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Accepted Manuscript Tubular titanium oxide/reduced graphene oxide-sulfur composite for improved performance of lithium sulfur batteries Junhua Song, Jianming Zheng, Shuo Feng, Chengzhou Zhu, Shaofang Fu, Wengao Zhao, Dan Du, Yuehe Lin PII:

S0008-6223(17)31156-9

DOI:

10.1016/j.carbon.2017.11.042

Reference:

CARBON 12572

To appear in:

Carbon

Received Date: 23 September 2017 Revised Date:

13 November 2017

Accepted Date: 14 November 2017

Please cite this article as: J. Song, J. Zheng, S. Feng, C. Zhu, S. Fu, W. Zhao, D. Du, Y. Lin, Tubular titanium oxide/reduced graphene oxide-sulfur composite for improved performance of lithium sulfur batteries, Carbon (2017), doi: 10.1016/j.carbon.2017.11.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Tubular Titanium Oxide/Reduced Graphene Oxide-Sulfur Composite for Improved

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Performance of Lithium Sulfur Batteries

Junhua Song, 1,2 Jianming Zheng, 2* Shuo Feng,1 Chengzhou Zhu,1 Shaofang Fu,1 Wengao Zhao,2 Dan Du1 and Yuehe Lin1*

School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington

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99164, United States

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

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Abstract

Lithium sulfur (Li-S) batteries are promising alternatives to conventional Li-ion batteries in terms of specific capacity and energy. However, the technical challenges raised from the soluble polysulfide (PS) in organic electrolyte deter their implementation in practical

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applications. Nanoengineered structure and chemical adsorptive materials hold great promise in mitigating the PS migration problem. Here, we develop a tubular titanium

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oxide (TiO2)/reduced graphene oxide (rGO) composite structure (TG) as a sulfur hosting material for constructing better performed Li-S batteries. The TG/sulfur cathode (TG/S) is able to deliver ~1200 mAh g-1 specific capacity with stable operation for over 550 cycles. Moreover, the TG/S composite cathode shows stable Coulombic efficiencies of over ~95% at various C rates, which are ~10% higher than those of the rGO/sulfur (G/S) counterparts. The superior electrochemical performances of TG/S could be ascribed to

*Corresponding author: Email: [email protected] (Yuehe Lin); [email protected] (Jianming Zheng)

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the synergistic effects between the conductive rGO support and the physically/chemically absorptive TiO2, that is, the spatial tubular structure of TiO2 provides intimate contact and physical confinement for sulfur, while the polar TiO2 in TG/S shows strong chemical

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interaction towards the sulfur species.

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1. Introduction

The booming electrified automobile market and increasing energy demand have triggered

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tremendous research interests in energy storage and conversion technologies. Currently, most electric vehicles rely on rechargeable lithium-ion batteries (LIBs) with high energy density.[1] The range anxiety, however, is still one of the main obstacles that distance consumers from owning gasoline-free cars. Developing high energy density batteries becomes indispensable to accelerate the market penetration of battery-powered vehicles.

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Many alternative solutions have been investigated based on the unique merits of various battery systems, including sodium ion batteries,[2, 3] lithium air batteries,[4, 5] and silicon-based batteries.[6, 7] Among the emerging battery chemistries, lithium sulfur (Li-S) batteries stand out with its high theoretical capacity (1675 mAh g-1) and energy

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density (2600 Wh kg-1).[8] Although being attractive in terms of outstanding energy storage capability, Li-S batteries cannot satisfy the working conditions in practical

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applications because of their poor cycling stability. During discharge in organic electrolyte, the sulfur cathode undergoes multi-step conversion reactions with lithium, forming a series of lithium polysulfide (PS) species (Li2Sn, n= 4≤n≤8).[9, 10] These intermediate PS species are soluble in the organic electrolyte and can shuttle across the porous membrane to irreversibly react with lithium anode.[11] Thus, the continuous consumption of active sulfur upon cycling leads to fast capacity decay. To mitigate the dissolution of PSs, chemical adsorption has been identified as an effective strategy by

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providing chemical bonding between hosting materials and active sulfur and their intermediate species.[12] The chemical interaction can trap sulfur species from escaping out of the host structure and increase the efficiency of their utilization.[13] Many polar

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materials have shown active interactions with PSs, including V2O5,[14] SnS2,[15] CoS2 [16], TiS2 [17], C3N4 [18] and MnO2 [19]. Titanium oxide (TiO2) has also demonstrated its strong electrochemical interaction with PSs and hence enabled an improved cycling performance.[20, 21] The semiconducting nature of TiO2, however, itself is not desirable

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as a host structure for poorly conducting sulfur, especially for the corresponding Li-S cells to operate at high current rate. In order to increase sulfur utilization, improving

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electronic conductivity and having an intimate contact between hosting materials and sulfur are of vital importance. Yet, composite materials that integrate strong chemical adsorption, high sulfur utilization and good electronic conductivity are less explored.

