Sulfur-doped anatase TiO2 as an anode for high performance sodium

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Sulfur-doped anatase TiO2 as an anode for high performance sodium-ion batteries Weifeng Zhang, Ningjing Luo, Shuping Huang, Nae-Lih Wu, and Mingdeng Wei ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00471 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Energy Materials

Sulfur-Doped Anatase TiO2 as an Anode for High Performance Sodium-Ion Batteries Weifeng Zhang,a,b Ningjing Luo,b,c Shuping Huang,*,b,c Nae-Lih Wu,d and Mingdeng Wei*,a,b a Institute

of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China c College of Chemistry, Fuzhou University, Fuzhou, Fujian 350002, China d Department of Chemical Engineering, Taiwan University, Taipei 106, Taiwan. KEYWORDS sulfur doping, TiO2, anode, sodium ion batteries, electrochemical performance b State

ABSTRACT: Sulfur-doped anatase TiO2 was prepared through a calcination conversion route for the first time. The grain size of TiO2 with S doping decreased obviously after S doped, manifesting that the introduction of S species could inhibit the crystal growth. Applied as an anode material for sodium-ion batteries, this material exhibited an impressive specific capacity of 174.4 mA h g-1 at a high current density of 10 C after 10000 cycles. The remarkable performance results from the unique crystal structure of anatase TiO2 with bidirectional pore channels for sodium ions intercalation, and S doped TiO2 could increase the electronic conductivity, as well as enlarge the channels structure. Furthermore, density functional theory calculations manifested that the Sdoping increases the volume of lattice slightly, leading to the ease of insertion for sodium ions into anatase TiO2 and a reduced bang gap with higher electronic conductivity. Therefore, S-doped TiO2 showed high reversible capacities and excellent long-term cycling performance.

Introduction Sodium-ion batteries (SIBs) have drew increasing attention and became worldwide study focus as promising alternatives to lithium-ion batteries (LIBs), benefiting from relative abundance of sodium resources and similar “rocking chair” operation principle.1-3 However, sodium ion (1.06 Å) has a high ionization potential and its diameter is obviously larger than that of lithium ion (0.76 Å),4 requiring tolerant structures to remittance its sluggish kinetics for SIBs.5 The poor Na-ion transport and volumetric expansion currently limit the large and stable capacity in most anodes. Such as graphite,6,7 hard carbon8,9,10 and alloying anodes11,12 are the common anode materials for LIBs,13,14 but they suffer from some serious issues including the obvious volume variation during the sodium cycling and irreversible capacity loss when applied in SIBs. Accordingly, it is necessary to obtain suitable anode materials with open and tolerant host structures for achieving acceptable ion diffusion and specific capacity. TiO2 is believed as an hopeful anode material for SIBs, owing to its natural abundance, environmental friendliness.15,16 Unfortunately, like the common issues existed in metal oxide anodes, TiO2 anodes inherently suffer from inferior rate performance and poor cyclability, because of its low electron conductivity and slow Na-ion diffusion.17 To date, various strategies have been employed for preparing TiO2-based anodes with high performance in SIBs. For example, conductive carbon materials (graphene,18,19 carbon nanotubes20 and amorphous carbon21), transition metal additives,22,23 nonmetals,24,25 and so on, have been integrated into TiO2-based composites so as to enhance electron transfer from the surface part to the bulk. On the other hand, the defects in electrode materials often exhibit prominent effects that could lead to

