered at 400 ˚C for 2 hours in the atmosphere, they became crystallized, and the carbon was oxidized to carbon dioxide. From the FESEM image of a typical broken free-standing, sintered TiO2 nanoparticle (inset of Figure 1(c)), it can be seen that it presents a hollow nanosphere architecture, with a shell wall ~ 20 nm in thickness between the inner and outer surfaces. The corresponding TEM image of the crystallized TiO2 hollow nanospheres (Figure 1(f)) confirms their hollow nature, as different contrast from the outside of a nanosphere to its inside can be readily observed. Furthermore, through the electron signal on the cross-section of a TiO2 nanosphere (inset of Figure 1f), it can be seen that the inner part of the nanosphere presents fewer counts, which indicates the vacant inner space. It also can be observed that pores are present in

Figure 1. (a) Schematic diagram of the transformation from amorphous TiO2@C nanospheres to rutile and anatase TiO2 hollow nanosphere composite, and then to pure anatase TiO2 hollow nanospheres. (b-d) FESEM images of amorphous TiO2@C nanospheres, rutile and anatase TiO2 hollow nanosphere composite, and pure anatase TiO2 hollow nanospheres, with the insets showing enlargements of single nanospheres. (e-g) TEM images of amorphous TiO2@C nanospheres, rutile and anatase TiO2 hollow nanosphere composite, and pure anatase TiO2 hollow nanospheres. Insets are the electron signal on the cross-section of a TiO2 nanosphere. (h-j) SAED patterns of amorphous TiO2@C nanospheres, rutile and anatase TiO2 hollow nanosphere composite, and pure anatase TiO2 hollow nanospheres. (km) Lattice resolution HRTEM images of amorphous TiO2@C nanospheres, rutile and anatase TiO2 hollow nanosphere composite, and pure anatase TiO2 hollow nanospheres.

the TEM image, indicating that intercrystal voids exist within the shell. The SAED pattern (Figure 1(i)) of the crystallized TiO2 hollow nanospheres after the first sintering can be indexed to two phases of TiO2, which are rutile TiO2 ((101), (110), and (200)) and anatase TiO2 ((101), (103), (200), and (202)). The lattice-resolved HRTEM image (Figure 1(l)) clearly demonstrates the crystal lattice fringes of the rutile TiO2 (110) crystal planes (d = 3.264 Å) and of the anatase TiO2 (101) crystal planes (d = 3.562 Å) within the primary nanocrystals. When the sintering temperature was increased to 800 ˚C under protection of 5% H2 in Ar gas flow, the as-prepared TiO2 nanoparticles preserved the nanosphere architecture, as shown in Figure 1(d), and the individual TiO2 nanosphere shown in Figure 1(g) presents an integrated nanospherical shape with porous architecture, which was confirmed by the electron signal counts measurement (inset in Figure 1(g)). Its corresponding SAED pattern (Figure 1(j)) suggests that the TiO2 sintered at 800 ˚C with protection gas only



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presents the diffraction of pure anatase TiO2 phase ((101), (103), (200), (202), and (204) planes). Furthermore, the lattice resolved HRTEM image (Figure 1(m)) clearly shows the (101) crystal planes of anatase TiO2 (d = 3.562 Å).

insets of Figure 2. These tunnels can provide the space for accommodating the Na ions, making rutile and anatase candidates as electrodes for Na-ion batteries. When the rutile and anatase TiO2 hollow nanosphere composite was further sintered at 800 ˚C under the protection of 5 % H2/Ar gas flow, the XRD pattern of the product can be fully refined to pure tetragonal anatase TiO2 with satisfied convergence factors (Rwp = 11.22 %, Rp = 7.27 %), as shown in Figure 2(b). 3.2 Electrochemical properties nanospheres for Na-ion batteries

Figure 2. Rietveld refinement patterns of X-ray diffraction data for rutile and anatase TiO2 hollow nanosphere composite (a), and pure anatase TiO2 hollow nanospheres (b). Insets are the crystal structures of refined rutile TiO2 and anatase TiO2. The Ti and O atoms are colored light green and red, respectively.