Herein, a tubular TiO2 and reduced graphene oxide (rGO) composited host structure (TG)

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was synthesized using a unique chemical reaction of TiO2 in alkaline solution, and spontaneously self-assembling and reduction of GO under hydrothermal condition. During the synthesis, TiO2 forms tubular nanostructure with large hollow space that is favorable for physically confining infused sulfur and later PSs generated during cell

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cycling. Meanwhile, GO transforms into interconnected structure to provide a conductive network to boost sulfur utilization. In addition, the large contact area of TiO2 nanotube

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enables intimate contact with active sulfur, which helps establish strong chemical binding beneficial for alleviating PS migration. The synergistic effect associated with TG structure leads to good electrochemical performance in terms of cycling stability, Coulombic efficiency (CE), and rate capability. The rationally designed sulfur cathode (TG/S) can deliver a high specific capacity of ~ 1200 mAh g-1 at 0.1 and 0.2 C rates (1C = 1675 mA g-1). The average CE of the TG/S reaches > 95% during cycling at various C rates, which is ~ 10% higher than that of the rGO/sulfur (G/S). The hierarchical structure

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of TG/S composite also enables superior long-term cycling performance with high capacity retention of 72%, 76% and 80% in the 0-50th, 50-300th and 300-550th cycles, respectively, which are better than the 64% (0-50th) and 37% (50-300th) of the G/S

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counterpart.

2. Experimental section

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2.1 Synthesis of graphene oxide (GO) sheet

Graphene oxide was synthesized by a modified Hummers method using commercial

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graphite (from Alfa Aesar).[22] To obtain GO sheet, the prepared graphite oxide was ultrasonicated and mechanically stirred for 1 hour. The final concentration of GO sheet solution was controlled to be 2 mg ml-1.

2.2 Synthesis of tubular titanium oxide/graphene oxide (TG) composite

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TG was synthesized by adding 45 mg of TiO2 powder (from Sigma-Aldrich) and 15 ml of GO solution into 2.5 ml NaOH solution (10 M). The mass ratio between TiO2 and GO is 3:2. The mixture was stirred and ultrasonicated for 30 minutes before being transferred and sealed in a 23 ml Teflon-lined stainless-steel autoclaves and kept at 180 °C for 24

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hours. Subsequently, the reaction product was treated with 0.1 M HNO3 solution for 24 hours under magnetic stirring and then washed and filtrated with a copious amount of

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water and vacuum dried overnight. To obtain the final TG product, the powder was further annealed at 420 °C for 2 hours under nitrogen flow in a tube furnace.

2.3 Synthesis of TG/sulfur (TG/S) Sulfur infusion was achieved by mixing sulfur particles (Sigma-Aldrich) with TG powder with a mass ratio of 2:1. To obtain TG/S, the mixed powder was then heat treated at 155 °C for 12 hours under nitrogen atmosphere. As a control, GO/S was also prepared in

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a similar manner with the same sulfur loading in TG/S.

2.4 Materials characterization

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SEM images were taken on a FEI Sirion field emission scanning electron microscope (FESEM). Transmission electron microscopy (TEM) images were obtained by Philips CM200 UT (Field Emission Instruments, USA). High-resolution TEM (HRTEM) images were obtained with a JEOL JEM 2100F TEM at an accelerating voltage of 200 kV. For

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the X-ray spectroscopy analysis (XPS), a Physical Electronics Quantera Scanning X-ray Microprobe was used, which was equipped with a focused monochromatic Al Kα X-ray

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(1486.7 eV) source for excitation and a spherical section analyzer. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images were acquired by energy dispersive X-ray spectroscopy (EDS) using a JEOL-2100F electron microscope equipped with a STEM unit. X-ray Diffraction (XRD) characterization was carried out by a Rigaku Miniflex 600, which was operated at

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40 kV accelerating voltage and 15 mA current. The sulfur content was estimated by thermogravimetric analysis (TGA) using the TA instrument from room temperature to 600 °C under nitrogen atmosphere.

