Polypyrrole as a

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Rational Design of TiO-TiO2 Heterostructure/Polypyrole as Multifunctional Sulfur Host for Advanced Lithium Sulfur Batteries Guilin Chen, Wentao Zhong, Yunsha Li, Qiang Deng, Xing Ou, Qichang Pan, Xiwen Wang, Xunhui Xiong, Chenghao Yang, and Meilin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19501 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Applied Materials & Interfaces

Rational Design of TiO-TiO2 Heterostructure/Polypyrole as Multifunctional Sulfur Host for Advanced Lithium Sulfur Batteries Guilin Chena,b, Wentao Zhonga,b, Yunsha Lia,b,Qiang Denga,b, Xing Oua,b, Qichang Pana,b, Xiwen Wanga,b, Xunhui Xionga,b, Chenghao Yanga,b*, Meilin Liuc a Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, P. R. China b Guangdong Engineering and Technology Research Center for Surface Chemistry of Energy Materials, New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China c School of Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332-0245, USA * Corresponding authors. E-mail addresses: [email protected] (C. Yang) Abstract Despite outstanding theoretical energy density (2600 Wh kg-1) and low cost of lithium-sulfur (Li-S) batteries, their practical application is seriously hindered by inferior cycle performance and low Coulombic efficiency due to the “shuttle effect” of lithium polysulfides (LiPSs). Herein, we proposed a strategy that combines TiO-TiO2 heterostructure materials (H-TiOx, x = 1, 2) and conductive polypyrole (PPy) to form a multifunctional sulfur host. Initially, the TiO-TiO2 heterostructure can enhance the redox reaction kinetics of sulfur species, and improve the conductivity of sulfur cathode together with the PPy coating layer. Moreover, the defect abundant

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H-TiOx matrices can trap LiPSs by the formation of Ti-S bond via the Lewis acid-base interaction. Furthermore, the PPy coating can physically hinder the diffusion of LiPSs, as well as chemically adsorb LiPSs by the polar-polar mechanism. Benefiting from the synergistic effect of H-TiOx and PPy layer, the novel cathode delivered high specific capacities at different current rates (1130, 990, 932, 862 and 726 mAh g-1 at 0.1, 0.2, 0.3, 0.5 and 1 C, respectively) and an ultra-low capacity decay of 0.0406% per cycle after 1000 cycles at 1 C. This work can not only indicate effectiveness of employing the H-TiOx materials to realize the LiPSs immobilization, but also enlighten the feasibility of combining different materials to achieve the multifunctional sulfur hosts for the advance Li-S batteries. KEYWORDS: lithium-sulfur batteries; TiO-TiO2 heterostructure; synergistic effect; Lewis acid-base interaction; LiPSs immobilization


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1. Introduction Li-S batteries are deemed as the most promising candidate for the next generation of energy storage systems, benefiting from their low cost sulfur cathode, attractive high theoretical specific capacity (1675 mAh g-1) and environmental benignity.1-3 Unfortunately, their disillusionary short cycle life and poor rate performance result from the insulation of sulfur and discharged products (Li2S2/ Li2S), notorious

“shuttle

effect”

and

great

volume

change

of

cathode

during

lithium-delithium process, greatly impede their commercial application.4-6 To address the mentioned problems, much efforts have been made to explore the appropriate sulfur host materials. Conductive carbon matrices, such as graphene,7 carbon nanotubes,8 microporous,9 mesoporous10 and macroporous11 carbons are considered to be the promising sulfur hosts. However, their lack of polarity means that the migration of LiPSs can only be inadequately suppressed by physical confinement rather than chemical interaction, which will further cause catastrophic “shuttle effect”.12 Doping is an effective approach to enhance the polarity of carbon materials. For example, nitrogen,13 oxygen14 and sulfur15 doped graphene materials all exhibit better chemical affinity between the host and LiPSs than the undoped ones. Unfortunately, their chemisorption on LiPSs are mainly via weak polar-polar interaction, which leads to the limited inhibition of the “shuttle effect”. In order to further enhance the chemical interaction (via Lewis acid-base interaction) between the host and polar LiPSs, polar metallic compound such as metal oxidation,16,17 metal sulfides,18–22 metal carbide,23 metal selenide,24 metal hydroxide25 and metal nitride26,27