Through the Rietveld refined XRD analysis (Figure 2), the phases of the as-prepared TiO2 nanospheres were further identified. For the TiO2@C solid nanospheres, there are no diffraction peaks (not shown), indicating their amorphous nature. After being sintered at 400 ˚C in atmosphere, the XRD patterns can be quantitatively refined to 40.15 wt. % rutile TiO2 and 59.85 wt. % anatase TiO2 with suitable convergence factors (weighted profile R-factor, Rwp = 15.76 %, profile R-factor, Rp = 12.72 %). The refined crystal structures are presented as insets in Figure 2(a). The rutile TiO2 was refined as tetragonal symmetry (P42/mnm space group (136)), with lattice parameters a = b = 4.5933 Å and c = 2.9592 Å, while the anatase TiO2 also presents tetragonal symmetry, but with I41/amd space group (141), and the lattice parameters were refined to be a = b = 3.7852 Å, c = 9.5139 Å. In the rutile TiO2 crystal structure, the titanium cations have a coordination number of 6, meaning that they are surrounded by an octahedron of 6 oxygen atoms. The oxygen anions have a coordination number of 3, resulting in trigonal planar coordination. The crystal structure of anatase TiO2 also consists of TiO6 octahedra, sharing four edges. Each Ti atom has six O neighbours, but only four Ti next-nearest neighbours. Both rutile and anatase TiO2 have tunnels formed by the TiO6 octahedra, as demonstrated by the

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We compared the electrochemical performances of the as-prepared amorphous TiO2@C nanospheres, rutile and anatase composite TiO2, and anatase TiO2 as anode materials for Na-ion batteries. The galvanostatic chargedischarge cycling results are shown in Figure 3. Figure 3(a, b, and c) presents the 2nd, 5th, 50th, 100th, and 200th charge and discharge profiles of the amorphous TiO2@C nanospheres, the rutile and anatase composite TiO2, and the anatase TiO2 at current density of 20 mA g-1, respectively (with Figure S3 (SI) showing their 1st cycle discharge profiles). There are obvious differences in their electrochemical performance. The amorphous TiO2@C nanospheres showed the lowest discharge capacity (196 mA h g-1 at the 2nd cycle), and after 50 cycles, the capacity stabilized at ~ 70 mA h g-1 for 200 cycles. The crystalline TiO2 demonstrated improved electrochemical performance. The rutile and anatase TiO2 hollow nanosphere composite yielded 242 mA h g-1 discharge capacity at the 2nd cycle, and it sustained its capacity very well. After 100 cycles, 81 % discharge capacity can be achieved, and after 200 cycles, the rutile and anatase TiO2 hollow nanosphere composite still maintained 185 mA h g-1 discharge capacity (76 % of the 2nd cycle discharge capacity). Among them, the pure anatase TiO2 achieved the highest discharge capacity, which is 295 mA h g-1. The anatase TiO2 also presented well-defined cyclability, as it could yield 218 and 212 mA h g-1 after 100 and 200 cycles, respectively. The cycling performances and corresponding coulombic efficiency of the samples are shown in Figure 3(d). It can be clearly seen that the crystalline anatase and rutile TiO2 hollow nanospheres achieved much higher capacity and coulombic efficiency than the amorphous material. The capacity of the amorphous TiO2@C nanospheres quickly decreased over the first 30 cycles, but after that, the capacity was also maintained at a stable level. To characterize the better electrochemical performance of anatase TiO2 hollow nanospheres, we tested the as-prepared materials at different current densities, as shown in Figure 4. It shows that the anatase TiO2 hollow nanospheres can achieve high discharge capacities at 40, 80, 160, 320, and 640 mA g-1 (266, 245, 226, 192, and 139 mA h g-1) at the 2nd cycle, respectively, Figure 4(a)). The long cycling performances of anatase TiO2 at different current densities are shown in Figure 4(b). The cycling results readily show the ultra-stable cyclability of the anatase TiO2. This material can maintain its high capacity after 500 cycles. 192 mA h g-1 discharge


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Figure 3. (a) 2 , 5 , 50 , 100 , and 200 cycle discharge and charge profiles of amorphous TiO2@C nanospheres (a), rutile and -1 anatase TiO2 hollow nanosphere composite (b), and pure anatase TiO2 hollow nanospheres (c) at 20 mA g current density. (d) -1 Discharge capacity and coulombic efficiency versus cycle number at current density of 20 mA g for amorphous TiO2@C -1 nanospheres, rutile and anatase TiO2 hollow nanosphere composite, and pure anatase TiO2 hollow nanospheres at 20 mA g current density.