substantial advancement in the scientific field. Therefore, synthesizing TiO2 host material with defect structure is another effective way for enhancing electric conductivity owing to the plentiful oxygen vacancies. For example, shin et al.26 reported that oxygen-deficient TiO2-δ prepared by hydrogen reduction can obtain a high-performance of lithium storage. Wang et al.27 prepared a boron-doped anatase TiO2 with an enhanced rate performance. Moitzheim et al.28 developed chlorine doping of amorphous TiO2 for increasing lithium storage. Therefore, heteroatom doping could narrow the bandgap and enhance electronic conductivity. In this work, sulfur doped TiO2 (S-TiO2) with high electronic conductivity has firstly been synthesized based on a facile thermal conversion reaction using titanate as a precursor. In such a unique structure, S-TiO2 nanoparticles with a high surface area of 254.4 m2 g-1 increase the contact interface of electrode and electrolyte, while the sulfur-doping enhances the electron transport ability, and offers additional storage sites for Na-ions. Used as an anode for SIBs, S-TiO2 showed a superior discharge capacity of 174.4 mA h g-1 after 10000 cycles at 10 C, demonstrating its remarkable long-term cycling stability and reversible capacity. The ultrahigh sodium-storage performance can be explained by the synergetic effects, such as small size of TiO2 nanoparticles and sulfur-doping. Experimental Section Preparation of titanate precursor: The titanate precursor was synthesized according to the earlier literature.29 In short, 0.5 g of TiO2 (P25) was added into the 100 mL of ptfe lining containing 50 mL of 10 M aqueous KOH solution under the vigorously stirring. After 20 minutes, the autoclave was placed in the oven with a temperature of 180 oC for 72 h. Then the obtained sediment was washed with HCl (0.1 M) for several

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times until PH 1-2, and then kept vigorously stirred for four days. The as-prepared sample was then collected after washed by deionized water and then placed at air oven with a temperature of 70 oC for 12 h. Thereafter the obtained titanate precursor was immerged in the mix solution including 15 mL of ammonium hydroxide (NH3•H2O) and 5 mL of hydrogen peroxide (H2O2) for 3 h, and then washed by ethanol and deionized water 3 times before dried in vacuum oven at 70 oC. Synthesis of sulfur doped anatase TiO2: S powder (99.9%, Sinopharm) and titanate precursor were placed on opposite sides of crucible in the furnace carefully, after calcination at 500 oC for 3 h with a ramp rate of 2 oC min-1 in the Ar gas atmosphere. The as-prepared S-TiO2 was obtained after rinsed with carbon disulfide and deionized water, respectively, and dried at 70 °C in the vacuum oven. As a reference, the titanate precursor was annealed at 500 oC for 3 h to obtain pure TiO2. Structural characterization: Transmission electron microscopy (TEM) was performed on a FEI F20 S-TWIN 0 instrument. Scanning electron microscopy (SEM) was performed on a Hitachi S4800 instrument. X-ray photoelectron spectroscopy (XPS) was detected on a Thermo Scientific Escalab 250Xi spectrometer. X-ray diffraction (XRD) and Raman spectroscopy was performed to obtain crystal structure information. N2 adsorption-desorption isotherms were detected by a Micro-meritics ASAP 2020 instrument, pore volumes were determined using the adsorbed volume at a relative pressure of 0.99. The pore size distribution of the S-TiO2 and pure TiO2 were evaluated by a Barrett-Joyner-Halenda (BJH) method based on the adsorption branch. Electrochemical measurements: For the typical electrochemical characterization, the active materials (S-TiO2 and pure TiO2) (70%), poly(vinyl difluoride) (PVDF) (20%) and carbon black (10%) were mixed using N-methyl-2pyrrolidone (NMP) as solution to form a slurry, and then load on the Cu foil used as working electrodes. After dried at 110 oC in a vacuum environment for 12 h, Cu foils with active materials were transferred into the vacuum glove box and assembled as 2025-type coin cells with Na metal (Aldrich) as the counter electrode in the presence of 1.0 M NaClO4 in PC electrolyte solution. The Celgard2400 (America) membranes were used as the separator film. The charge-discharge profiles were performed on a Land automatic batteries tester (Land CT 2001A, Wuhan, China). Cyclic voltammetry (CV) was conducted on an electrochemistry workstation (CHI660c) at 0.5 mV s-1. Density Functional Calculations: The DFT calculations have been performed with generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE)30 exchange-correlation functional by the Vienna ab initio simulation package (VASP).31,32 The projector-augmented-wave (PAW) potentials30 provided in the VASP package were used to represent core electrons and a cut-off energy of 600 eV for plane-wave