are used as sulfur hosts. Strong chemical binding ability on LiPSs by these sulfur host makes it possible for the alleviation of the LiPSs diffusion, thus greatly prolong the cycle life of the Li-S batteries. Recently, intensive attention has been captured by titanium dioxide (TiO2) in the field of Li-S batteries, due to its nontoxicity, tunable morphology, and certain LiPSs immobilization ability.28–31 However, suffering from its essential low electrical conductivity and relatively low polarity, when used as the sulfur host, the sulfur utilization and LiPSs entrapment are quite inadequate. Defects abundant titanium oxides, such as Magnéli-phase TinO2n-1 (3 < n < 10) materials and nonstoichiometric titanium monoxide TiOy (y = 0.70-1.25), have stimulated great research interests due to their enhanced conductivity and polarity.32-34 As kind of nonstoichiometric titanium monoxide TiOy, TiO is of outstanding conductivity and superior polarity due to its abundant vacancies (about 15 at%) in the titanium and oxygen sublattices.35 Its high conductive bulk and Ti-sites (Lewis-acid) rich surface enable the adequate entrapment of the LiPSs (soft Lewis-base) by the Lewis acid-base interaction.36,37 In addition, heterostructures constructed by coupling nanocrystals with different bandgaps have been proved an effective approach to accelerate charge transport and enhance surface reaction kinetics. For example, NiO-NiCo2O4, VO2-VN and MXene-MoS2 heterostructure sulfur hosts all exhibit significant improvement on conductivity and dynamical enhance on LiPSs redox reaction.38-40 Inspired by the unique advantages of TiO and heterostructures, we propose a rational integration of TiO with TiO2 to afford TiO-TiO2 heterostructure material, and applied it to the field of Li-S batteries for the


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first time. On the other hand, previous reports have revealed that conductive polymer coating of sulfur can block the diffusion of LiPSs physically and chemically, thus restraining the “shuttle effect” to some extent.41,42 Therefore, combining TiO-TiO2 hetreostructure (H-TiOx, x = 1, 2) materials and conductive polymer to fabricate multifunctional sulfur host could be a promising approach to synergistically boost the performance of Li-S batteries. In this work, novel H-TiOx@S/PPy composites have been developed, which was consisted of H-TiOx matrices, sulfur and PPy layer. Specially, sublimed sulfur was firstly loaded into porous H-TiOx matrices, then coated with PPy to afford a sandwich-type H-TiOx@S/PPy composites. Initially, the inner H-TiOx matrices can chemically adsorb the LiPSs by Lewis acid-base interaction and promote the surface reaction kinetics via TiO-TiO2 heterostructures. Moreover, the outer PPy layer can not only act as the physical barrier of the LiPSs diffusion, but also the adsorbent for the chemisorption of LiPSs, due to the polar-polar bonding between polar N-atom and Li+.43 As a result, the H-TiOx@S/PPy composite cathode exhibits a superior comprehensive

electrochemical

properties

compared

with

the

counterparts

(H-TiOx@S, TiO2@S, CNT@S and PPy@S). The electrochemical test results show that such hierarchical structure simultaneously realize the significant improvement on rate performance, great promotion of capacity and outstanding cycle stability of Li-S batteries.

2. Experimental Section 2.1. Synthesis of H-TiOx. Different reduced H-TiOx materials were prepared with



varying molar ratio of Mg: TiO2 (1, 2 and 3). The certain amount of TiO2 nanoparticles (40-50nm, 99.9%, Aldrich) and Mg powders (100-200 mesh, Sinopharm Chemical Reagent) were mixed by ball mill for 2 h. Then the TiO2/Mg mixture was sealed in a quartz tube and heated in argon atmosphere at 750 ℃ for 6 h. After the annealing treatment, the resultant black powder was stirred for 12 h in 2.0 M HCl solution to remove the residual Mg and etch the H-TiOx particles. Then the black precipitate was obtained by centrifugation, and rinsed with sufficient amount of deionized (DI) water until the the pH of supernatant above 6. Finally, the resultant H-TiOx was dried in the vacuum oven at 60 ℃ for 12 h. The H-TiOx prepared under the condition of Mg: TiO2 = 3 was used as the sulfur host and LiPSs adsorbent in the following tests. 2.2. Synthesis of H-TiOx@S composite. H-TiOx and sublimed sulfur with the mass ratio of 3:7 were dispersed in a mixed solution of CS2 and N, N-dimethylformamide (V/V = 5:2) by ultra-sonication for 0.5 h, then the resultant intermixture was stirred at 45 ℃ for 12 h to evaporate the CS2 completely. The resultant suspension was filtered and rinsed twice with ethanol and DI water, respectively, then dried in the vacuum oven at 60 ℃ overnight. Finally, the mixture was heated under an argon flow at 155 ℃ for 6 h to afford the H-TiOx@S composite. In addition, the TiO2@S and CNT@S composites were obtained in the similar processes. 2.3. Synthesis of H-TiOx@S/PPy composite. 0.3 g H-TiOx@S composite, 20 mg lauryl sodium sulfate and 10 μL Triton X-100 (99.9%) were added into the mixed