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Moreover, the capacity can recover when the current density is reversed back to the initial low current density. In these results, the anatase TiO2 hollow nanospheres exhibit the highest capacities at all current densities. When the current densities were changed to higher levels, such as 40, 80, 160, and 320 mA h g-1, the discharge capacities still had high values. Surprisingly, the discharge capacity of the as-prepared anatase TiO2 hollow nanospheres was maintained at the original level if the testing current density was changed back to 20 mA g-1. This confirms that it is tolerant to high current charge/discharge.

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capacity can be obtained at 40 mA g-1 current density after 500 cycles. The corresponding values are 182, 162, and 146 mA h g-1 for the current densities of 80, 160, and 320 mA g1 after 500 cycles, respectively. Even when cycling the anatase TiO2 at the high current density of 640 mA g-1, 133 mA h g-1 discharge capacity still can be obtained after 500 cycles. The cycling performances of the amorphous TiO2@C nanospheres, the rutile and anatase hollow nanosphere composite TiO2, and the pure anatase TiO2 hollow nanospheres in electrodes at varied current densities were evaluated and are shown in Figure 4(c). The results show the excellent capacity retention of all three electrodes at the various current densities.

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Figure 4. (a) 2 cycle discharge and charge profiles of pure anatase TiO2 hollow nanospheres at various current densities. (b) -1 Discharge capacities versus cycle number at current densities of 40, 80, 160, 320, and 640 mA g . (c) Rate performance of amorphous TiO2@C nanospheres (blue triangles), rutile and anatase TiO2 hollow nanosphere composite (green rectangles), and pure anatase TiO2 hollow nanospheres (red circles) at varied current densities.



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To understand how the sodiation mechanism is related to the different crystal structures, an investigation of the electronic structures of rutile TiO2 and anatase TiO2, and of sodiated rutile and anatase TiO2 was conducted. The density of states (DOS) and partial DOS (PDOS) of Ti and O atoms in bulk rutile and anatase TiO2 are shown in Figure 5(a) and (b), respectively. From these results, it can be seen that in the range of -20 to 15 eV, there are four regions: the lower valence band (VB) located at about 18.51 eV to -15.89 eV, the upper VB at around -5.90 eV to 0.00 eV, the lower conduction band (CB) at around 1.82 eV to 4.57 eV, the middle CB between 4.57 eV and 7.91, and the higher CB at around 7.91 eV to 14.17 eV for the rutile, while the higher VB and lower VB for the anatase

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TiO2 are located at around -17.87 eV to -15.51 eV, and -5.32 eV to 0 eV, and the corresponding CB bands for anatase TiO2 are located within the ranges of 2.17 eV to 5.14 eV, 5.14 eV to 7.98 eV, and 7.98 eV to 12.71, respectively. The Ti orbitals make the main contribution to the VB, and the O orbitals mainly contribute to the CB. The band structure is highly consistent with an early result.33 The CB of the two phases can be decomposed into Ti eg (> 5 eV) and t2g (< 5 eV) states, which is due to the O octahedral crystal field.34 The differences in the DOS distributions between the rutile and the anatase TiO2 should be ascribed to the different packing of the O octahedra.

Figure 5. Total (black) density of states (DOS) and partial density of states (PDOS, with red, blue and green lines for O, Ti, and Na, respectively) calculated for rutile TiO2 (a), anatase TiO2 (b), sodiated rutile TiO2 (c), and sodiated anatase TiO2 (d), respectively.