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2.5 Electrochemical measurement The electrode was prepared by slurry casting using doctor blade. The TG/S composite

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was mixed with carbon black and PVDF with a mass ratio of 8:1:1 in N-methyl-2-pyrrolidone solvent. The obtained slurry was then cast on aluminum foil and dried under vacuum at 55 °C for 12 hours. The diameter of sulfur cathode was 8/16 inch. The R2032 coin cells (from MTI Corp.) were assembled in an argon-filled glove box with a separator (Celgard 2400), lithium foil (10/16 inch) as negative electrode and 1 M LiTFSI in 1,3-dioxolane (DOL)/1,2-dimethoxy ethane (DME) (1:1 in volume), 2 wt.% LiNO3 as electrolyte. The sulfur to electrolyte ratio (S/E) is kept at 50 g L-1 unless

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otherwise mentioned. The average sulfur loading is 1.0-1.5 mg cm-2. The electrochemical performance was tested on an Arbin BT-2000 battery tester at room temperature. The capacity is calculated based on the active sulfur component of the cathode.

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Electrochemical Impedance Spectra (EIS) was recorded using CHI660 electrochemical station in a frequency range from 100 kHz to 10 mHz with a perturbation amplitude of ±10 mV.

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3. Results and Discussion

As illustrated in Fig. 1, the TG composite structure was synthesized through a

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hydrothermal reaction followed by a thermal annealing process under an inert atmosphere. It is well studied that under hydrothermal condition, TiO2 nanoparticles can reassemble into tubular TiO2 in concentrated alkaline solution.[23, 24] At the same time, upon losing oxygenated functionalities and weakened π-π stacking, the simultaneously rGO sheets naturally anchor and wrap TiO2 nanotubes to form the three-dimensional (3D) TG

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composite.[25] The morphology and structure of TG were examined by TEM and the result is shown in Fig. 2. The average diameter of TiO2 nanotube is determined to be around 30-40 nm and a few hundred nanometers in length (Fig 2A). SEM gives a macroscopic view of the TG composite structure with the tubular TiO2 intertwiningly

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connected with rGO support (Fig. S1A). To confirm the location of sulfur after infiltration, elemental mapping was conducted. Compared to pristine TG, the increased

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contrast of TiO2 nanotubes in Fig. 2B indicates the successful sulfur impregnation into the inner space of the tubular structure. Indeed, in the elemental mapping analysis of TG/S composite (Fig. 2C-F), the sulfur distribution matches well with the Ti and O of TiO2. Sparsely scattered around TiO2, some sulfur might be homogeneously attached to the functional groups remained in the rGO sheets.[26] In Fig. S1B, negligible structure change after sulfur impregnation without bulk sulfur particles can be seen in the SEM image of TG/S. In contrast, sulfur particles are visually observed to be wrapped by the

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rGO sheet in G/S as shown in the Fig. S1C.

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Fig. 1. Schematic illustration of the synthetic method of TG/S composite as cathode for Li-S batteries.

The amount of sulfur loading is confirmed by the thermal gravimetric analysis shown in Fig. 3A. Consistent with previous studies, the weight percentage of sulfur loading is ~60 %

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in of G/S and TG/S as expected.[27, 28] To confirm the infusion of sulfur, X-ray diffraction (XRD) patterns of pure sulfur, TG/S and G/S are recorded and compared with pristine TG and GO (Fig. 3B). For pure sulfur, the XRD pattern shows sharp diffraction peaks that could be assigned to Fddd orthorhombic crystal structure of sulfur.[28] The

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sulfur characteristic peaks are also observable in TG/S and G/S but with considerably decreased diffraction intensity since most sulfur is buried inside the host structure. It is

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worth noting that the peak intensity of sulfur in TG/S is weaker than that of G/S, despite using the same amount of sample (same amount of S) during XRD scan. This can be easily seen at the position marked out by the squared dash lines as shown in Fig. 3B. The reduced peak intensity suggests that the TiO2 shields the sulfur signal by effectively encapsulating it inside the hollow tube, in contrast to the more exposed sulfur particles when wrapped by rGO. The TiO2 in pristine TG composite consists of mixed anatase and rutile phases.[29]

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Figure 2. TEM images of (A) pristine-TG, (B)TG/S; (C-F) Elemental mapping of Ti, O,

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C and S in TG-S.