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solvent of DI water and ethanol (V/V = 9:1) and stirred for 0.5 h. Then 20 μL pyrrole was dispersed in the resultant suspension by ultra-sonicationfor 5 min. After that, 10 mL of 0.1 M (NH4)2S2O3 solution was added drop-wise into the above suspension under the condition of ice bath and stirring for 4 h. The product was filtered and washed with ethanol and DI water 2 times, respectively, then vacuum-dried overnight at 60 ℃. In addition, the PPy@S composites were obtained in the similar processes. 2.4. Visualized LiPSs adsorption test. A Li2S4 solution (0.05 M) was homemade by dissolving 55 mg lithium sulfide (Alfa Aesar) and 200 mg sublimed sulfur into 25 mL mixed solvent of dimethoxyethane/dioxolane (DME/DOL,V/V = 1:1) under vigorous stirring overnight in an Ar-filled glovebox. 0.1 g H-TiOx/PPy (0.095 g H-TiOx + 0.05 g PPy), H-TiOx, TiO2 and CNT were slowly added into 5 mL Li2S4 solution, respectively. After 5 minutes and 12 h of soaking, 100 μL supernatant was taken out and diluted to 5 mL by DME for the evaluation of residual concentration of Li2S4 by UV-vis spectroscopy (UV lambda 750, Japan). After 12 h of soaking, the Li2S4 absorbed hosts were collected and dried in the Ar-filled glovebox to conduct the X-ray photoelectron spectroscopy (XPS) measurements (Thermo, K-Alpha+ spectrometer). To avoid the oxidation and pollution causing by oxygen and water, the above UV-vis spectroscopy and XPS samples were sealed in Ar-filled vials and quickly transferred to the test system. 2.5.

Materials

characterization.

The

morphologies

of

materials

were

characterized by a Hitachi SU8010 field-emission scanning electron microscopy (FESEM). The elemental mapping analysis was conducted by an energy dispersive



spectroscopy (EDS) detector. TEM images were obtained by using a JEM-2100F transmission electron microscope operating at 200 kV. X-ray diffraction (XRD) patterns were recorded in the range of 2θ = 5-80° in a Bruker D8 Advance powder X-ray diffractometer with Cu Kα radiatio, and the working current and voltage of the diffractometer were 40 mA and 40 kV, respectively. Thermogravimetry analysis (TGA) was conducted with a Mettler-Toledo TGA/DSC1, heating from 30 to 800 °C at a ramp of 5 °C min-1 under N2 flow (30 mL min-1). Brunauer-Emmett-Teller (BET) properties were collected by a Micromeritics ASAP 2020 analyzer. 2.6. Electrochemical characterization. Electrochemical measurements were conducted by CR2032 coin-type test cells assembled using lithium metal as the counter and reference electrodes in an argon-filled glovebox. Celgard 2400 membrane was used as the separator. Working electrodes involving different sulfur loading materials were prepared by mixing 70% of the active materials (H-TiOx@S/PPy, TiOx@S, TiO2@S and CNT@S), 20 wt% of super P, 10 wt% of sodium alginate binder and certain amount of DI water to form a homogeneous slurry. After that, the above slurry was equally coated on the carbon paper (Toray, TGP-H-120) by a blade and dried in a vacuum oven at 60 °C for 12 h. Subsequently, it was cut into circular pieces with a 14 mm diameter to afford the H-TiOx@S/PPy eletrodes. The general mass loading of sulfur on each electrode was 0.8-1.0 mg cm-2 The electrolyte was prepared by mixing 1 M bis-(trifluoromethane) sulfonamide lithium (LiTSI, TCI Japan) and 2 wt% LiNO3 with the solvent of DME and DOL (V/V = 1:1). The electrolyte/sulfur ratio was maintained at about 30:1 (μL mg-1). The galvanostatic


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experiments of the as-assembled cells were conducted on a LAND-BT2013A measurement system with a voltage range from 1.7 to 2.8 V at 25 ℃ after aging for 24 h. The electrochemical impedance spectroscopy (EIS) was recorded by a CHI660E workstation with a frequency ranged from 100 kHz to 0.01 Hz. The cyclic-voltammetry (CV) tests were carried out by a CHI660E workstation with a voltage window of 1.7-3.0 V at a scanning rate of 0.1 mV s-1.