The DOS and PDOS of sodiated rutile and anatase TiO2 are shown in Figure 5(c) and (d), respectively. It can be seen that when the Na ions are inserted, the DOS curves of rutile and anatase TiO2 are greatly changed. Both the CB and the VB are shifted negatively. In the case of rutile TiO2 (Figure 5(c)), the electrons contributed by Na are delocalized over the whole range of the DOS, which is consistent with the previous conclusion: although the Na ion has a noticeably larger radius, its barrier against diffusion is not larger, indicating that the ionic radius of the diffusing ion plays a less dominant role than its ability to polarize its surroundings.34

Furthermore, the higher CB is mainly contributed by Na orbitals, instead of the Ti orbitals in the bulk rutile and anatase TiO2. The PDOS clearly demonstrates the interaction of the Ti, Na, and O atoms. By comparison, anatase TiO2 has more overlapping electron orbitals between Ti, Na, and O, suggesting that anatase TiO2 can provide higher activity towards Na ions. As reported by some very recent publications,35-37 a range of oxides as anode materials for Na-ion batteries, including TiO235 and SnO2,36, 37 exhibit different behavior than in the more frequently investigated Li-ion chemistry. For TiO2, besides the Na ions inserted into the TiO2 lattice, Na ions


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also partially reduce the TiO2 and form metallic titanium, sodium oxide, and amorphous sodium titanate.35 We also conducted ex-situ XRD measurements on the as-prepared anatase TiO2 hollow nanospheres, and the results show that after discharge to 0.01 V, all anatase-related diffraction peaks disappeared, and in the following charge state, the peaks did not reappear. This confirms the reversible conversion reaction and the amorphous phase formation occurring in the as-prepared anatase TiO2 hollow nanospheres. Because anatase TiO2 can provide higher activity towards Na ions, as calculated above, it can promote the conversion reaction between the Na ions and TiO2. DFT calculations were also employed to study the Na ion insertion process (insets of Figure 5(c, d). The calculated energy barriers for insertion of 2 Na ions into unit cells of rutile TiO2 and anatase TiO2 are 20.65 eV and 11.10 eV, respectively, indicating much easier transmission of Na+ into anatase TiO2. Therefore, higher capacity will be achieved with the anatase TiO2. Furthermore, the crystal structure of anatase TiO2 presents twodimensional paths for Na+ diffusion, which are along the a-axis and b-axis respectively, as shown in Figure S4(a) and (b) (SI), while the rutile TiO2 only can provide one type of diffusion path for the Na+ (along the c-axis, as shown in Figure S5(c), SI). More available paths for Na+ intercalation give anatase TiO2 the ability to more easily accommodate the Na ions. Furthermore, the excellent cycling stability of the as-prepared anatase TiO2 also can be ascribed to the unique hollow nanosphere architecture, which provides tolerance for volume deformation during the Na+ insertion process. We have developed a finite element model (FEM) to calculate the diffusion induced von Mises stress of this kind of structure.38 It shows that the material in the shell is squeezed out in the radial direction via inelastic flow, and the expansion of the shell is mostly in the radial direction. The voids between the nanoparticles on the shell of the hollow nanospheres allow them to tolerate the tensile stress generated by the insertion of Na+, which can have benefits for long cycling performance. Moreover, the hollow porous nanosphere architecture enhances the cyclability, as was proven by ex-situ SEM observations on the electrodes (Figure 6). It can be seen that after 200 cycles, the spherical architecture is still retained very well. The diameters of individual nanospheres were measured to be around 200 nm, which is not too much of a change from the fresh material, indicating the stability of the hollow nanosphere architecture. Therefore, the asprepared anatase TiO2 hollow nanospheres can withstand extended cycling. 4 Conclusion A template method has been developed to synthesize amorphous TiO2@C nanospheres. Using a sintering approach, a composite of hierarchical rutile and anatase TiO2 hollow nanospheres and pure anatase TiO2 hollow nanospheres were synthesized. The electrochemical properties of the TiO2 nanospheres with different phases were compared for application in Na-ion batteries. The

results show that all of the different types of TiO2 nanospheres can be cycled more than a hundred times. Their capacity and rate capability show significant differences, however. Anatase TiO2 yielded the highest capacity, which is 295 mA h g-1 at the 2nd cycle, tested at 20 mA g-1 current density. It was revealed that anatase TiO2 can supply 2D diffusion paths for the Na ions and more accommodation sites. Furthermore, the lower energy barriers for Na+ insertion into anatase TiO2 imply that the Na+ intercalation can be substantially enhanced, as indicated by the DFT calculations. The anatase TiO2 hollow nanospheres also show excellent cyclability, and they can yield 218 and 212 mA h g-1 at 100 and 200 cycles, respectively, with only 2.8 % capacity decline after hundreds of cycles. The superior cycling stability can be ascribed to the unique hollow nanosphere architecture, which can readily tolerate volume expansion and contraction during cycling. As verified by the ex-situ FESEM analysis, the TiO2 hollow nanosphere structure can be maintained for hundreds of cycles.