+ S powder

basis set were adopted. We used a supercell Ti32O64 (2 × 2 × 2 repetition of the conventional cell (Ti4O8)) to simulate S-doped TiO2 anatase system (Ti32O63S). For geometry optimizations of convention cell and supercell, the Monkhorst-Pack grid33 with 7 × 7 × 3 k points mesh and Brillouin zone sampling limited to the Γ point were used, respectively. The Monkhorst-Pack grid with 2 × 2 × 2 k points was used for the accurate density of the electronic states34. We optimized both the coordinates of the atoms and the lattice constants. The energy criterion for selfconsistency was set to less than 10-6 eV/unit cell, and the force criterion in structure relaxation was set to less than 0.02 eV/Å per atom. Results and Discussion The unique structure of S-TiO2 was synthesized in a relatively simple process as schematically illustrated in Figure 1. In brief, S-TiO2 was prepared based on a facile heat-treatment method using titanate and S powder as raw material. The morphology and structural features of S-TiO2 were examined using SEM and TEM. As presented in Figure 2a, the sample reveals a tiny and irregular shape with an observable diameter of ~50 nm. The TEM image of S-TiO2 demonstrates the two different sizes of nanoparticles (Figure 2b). The HRTEM image displayed in Figure 2c shows well-organized nanocrystalline of anatase TiO2 with interplanar distances of 0.35 and 0.23 nm, corresponding to the d101- and d103-spacing, respectively. However, the smaller one could be classified as TiO2-B, which has been further confirmed by HRTEM. As indicated in Figure S1, the lattice fringe was calculated of about 0.208 nm, corresponding to the d003-spacing of TiO2-B.35 The d-spacing of 0.35 and 0.23 nm are corresponded to the (101) and (004) faces of anatase TiO2. Moreover, SAED pattern shown in Figure 2d reveals a set of diffraction rings, which could be indexed to (101), (103), (200), (105) and (213) planes of anatase TiO2. Elemental analysis further verifies that O, Ti, and S are homogeneously distributed in S-TiO2, manifesting that the sulfur atoms have been successfully doped into TiO2 bulk (Figure 2f-h).

Ar atmosphere 500 oC 3h Titanate

S-TiO2

Figure 1. Schematic illustration for the preparation of S-TiO2.

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Figure 2. (a) SEM image, (b) TEM image and (c) HRTEM image, (d) the corresponding SAED pattern, (e) STEM image as well as (f-h) elemental mappings of S-TiO2, respectively.


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Figure 3. (a) SEM image, (b) TEM image, (c) HRTEM image and (d) the corresponding SAED pattern of pure TiO2, respectively.

For comparison, pure TiO2 was calcined in the vacuum tube furnace without the additive of sulfur, the results are illustrated in Figure 3. SEM and TEM images shown in Figure 3a, b display a cube-like morphology of pure TiO2, which have smooth surface and uniform sizes at around 20 nm. The surface morphology of pure TiO2 is significantly different from that of S-TiO2, verifying fact that the presence of S species might influence the morphology of TiO2. The HRTEM image of pure TiO2 (Figure 3c) reveals that the lattice fringe of 0.35 nm was in consistent with (101) face of anatase TiO2. The SAED pattern shown in Figure 3d is the same as S-TiO2. Figure 4a shows the XRD pattern in a 2θ range of 10o-80o for S-TiO2, and all diffraction peaks are in complete accordance with the standard JCPDS No. 71-1167, indicating that S-TiO2 belongs to an tetragonal phase with a space group of I41/amd, accompanied by a minor phase of TiO2-B.36 The strongest peak at 25.3o corresponds to (101) crystal plane of the anatase TiO2.37 To further explore the defects and imperfections in STiO2 material, Raman spectroscopy, a more sensitive detection technology, was also applied. As indicated in Figure 4b, there are five groups of typical vibration modes located at 144 cm-1 (Eg), 196 cm-1 (Eg), 399 cm-1 (B1g), 513 cm-1 (A1g), and 639 cm-1 (Eg) in every spectrum, indicating that the crystalline phase of S-TiO2 and pure TiO2 is anatase.38,39 For pure TiO2, which shows the higher intensities of Raman spectra, indicating a high crystallinity and high purity of anatase TiO2. However, the defects of S doping can give more active sites and freedom for sodium insertion and thus improve the reversible capacity.

b

(101)

a

Eg(1)

(200)

(213)

(116) (220) (215)

#

(105) (211)

# TiO2-B (103) (004) (112)

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Eg(2)

B1g

A1g

Eg(3)

Figure 4. (a) XRD patterns and (b) Raman spectra of S-TiO2 and p-TiO2, respectively.

a

b

c

d

Figure 5. (a) Full-scale XPS spectra, (b) Ti 2p XPS spectra, (c) O 1s XPS spectra and (d) S 2p XPS spectra of S-TiO2, respectively.