3. Results and discussions

Figure 1. (a) Schematic illustration for the synthesis of H-TiOx@S/PPy composite. SEM images



of (b) H-TiOx with the Mg : TiO2 = 3; (c) H-TiOx@S; (d) H-TiOx@S/PPy, respectively. (e) High-resolution TEM image of H-TiOx. (f) TEM images for H-TiOx @S and (g) sandwich-type H-TiOx@S/PPy composites.

The synthesis process of the H-TiOx@S/PPy composite is illustrated as Figure 1a. TiO2 nanoparticles were used as the raw materials to prepare H-TiOx by a facile magnesiothermic reduction method. As shown in the scanning electron microscopy (SEM) picture in Figure S1a, the TiO2 nanoparticles are with a smooth surface, irregular shape and an average particle size of about 40-50 nm, showing obvious agglomeration. Firstly, magnesium was used as a reducing agent to reduce TiO2 to H-TiOx at the temperature of 750 ℃ for 6 h under the argon atmosphere. After that, the resultant black powder was stirred in the concentrated HCl solution to remove the residual Mg and etch the H-TiOx particles. Figure 1b, S1b and S1c show H-TiOx particles with unique porous structure and quite rough surface obtained by varying the ratio of Mg/TiO2 (from 1:1 to 3:1). Obviously, the more Mg was used, the rougher surface of H-TiOx nanoparticles were etched by concentrated HCl solution, which means more space for sulfur loading and more chemisorption sites for LiPSs immobilization (Figure S2a-c). It is worth noting that the size of TiO particles (about 1 μm) grew dramatically after calcining, due to the nucleation and growth of the TiO2 nanoparticles.44


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Figure 2. (a) XRD patterns for nano TiO2 (PDF#21-1272), H-TiOx (Mg : TiO2 = 1, 2, 3; PDF#65-2900), H-TiOx@S/PPy (PDF#08-0247). (b) TGA curves for H-TiOx@S/PPy, H-TiOx@S, TiO2@S and CNT@S. (c) XPS survey spectra for pure TiO2 and H-TiOx. (d) XPS spectra o f Ti 2p for pure TiO2 and H-TiOx (black hollow dots lines: experimental data, red solid line: entire fitted data, solid lines in other colors:fitted data for individual components).

Evidence of the formation of TiO-TiO2 heterostructure stems from the high-resolution TEM image of H-TiOx (Figure 1e), which shows distinct interlayer spacing of 0.1490 and 2.082 nm, corresponding to the (204) lattice facet of TiO2 and (200) lattice facet of TiO, respectively. Moreover, the clear interface (orange dotted line) between TiO and TiO2 can be can be easily recognized, which can not only accelerate electron transfer, but also act as the LiPSs absorption sites.40 In addition,



the XRD patterns (Figure 2a) of TiOx show diffraction peaks of TiO and TiO2, confirm the partial reduction of original TiO2 (PDF#21-1272) to TiO (PDF#65-2900). Moreover, the intensity of TiO peak in the XRD patterns increases with the rise of Mg ratio (from 1 to 3), which indicates a crystallization improvement of H-TiOx. It should be noted that the H-TiOx prepared under the condition of Mg: TiO2 = 3 was used as the sulfur host and LiPSs adsorbent in the following tests. On the other hand, XPS spectra of pristine TiO2 and H-TiOx (Figure 2c-d) show further evidence of the existence of TiO. Survey XPS spectra of TiO2 and H-TiOx (Figure 2c) both exhibit obvious Ti 2p signals. However, the signal of reduced Ti2+ can only be observed from the high-resolution Ti 2p XPS spectra for H-TiOx (Fiure 2d) at a binding energy of 453.9 and 4.57.9 eV,45 proving the reduction of Ti4+ to Ti2+. Besides, the peaks of Ti-O bond at 458.4 and 464.2 eV both undergo 0.3 eV positive shift after annealing, which further confirm the reduction of TiO2 and the formation of oxygen vacancies during the magnesiothermic reduction process.46 Sublimed sulfur was loaded onto the H-TiOx host by a modified “CS2 volatilization + sulfur melt-diffusion” method.47 SEM image (Figure 1c) exhibits that the pores of H-TiOx are fully filled with sulfur, and the surface of the H-TiOx@S composite is relatively smooth compared with the initial rough morphology of H-TiOx (Figure 1b). In addition, TEM image (Figure 1f) shows well-defined sulfur layer, indicating an effective sulfur infiltration. The specific surface area and pore volume of H-TiOx nanoparticles shown in Figure S2c are 20.34 cm2 g-1 and 0.125 cm3 g-1, respectively, which enable the infiltration of sulfur. In addition, to prevent the sulfur covered on the