Figure 6. Ex-situ FESEM images of anatase TiO2 hollow nanosphere electrode after 200 cycles.

ASSOCIATED CONTENT Supporting Information. FESEM image of carbon nanospheres. TG/DTA curves of amorphous TiO2@C st nanocomposite. 1 cycle discharge profiles of amorphous TiO2@C nanospheres, rutile and anatase hollow nanosphere composite TiO2, and pure anatase TiO2 hollow nanospheres


Chemistry of Materials -1

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at 20 mA h g current density. Crystal structure of anatase and rutile TiO2 projected along different directions.

AUTHOR INFORMATION Corresponding Author *Dawei Su: E-mail: [email protected] Tel: 61-02-4298 1424 Fax: 61-02-4221 5731 *Guoxiu Wang: E-mail: [email protected] Tel: 6-02-95141741 Fax: 61-02-95141460

Author Contributions The manuscript was written through the contributions of all the authors, and all authors have given approval to the final version of the manuscript.

Funding Sources Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC). Fundamental Research Funds for the Central Universities of China (NE2014301).

ACKNOWLEDGMENTS This original research was proudly supported by the Commonwealth of Australia through the Automotive Australia 2020 Cooperative Research Centre (AutoCRC) and the Fundamental Research Funds for the Central Universities of China (NE2014301). The authors acknowledge use of the facilities and the assistance of Dr. Gilberto Casillas at the UOW Electron Microscopy Centre. The authors would like to also thank Dr. Tania Silver for critical reading. REFERENCES 1. Yoshida, H.; Yabuuchi, N.; Kubota, K.; Ikeuchi, I.; Garsuch, A.; Schulz-Dobrick, M.; Komaba, S., P2-type Na2/3Ni1/3Mn2/3− xTixO2 as a new positive electrode for higher energy Na-ion batteries. Chem. Commun. 2014, 50, 3677-3680. 2. Li, S.; Dong, Y.; Xu, L.; Xu, X.; He, L.; Mai, L., Batteries: Effect of Carbon Matrix Dimensions on the Electrochemical Properties of Na3V2(PO4)3 Nanograins for High-Performance Symmetric Sodium-Ion Batteries Adv. Mater. 2014, 26, 33583358. 3. Qian, J.; Xiong, Y.; Cao, Y.; Ai, X.; Yang, H., Synergistic NaStorage Reactions in Sn4P3 as a High-Capacity, Cycle-stable Anode of Na-Ion Batteries. Nano Lett. 2014, 14, 1865-1869. 4. Ji, L.; Gu, M.; Shao, Y.; Li, X.; Engelhard, M. H.; Arey, B. W.; Wang, W.; Nie, Z.; Xiao, J.; Wang, C., Controlling SEI Formation on SnSb-Porous Carbon Nanofibers for Improved Na Ion Storage. Adv. Mater. 2014, 26, 2901-2908. 5. Park, Y. U.; Seo, D. H.; Kim, H.; Kim, J.; Lee, S.; Kim, B.; Kang, K., A Family of High-Performance Cathode Materials for Na-ion Batteries, Na3(VO1− xPO4)2F1+ 2x (0≤ x≤ 1): Combined First-Principles and Experimental Study. Adv. Funct. Mater. 2014, 24, 4603–4614. 6. Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B., A Superior Low-Cost Cathode for a NaIon Battery. Angew. Chem. In Edit. 2013, 52, 1964-1967. 7. He, H.; Jin, G.; Wang, H.; Huang, X.; Chen, Z.; Sun, D.; Tang, Y., Annealed NaV3O8 nanowires with good cycling stability as a novel cathode for Na-ion batteries. J. Mater. Chem. A 2014, 2, 3563-3570.

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Anatase TiO2: Better Anode Material than Amorphous and Rutile Phases of TiO2 for Na-ion Batteries


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Anatase TiO2: Better Anode Material Than Amorphous and Rutile

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