XPS is a useful characterization to determine the chemical state of S-TiO2. As indicated in Figure 5a, the survey XPS spectrum of S-TiO2 is consist of Ti 2p, O 1s, S 2p and C 1s. Figure 5b shows the Ti 2p XPS spectra, in which the Ti 2p3/2 peak centred at 458.7 and Ti 2p1/2 peak centred at 464.4 eV, corresponding to the Ti4+ state.40,41 The O 1s spectra of the STiO2 composite shown in Figure 5c can break up into three bands at 533.5 (O-S), 532.1 (O-Ti-S) and 530.1 eV (O-Ti) with a little bit shifts to higher binding energy, this phenomenon might be caused by the different surface states, including the defects and less electron density.42 Moreover, Figure 5d shows the S 2p spectra, which demonstrated that S was successfully doped into the TiO2 bulk. The S2- species with different peak positions can be attributed to S-Ti (161.2 and 162.4 eV) and S-Ti-O (163.7 and 165.3 eV), respectively. Furthermore, the peak located at 168.8 eV can be ascribed to the fact that S4+ ions replace Ti4+ in the TiO2 bulk, 43 the total content of sulfur was found to be about 3.17%. The presence of defects (Ti3+ or oxygen vacancies) was investigated by electron paramagnetic resonance (EPR) measurements. A signal of g-factor (g = 2.001) was observed for S-TiO2, which results from the unpaired electrons trapped by oxygen vacancies (Figure S2).44-46 BET surface area and BJH pore size distribution (inset) of STiO2 and pure TiO2 are displayed in Figure S3. The N2 adsorption-desorption isotherms of S-TiO2 and pure TiO2 reveal a typical type IV isotherms, with a distinct hysteresis loop at relative pressure ranging from 0.8 to 1. The superior BET surface area for S-TiO2 can be ascribed to its particular nanostructure. Specially, the surface area of S-TiO2 can be calculated of about 254.4 m2 g-1, which is larger than the surface area of pure TiO2 (77.1 m2 g-1). Furthermore, there is a concentrated pore size distribution with ca. 30 nm for S-TiO2, could be attributed the piling of lots of nanoparticles.47 Based on the previous experience, the electrode materials with large specific surface area might generate more active sites for sodium storage, and hence improve the contact of electrode/electrolyte. These advantages are favorable to boost the electrochemical properties of S-TiO2 anode. To demonstrate the effect of the sulfur doping in improving



a

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b

Figure 7. (a) The supercell of TiO2 anatase (Ti32O64). (b) The configuration of S-doped TiO2 anatase supercell. Blue, red, and yellow spheres represent Ti, O, and S atoms, respectively. The unit of bond length is angstrom.

Figure 6. Electrochemical properties of S-TiO2: (a) Chargedischarge profiles, (b) rate capabilities of S-TiO2 and pure TiO2, (c) cycling performance at 10 C for 10000 cycles, (d) CV curves from 0.5 to 10 mV s-1 and the corresponding (e) capacitive contribution.

the sodium storage performance, the electrochemical properties of S-TiO2 and pure TiO2 were explored. The charge-discharge profiles of S-TiO2 anode at a current density of 0.5 C were displayed in Figure 6a. In the first cycle, the large capacity loss is attributed to the formation of solid electrolyte interface (SEI) and the electrolyte decomposition. A plateau at around 0.25 V was corresponded to the reversible reaction of Ti4+/Ti3+,48 which is in conformity with the CV curves (Figure S4). Moreover, S-TiO2 delivers discharge and charge capacities of 492.1 and 198.3 mA h g-1 in the initial cycle with a Coulombic efficiency of 40.2%. In the subsequently cycling, the charge-discharge profiles almost were overlapped, indicating the excellent reversibility of STiO2 anode. The rate capabilities of S-TiO2 at various current densities were further explored. As revealed in Figure 6b, S-TiO2 exhibited a discharge capacity of 220.9 mA h g-1 at 0.5 C, corresponding to about 0.65 Na+ insertion. With the increasing of rate, the reversible capacities of 186.6, 169.4, 156.5, 144.9 and 136.2 mA h g-1 are obtained at 1, 2, 5, 10 and 15 C, respectively. Especially, a capacity of 182.2 mA h g-1 can be achieved while the rate got back to 1 C which is close to its original value, delivering a superior rate performance. Furthermore, long-term cycling performance of S-TiO2 was investigated at 10 C over 10000 cycles. As displayed in Figure 6c, the specific capacity of S-TiO2 is 174.4 mA h g-1 after a activation process, then the capacity keeps stable in the following cycles, showing the structure advantages of the material with an remarkable cycling stability, which is more excellent than that of p-TiO2 (Figure S5). Such phenomenon has been reported in other literature.49 Furthermore, the Coulombic efficiency increases to 98.8% after a few rounds of cycles and remains greater than 99.5% thereafter. Surprisingly, the performance of