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surface of H-TiOx from being directly exposed to the electrolyte, coating PPy on the H-TiOx@S to afford the H-TiOx@S/PPy material is necessary. Besides, TGA analysis (Figure 2b) reveals that the sulfur content in H-TiOx@S composite is 66.98 wt%. The polar conductive PPy layer was coated on the surface of H-TiOx@S composite by an in-suit polymerization strategy.48 SEM image shows rough surface of the H-TiOx@S/PPy composite (Figure 1d), and the specific surface area and pore volume of the H-TiOx@S/PPy composite shown in Figure S2d are 5.12 cm2 g-1 and 0.027 cm3 g-1, implying the effective coating of PPy. On the other hand, TEM image (Figure 1g) reveals a sandwich-type hierarchical structure of H-TiOx@S/PPy, whose PPy layer thickness is about 10-20 nm, and its energy dispersive spectrometry (EDS) in Figure S3 further comfirms the homogeneous distribution of Ti, S, N, O and C, indicating a successful incorporation of H-TiOx, S and PPy. The XRD pattern of H-TiOx@S/PPy (Figure 2a) shows intensive diffraction peaks of sulfur (PDF#08-0247), which verifies that sulfur is the primary crystalline phase. The sulfur content of H-TiOx@S/PPy composite was determined by TGA analysis (Figure 2b), a slight decrease of 66.98% to 66.33% can be ascribed to the dissolution of sulfur during the polymerization process in the solution.



Figure 3. (a) EIS plots of cells with H-TiOx@S/PPy, H-TiOx@S, TiO2@S and CNT@S cathodes at open-circuit voltage conducted from 100 kHz to 10 mHz with an amplitude of 5 mV. (b) CV profiles of H-TiOx@S/PPy cathode with a voltage window of 1.7-3.0 V at a scanning rate of 0.1 mVs-1. (c) Charge/discharge profiles for the first cycle at 0.5 C, (d) rate performance profiles and (e) Cycling performance at 0.5 C of cells with H-TiOx@S/PPy, H-TiOx@S, TiO2@S and CNT@S cathodes, respectively. (f) Cycling performance of cell with H-TiOx@S/PPy cathode at 1 C.

To verify the feasibility of the strategy that combines H-TiOx matrices and PPy coating for advance Li-S batteries, coin cells using Li metal as anode materials were assembled. EIS plots of cells with H-TiOx@S/PPy, H-TiOx@S, TiO2@S and CNT@S cathodes at open-circuit voltage measured from 100 kHz to 10 mHz with an amplitude of 5 mV are shown in Figure 3a. The Nyquist plots of tested fresh cathodes


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share the similar characteristics of a depressed semicircle at high-medium frequency and a linear tail at low-frequency. According to the previous reports, the charge transfer resistance (Rct) can be measured by the diameter of semicircle at high-medium frequency region.49,50 In this case, Rct order of different cathodes is: H-TiOx@S/PPy (14.49 Ω)< H-TiOx@S (15.1 Ω)< CNT@S (25.79 Ω)< TiO2@S (42.23 Ω). This result implies that the conductivity of H-TiOx@S composite is better than that of CNT@S and TiO2@S composite, benefiting from the defects abundant TiO and electron transfer enhanced TiO-TiO2 heterostructure. It is worth noting that the conductivity of H-TiOx@S composite can be further improved by the coating of conductive polymer PPy, in generally, the enhanced conductivity means faster electron transport during the electrochemical reactions, leading to higher sulfur utilization ratio and batter rate performance of Li-S batteries.51 The cyclic-voltammetry (CV) profiles of H-TiOx@S/PPy cathode conducted from 1.7 to 3.0 V at a scan rate of 0.1 mV s-1 are presented in Figure 3b. In the cathodic scan curves, two well-defined reduction peaks are ascribed to the multistep lithiation of elemental sulfur to soluble high-order LiPSs (i.e. Li2Sx, 4≤ x≤ 8) and their subsequent conversion to solid-state discharged products (i.e. Li2S2/ Li2S), respectively. In the following anodic scan, the well distinguished oxidation peaks are correspond to delithiation of Li2S2/ Li2S to elemental sulfur.52 Different from irregular peaks intensity fluctuations and position shift in CV profiles of CNT@S, TiO2@S and H-TiOx@S cathodes (Figure S4a-c), there are no obvious current or potential variation in the cathodic and anodic peaks after repeated scans and the curves are