PSs dissolution is one of the major issues costing the performance of Li-S batteries. A strong chemical interaction between the host structure and PSs is important to mitigate the detrimental shuttling effects of the dissolved PSs during battery cycling. To

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investigate the possible interaction, X-ray photoelectron microscopy (XPS) was used to analyze the bonding environments between TiO2 and sulfur. In Fig. S2, the Ti 2p spectra

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show typical Ti4+ characteristics with deconvoluted peaks of Ti 2p1/2 (464.8 eV) and Ti 2p2/3 (458.9 eV).[30] The O1s spectra in Fig. 4A reveals an apparent difference between TG composite before and after sulfur infusion. For pristine TG, its O1s spectra can be deconvoluted into three distinct peaks. The peak at 530.6 eV corresponds to oxygen bound to Ti4+ (Ti-O) in TiO2 as well as the C=O bond in rGO.[31, 32] The other two peaks at 531.2 eV and 533.8 eV can be referred to the hydrated oxygen (OH-C) and epoxide (C-O-C), respectively.[33] As for TG/S, the sulfur infusion has resulted in an

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extra characteristic peak at 532.6 eV, which can be assigned to oxygen bound to sulfur (O-S). This is clear evidence proving that the loaded sulfur chemically interacts with the oxygen in TiO2 and oxygenated functional groups of rGO. In the S 2p region,

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carbon-sulfur (C-S) bonds at 163.8 eV and 164.9 eV, and sulfur oxygen (S-O) bonds at 164.4 eV and 165.4 eV further corroborate the strong chemical interaction between TG

(A) TGA curve of G/S, TG/S and pure sulfur. (B) XRD patterns of pure sulfur,

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Fig. 3.

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composite and the infused sulfur.[34]

TG/S, TG, G/S and GO.

The TG/S delivers superior electrochemical performance. Fig. 5A shows the 1st cycle

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charge-discharge profiles of TG/S and G/S electrodes at 0.1 C rate. The discharge capacities are ~1217 mAh g-1 and ~1300 mAh g-1 for TG/S and G/S, respectively. It

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should be noted that the voltage window was intentionally chosen between 1~3 V to ensure complete reduction and oxidation of sulfur during cycling. The complete conversion of sulfur is supposed to maximize “shuttle-effect” upon PSs generation if the host materials cannot effectively confine their movement. Although there is about 50~100 mAh g-1 capacity contribution from the TiO2 nanotube in TG, it is much less than the rechargeable capacity of sulfur. The comparison of the cycling performance at 0.1 C rate is plotted in Fig. 5B. The TG/S shows apparently higher discharge capacity than G/S after

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25 cycles and retains a capacity of 518 mAh g-1 after 200 cycles, much higher than a capacity 366 mAh g-1 retained by G/S. More impressively, the TG/S shows much improved capability in preventing polysulfide migration/shuttling, evidenced by the

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significantly higher average Coulombic efficiency (CE) of ~97%, compared to the ~ 82% of G/S (Fig. 5C). A similar trend is also observed for cells cycled at 0.2 C rate. The discharge capacities are ~1176 mAh g-1 and ~1260 mAh g-1 for TG/S and G/S (Fig. 5D). Again, the stabilized discharge capacity of TG/S after 25 cycles is ~ 200 mAh g-1 higher

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than that of G/S with (Fig. 5E). At 200th cycle, the TG/S can maintain a capacity of ~624 mAh g-1, compared to G/S’s ~439 mAh g-1. In Fig. 5F, despite the CE of G/S gradually

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approaching 90% during cycling, it is always lower than TG/S (>95%).

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Fig. 4. High-resolution XPS characterization of TG and TG/S; High-resolution spectra of

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(A) O 1s and (B) S 2p.