S-TiO2 has a good performance advantages compared with other anatase TiO2 anodes in SIBs (Table S1). The morphologies of S-TiO2 at 5 C are well maintained after 500 cycles, revealing a remarkable cycling stability (Figure S6). The CV curves with various scan rates were executed to determine the electrode process, which includes the kinetics of capacitive behavior and diffusion-controded. The high surface area of S-TiO2 is apt to capacitive Na+ storage behavior. In general, the capacitive contributions and diffusion-controded can be expressed by the following power equation (1)50, in which a and b are both arbitrary coefficients, i and v are current and scan rate, respectively. Among them, b value is confirmed from the relationship between log(v) and log(i). i = avb

(1)

Normally, for rechargeable battery electrode materials, the b value of about 0.5, representing an ideal diffusion limiting process, while for pseudocapacitor materials, the b value of about 1.0, representing a surface limited process. The greater the b-value is, the more the contribution of the capacitive process. For example, as indicated in Figure 6d, the b-values of oxidation peaks are 0.87, indicating that the current of STiO2 was dominated preferentially by the capacitive process. Furthermore, the total capacitive contribution can be quantified by the following equation (2)51 i (V) = k1v + k2v0.5

(2)

in which the measured current (i) related to a potential sweep rate (v) can be acquired directly, the capacitive and diffusioncontrolled contributions can be expressed by k1v and k2v0.5, respectively. For instance, it can be calculated that the capacitive contribution can reach to 71.3% at 2 mV s-1. The high values in capacitive contributions might be attributed to their small particle sizes, which have been confirmed to have obvious capacitive behavior.52,53 Similarly, during the other scan rates, the contribution ratios between diffusion-controded and capacitive processes were also quantified. As indicated in Figure 6e, the capacitive contribution of S-TiO2 increases obviously with the increase of scan rate, which can promote the total stored charge practically raise and achieve a maximum value of 83.5% at a scan rate of 5 mV s-1.


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Figure 8. Band structures of (a) anatase TiO2 supercell and (b) S-doped anatase TiO2 supercell. (c) The total and partial density of states of S-doped anatase TiO2 supercell. Γ (0.000 0.000 0.000), X (0.500 0.000 0.000), Z (0.000 0.000 0.500), R (0.500 0.500 0.500).

Therefore, the S-TiO2 can effectively promote the rate capabilities, due to the fact that large surface area and lessened distance benefit for the fast surface adsorption of Na+ ions. Figure S7 showed the EIS of S-TiO2 and p-TiO2 anodes after 50 cycles at 1 C, the charge transfer resistances of the S-TO2 and p-TiO2 are 55.2 and 74.8 Ω, respectively, which indicates that S doping can effectively reduce charge transfer resistance and hence improve the rate capability. Figure 7 shows the anatase TiO2 belonged to tetragonal crystal system with I41/amd space group. The calculated crystal parameters of the unit bulk anatase TiO2 are a = b = 3.803 Å and c = 9.694 Å, which closely agree both with the experimental values of a = b = 3.785 Å, c = 9.514 Å and previous theoretical calculation results.54,55 For pure anatase TiO2, the calculated lengths of Ti-O bonds vertical to z-axis are 1.95 Å and those parallel to z-axis are 2.00 Å. We used a supercell Ti32O64 simulating the S-doped anatase TiO2 system by substituting one O atom with one S atom. The S-doping can cause structural changes. The calculated volume of supercell after S-doping increases from 1122 Å3 to 1139 Å3. The calculated lengths of Ti-O bond vertical to z-axis decrease by 0.14 Å to 1.81 Å and the ones parallel to z-axis have changed from 2.00 Å to 1.97 Å. To analyze the electronic properties of S-doped anatase TiO2, the band structures and density of states (DOS) were also calculated. Figure S8 shows the band structure and DOS of TiO2 anantase convention cell, respectively. The calculated band structure by GGA-PBE shows that anatase TiO2 has a direct band gap of 2.20 eV at Γ point, which is smaller than the experimental value of 3.20 eV. This is derived from the self-interaction problem in GGA which generally underestimates the energy gap. The major contributions of the top of the valence band are from O species, while the bottom of the conduction bands is originated from Ti species. The calculated band structures show that the direct band gap of anatase TiO2 supercell is 2.15 eV (Figure 8a) and the direct band gap of S-doped TiO2 anatase supercell is 1.83 eV (Figure 8b). After doping, the band gap has decreased by 0.32 eV than perfect anatase TiO2 supercell. As shown in Figure 8b, the top