almost overlapped within four cycles in the CV profiles of H-TiOx@S/PPy cathode. In addition, the sharp cathodic and anodic peaks signify rapid redox kinetics of sulfur and LiPSs, benefiting from the dynamical enhance on sulfur species redox reaction by TiO-TiO2 heterostructure.53 The CV result suggests excellent reversibility and stability of the cell with H-TiOx@S/PPy cathode, which could be attributed to the improvement on conductivity and enhancement of LiPSs diffusion restriction ability by the combination of H-TiOx and PPy. Corresponding to the CV curves, two typical discharge voltage plateaus for Li-S batteries are observed in the charge/discharge voltage profiles of H-TiOx@S/PPy, H-TiOx@S, TiO2@S and CNT@S cathodes at a rate of 0.5 C (Figure 3c).54 Because of the improved conductivity and redox kinetics, the H-TiOx@S/PPy cathode shows a relative high initial discharge/charge capacities of 1087/1050 mAh g-1 in comparison with the other counterparts, with a high initial Coulombic efficiency of 96.60%. Such results indicate favorable utilization of the active sulfur species during the redox process.55 To further examine the facilitation of H-TiOx and PPy on the electrode conductivity and redox kinetics, rate performance tests of H-TiOx@S/PPy, H-TiOx@S, TiO2@S and CNT@S cathodes were conducted by gradually increasing the charge/discharge current density from 0.1 to 1 C (every 10 cycles) and reverting to 0.1 C, the test results are shown in Figure 3d. When the current rates rise from 0.1 to 1 C, the specific capacities of H-TiOx@S/PPy at 0.1, 0.2, 0.3, 0.5 and 1 C are 1130, 990, 932, 862 and 726 mAh g-1, respectively, which are generally higher than those of


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H-TiOx@S, TiO2@S, CNT@S cathodes. More importantly, after undergoing a series of different current rates, a high reversible specific capacity of 1120 mAh g-1 is obtained when the current rate is gradually switched back to 0.1 C, which is close to the initial value at 0.1 C (99.12% retention). In contrast, cells with the TiO2@S, CNT@S cathodes show inferior specific capacity at high current (1 C), and obvious specific capacity fade after being tested under different rates. Specially, when the current rate switches to 1 C, the specific capacity of TiO2@S, CNT@S cathodes are 302 and 503 mAh g-1, respectively. When the current rate returned to 0.1 C, their specific capacity retention rate are 92.64% and 58.16%, respectively, suffering from their poor conductivity and inadequate LiPSs adsorption ability. These results demonstrate that H-TiO@S/PPy possesses outstanding advantages in aspects of both capacity retention rate and high rate performance.56 For the purpose of inspecting the long cycle performance of different cathodes, the above cathodes are measured at a current rate of 0.5 C for 500 cycles (Figure 3e). Profiting from the high utilization ratio of sulfur and sufficient LiPSs trapping, the H-TiOx@S/PPy cathode shows a high initial discharge specific capacity of 1050 mAh g-1 and keeps at 590 mAhg-1 after 500 cycles, exhibiting an average specific capacity decay rate of merely 0.0876% per cycle. Moreover, the Coulombic efficiency of H-TiOx@S/PPy cathode remains at more than 99%, reflecting remarkable cycle stability. In the meantime, after 500 cycles, the H-TiOx@S, TiO2@S, CNT@S cathodes suffer the average specific capacity decay of 0.108%, 0.109% and 0.0931%, respectively. Such performances can be attributed to their poor inhibition on the