Rate capacity is another examining factor that can gauge the host structure’s confining ability towards PS’s dissolution. Thermodynamically, the long discharge time at low discharge rate aggravates the dissolution of polysulfide and usually results in poor capacity retention and low CE. This phenomenon is already presented in Fig. 5. To further examine the ability of TG composite in deterring PS diffusion, we test and compare it with G/S electrode at different discharge rates. As shown in Fig. 6A, at 0.1 C, 0.2 C, 0.5 C and 1 C rate, TG/S delivers capacities of 1285 mAh g-1, 984 mAh g-1, 898 10

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mAh g-1 and 803 mAh g-1, which are higher than the 1281 mAh g-1, 812 mAh g-1, 723 mAh g-1 of G/S. At 2 C rate, however, the G/S shows superior discharge capacity of 686 mAh g-1 than 606 mAh g-1 of TG/S. This is due to the presence of a significant amount of

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semiconducting TiO2, which decreases the overall electrical conductivity of TG composite and leads to the lower sulfur utilization.[35] Nevertheless, when switched back to 0.1C, TG/S recovers the discharge capacity back to 907 mAh g-1, ~150% of its capacity at 2C. In contrast, G/S can only recover a capacity of 738 mAh g-1

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corresponding to ~107% of its capacity at 2C. The superior rate recovery of TG/S might be attributed to the enhanced PSs absorption on TiO2 upon the addition of conductive

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rGO.[36, 37] At a discharge rate of 0.5C, the PS dissolution is thermodynamically less prominent with the fast discharge current density and short discharge time. Indeed, the capacity difference between TG/S and G/S are narrowed as compared to the electrodes discharged at 0.1C and 0.2C rates. After cycling 200 times at 0.5C rate, TG/S is still able to deliver 631 mAh g-1, which is higher than 542 mAh g-1 obtained for G/S. Despite the

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average CE of G/S is also increased to > 87%, TG/S maintains a pretty high CE of ~97%, consistent with its performance at lower C rates. Long-term cycling was further

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conducted to demonstrate the structural advantages of TG composite.

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Fig. 5. (A-C) Electrochemical performance of TG/S and G/S at 0.1C rate: (A)

Charge-discharge profile; (B) Cycling performance and (C) Coulombic efficiency; (D-E) Electrochemical performance of TG/S and G/S at 0.2C rate: (D) Charge-discharge profile;

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(E) Cycling performance and (F) Coulombic efficiency.

To better compare the electrochemical performance of G/S and TG/S, the discharge/charge voltage range narrowed to 1.7-2.8 V so as to limit the LiNO3

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consumption. The cycling performance is divided into three sections in Fig. 6D. In the first 50 cycles, the G/S shows higher discharge capacity, but exhibits an inferior capacity

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retention of 64%, in comparison with the 72% obtained for TG/S. Since no additional lithium polysulfide was added to the electrolyte, there is a large PS concentration gradient at the electrode/electrolyte interface.[38] The initial capacity decay is mainly caused by the dissolution of active sulfur species in order to quickly reach a concentration equilibrium with lithium polysulfides in the electrolyte. For the following 250 cycles

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(50th to 300th cycle), the TG/S shows over two times better capacity retention (76%) than the G/S counterpart (37%). At 300th cycle, the capacity of G/S drops to 363 mAh g-1, only 54% of the capacity of TG/S (666 mAh g-1). From 300th to 550th cycles, the TG/S electrode further achieves a high capacity retention of 80%. The stable cycling

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performance substantiates the synergistic effect of TG composite as sulfur cathode. Specifically speaking, when coupled with electrical conductive rGO, TiO2’s strong

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polysulfide confinement and chemical adsorption ability can prevent the highly soluble PS species from shuttling between electrodes, securing the sustainable operation of TG/S composite cathode.

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Fig. 6. Rate and long-term cycling performance of TG/S and G/S electrodes: (A) Rate

performance; (B) cycling performance at 0.5 C rate; (C) Coulombic efficiency at 0.5C

4. Conclusion

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rate; (D) Long-term cycling at 0. 2 C rate.

In summary, TiO2/rGO composite has been successfully synthesized and used as sulfur hosting materials, demonstrating a positive effect for improving Li-S batteries’

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performance. Physical confinement of sulfur and chemical interactions between TG and PSs are identified to be the main reasons for the superior electrochemical behaviors.

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Compared to G/S, TG/S electrodes show promising capability in trapping PS from shuttling between electrodes at various current densities, achieving with higher stabilized discharge capacities at various current densities along with greatly improved CE. The findings of this work provide insights on the synergistic influence of sulfur host structure on the performance Li-S batteries. In addition, the knowledge gained in this study can be generalized for fabricating advanced sulfur cathode of similar hierarchical structures for constructing high-performance Li-S batteries.

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Acknowledgement

Professor Yuehe Lin would like to thank the Start-Up fund from Washington State

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University.

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