of valence band around the fermi level leads to the reducing of the band gap. And from the partial DOS (PDOS) in Figure 8c, it is evident that the decrease of band gap mainly originates from the S doping for the top of valance band is originated from S species. Therefore, the S-doping can obviously decrease the band gap of anatase TiO2, and hence increase the electronic conductivity of the materials. Conclusions In summary, sulfur-doped anatase TiO2 was prepared through a calcination conversion route for the first time and exhibited a large capacity of 174.4 mA h g-1 after 10000 cycles at a current density of 10 C when applied as an anode for SIBs. The remarkable electrochemical properties of S-TiO2 can be ascribed to the S doping, which could inhibit the crystal growth to a smaller size and enlarge its channels structure for fast Na ions transport. Furthermore, based on DFT calculation, it can be found that the volume of lattice was slightly increased owing to S doping, which could result in more oxygen vacancies and reduce bang gap with higher electrical conductivity, leading to the ease of insertion for sodium ions into anatase TiO2. Therefore, the superior electrochemical performance might be summarized to the facts: i) enhancement of electronic conductivity; ii) the presence of oxygen vacancies; iii) decrease of Na+ ion transport distance; iv) increase of contact area with electrolyte. As a result, STiO2 anode enables superior sodium storage in terms of large capacity, ultrastable cycling, and robust rate capability. Thus, S-TiO2 is a competitive anode candidate for SIBs in the future.

ASSOCIATED CONTENT Supporting Information. Figure S1: TEM image of S-TiO2, the inset shows the corresponding intensity profile for the line across the lattice fringe; Figure S2: EPR spectra of of S-TiO2 and pTiO2; Figure S3: Nitrogen adsorption-desorption isotherms of (a) S-TiO2 and (b) p-TiO2 (inset shows the corresponding BJH pore size distribution curve); Figure S4: CV curves of S-TiO2 with a scan rate of 0.5 mV s-1; Figure S5: Long-term cycling performance of p-TiO2 at current density of 10 C; Figure S6: SEM image of S-TiO2 at 5 C after 500 cycles; Table S1: Electrochemical properties of anatase phase TiO2 anodes in SIBs; Figure S7: Impedance plots of S-TiO2 and p-TiO2 after 50 cycles



at 1 C; Figure S8: (a) Band structure and (b) DOS using the PBE functional for the conventional unit cell of TiO2 anatase. Γ (0.000 0.000 0.000), X (0.500 0.000 0.000), Z (0.000 0.000 0.500), U (0.500 0.000 0.500), Y (0.000 0.500 0.000), S (0.500 0.500 0.000), T (0.000 0.500 0.500), R (0.500 0.500 0.500). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Mingdeng Wei. Email: [email protected] *Shuping Huang. Email: [email protected]

ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (NSFC U1505241 and 21703036).

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+

Ar atmosphere 500 oC 3h Titanate

S-TiO2

400

120 100

300

80

10 C

200

60 40

100

20

0 0

2000

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6000

Cycle number

8000


0 10000

Coulombic efficiency (%)

S powder Capacity (mA h g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Sulfur-doped anatase TiO2 as an anode for high performance sodium

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