“shuttle effect”, proving the LiPSs immobilization advantages of H-TiOx@S/PPy cathode from the reverse side.57 Long cycle performance of Li-S batteries under high current rates is of great significance for their practical applications. As shown in Figure 3f, after 5 cycles of activation under a rate of 0.1 C, the H-TiOx@S/PPy cathode (sulfur loading = 1.0 mg/cm2) delivers an initial discharge capacity of 693.7 mAh g-1 at 1 C, and slowly decline to 541.0 mAh g-1 after 400th cycles. Then, a mild decrease is observed between 400 and 800 cycles. After that, the specific capacity keep almost unchanged till 1000 cycles, and a high specific capacity of 411.7 mAh g-1 was achieved at the 1000th cycle, corresponding to a capacity retention of 59.35% with the capacity decay of only 0.0406% per cycle. Furthermore, the Coulombic efficiency of H-TiOx@S/PPy cathode under 1 C are nearly 100% even after 1000 cycles, implying an effective suppression of the “shuttle effect”.58 Furthermore, favorable electrochemical performance are still achieved when the sulfur loading of the H-TiOx@S/PPy electrode increased to 2.4 mg/cm2 (Figure S5). Besides, it is worth noting that the electrochemical performance of H-TiOx@S/PPy cathode at 0.5 C and 1 C are comparable with those reported in state-of-the-art doped carbon materials@S, conductive polymer@S, metal oxide@S, defective compound@S and heterostructure materials@S composite sulfur cathode (Table S1 shows more details), and better than those of PPy@S cathode (Figure S6). To figure out the reasons for different electrochemical performances in the above cathode, further study of interactions between different sulfur host materials


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(H-TiOx/PPy, H-TiOx, TiO2 and CNT) and LiPSs were conducted, using a combination of ex-situ UV-Vis spectroscopy, visual LiPSs absorption ability test and XPS analysis. Li2S4 was employed as the representative LiPSs. A 0.05 M Li2S4 solution was prepared by previously reported approach.59 In the test, the same mass of TiOx/PPy, TiOx, TiO2 and CNT were added into a brown yellow Li2S4 solution, different LiPSs absorption ability of sulfur host materials is revealed directly by the decoloring degree of Li2S4 solution. As shown in the Figure 4a-b, Li2S4 solution with H-TiOx/PPy faded obviously after 5 min and became almost transparent after 12 h, exhibiting outstanding adsorption ability to LiPSs. While Li2S4 solution with H-TiOx decolored within 5 min and insufficiently faded even after 12 h. In addition, Li2S4 solution with TiO2 shows slightly decoloration after 12 h, and Li2S4 solution with CNT exhibited almost no change in color, due to their relatively weak interaction with Li2S4.



Figure 4. Physical appearances of resultant Li2S4 solution after soaking 0.1g H-TiOx/PPy, H-TiOx, TiO2 and CNT, (a) for 5 min, (b) for 12 h. Corresponding UV-vis spectra of the Li2S4 solution, (c) for 5 min soaking, (d) for 12 h soaking.

The supernatant of above solution were diluted 50 times and used to detect residual Li2S4 concentrations by an ex-situ UV-Vis spectroscopy, the results are shown in Figure 4c-d. The characteristic band around the region of 300-325 nm can be ascribed to Li2S4 in the solution.60 Either after 5 min or after 12 h, the intensity order for the absorption band is Blank> CNT> TiO2> H-TiOx> H-TiOx/PPy, which indicates that the concentration of Li2S4 in the corresponding supernatant decrease in turn. The analysis results show that H-TiOx/PPy possesses the best LiPSs absorption ability, fitting with the visual observation results.


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Figure 5. High-resolution XPS of (a) S 2p spectra of Li2S4 and (b) Li2S4 soaked H-TiOx/PPy. (c) Ti 2p spectra of Li2S4 soaked H-TiOx/PPy. (d) Li 1s spectra of Li2S4 soaked H-TiOx/PPy (Black hollow dots lines: experimental data, red solid line: entire fitted data, solid lines in other colors: fitted data for individual components).

In order to further probe into the surface chemical interaction between LiPSs and H-TiOx/PPy, XPS analyses were conducted on the dry powder of Li2S4 and Li2S4 soaked H-TiOx/PPy. As shown in Figure 5a, S 2p spectra of Li2S4 exhibit three well defined peaks at 161.5, 163.1 and 167.0 eV, which can be ascribed to the terminal S (ST-1), bridging S (SB0) and thiosulfate, respectively.61,62 However, after soaking H-TiOx/PPy for 12 h, the spectra of S 2p (Figure 5b) change dramatically, where the peaks of ST-1, SB0 and thiosulfate all undergo certain positive shift and intensity change. Specially, peaks of SB0, thiosulfate experienced a + 0.5 eV, + 0.7 eV shift to 163.6 eV and 167.7 eV, respectively, and the positive shift of ST-1 peak even reached 1.7 eV. The notable shift of SB0 and ST-1 peaks can be attributed to the formation of the SB0-Ti and ST -1-Ti bonds, due to interaction between SB0, ST-1 with higher electron density and TiO with abundant electropositive Ti and/or O vacancies.33 More specifically, their bonding lead to the deviation of electrons away from the electronegative SB0 and ST-1 atoms to the electropositive Ti and/or O vacancies, which subsequently cause the positive shift of ST-1 peak. Simultaneously, the electron donation from thiosulfate to TiO contributes to the shift of later to a higher binding energy.16,63 The synergistic effect of H-TiOx and PPy on LiPSs can be further confirmed by



the Ti 2p, Li 1S and N 1S spectra of Li2S4 soaked H-TiOx/PPy (Figure 5c-d, S7a) and Li 1S spectra of pure Li2S4 (Figure S7b). The Ti 2p spectra of Li2S4 soaked H-TiOx/PPy (Figure 5c) exhibit a slightly negative shift of 0.1-0.2 eV to a lower binding energy compared with the without soaking H-TiOx (Figure 2d), implying an enhanced electron density of Ti atoms.64 Moreover, Figure 5c shows new peaks at 456.4 and 463.3 ev, which can be ascribed to the formation of Ti-S bond.59 Such results are consistent with the S 2p spectra of Li2S4 soaked H-TiOx/PPy, further confirmed the Lewis-base interaction between TiO and LiPSs. Besides, three characteristic peaks of C=N (iminic N), -NH- (Pyrrolic N) and C=NH+ (protonated amine N) bonds can be easily discerned in the N 1s spectra of Li2S4 soaked H-TiOx/PPy (Figure S7a).65-67 Among them, electron-rich pyrrolic N groups of -NHcan bond with electropositive Li+ of LiPSs by the polar-polar interaction.68 Therefore, the well distinguished peak in the Li 1S spectra of Li2S4 soaked TiO/PPy (Figure 5d) at 55.8 eV can be ascribed to the formation of Li-N bond, and the electron donation from pyrrolic N groups to Li+ lead to the shift of later to a lower binding energy (Figure S7b). The above results all suggest the chemical adsorption of LiPSs by the PPy layer.69,70 In conclusion, the XPS analyses are backing the idea that combines H-TiOx and PPy to chemically adsorb LiPSs, corresponding to the results of visual LiPSs absorption ability test and ex-situ UV-Vis spectroscopy.

4. Conclusion In summary, we have prepared a H-TiOx materials with high conductivity and high polarity by a facile magnesiothermic reduction approach, and combined it with the


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conductive polymer PPy coating to fabricate a high performance Li-S batteries cathode. Initially, the high conductivity of H-TiOx and PPy make up for the insulation of sulfur, thus greatly improve the sulfur utilization and rate performance of Li-S batteries. Moreover, the TiO-TiO2 heterostructure enables the enhanced redox reaction kinetics of sulfur species. Furthermore, due to the synergistic effect of H-TiOx and PPy, the LiPSs diffusion is adequately suppressed by the formation of Ti-S bond (Lewis acid-base interaction), Li-N bond (polar-polar interaction) and the physical restriction of PPy layer, which lead to a significant prolongation of Li-S batteries cycle life. The high initial discharge/charge capacities of 1087/1050 mAh g-1 at 0.5 C and the ultra-low capacity decay of 0.0406% per cycle after 1000 cycles at 1 C confirm the validity of the synergistic effect of H-TiOx and PPy on the improvement of Li-S batteries electrochemical properties. Conclusively, our work in this paper provides a new idea for the preparation and application of H-TiOx materials in Li-S batteries, and enhances the understanding of surface chemical interaction between polar host and LiPSs. It can also have a directional effect on the design of multifunctional cathode materials for Li-S batteries in the future.

Acknowledgments We gratefully acknowledge the financial support from the Science and Technology Planning Project of Guangdong Province, China (No.2017B090916002), National Natural Science Foundation of China (51872098), Guangdong Natural Science Funds for Distinguished Young Scholar (2016A030306010), Guangdong Innovative and



Entrepreneurial Research Team Program (2014ZT05N200) and Fundamental Research Funds for Central Universities, China (2017ZX010).

Supporting Information Figure S1: SEM images of TiO2 nanoparticles and H-TiOx with the Mg : TiO2 = 1, 2. Figure S2: N2 adsorption/desorption isotherms and corresponding pore distributions (inset diagram) of H-TiOx with Mg: TiO2 = 1, 2, 3 and 3H-TiOx@S/PPy. Figure S3: Energy dispersive spectrometry (EDS) mapping of H-TiOx@S/PPy. Figure S4: CV profiles of CNT@S, TiO2@S and H-TiOx@S cathodes with a voltage window of 1.7-3.0 V at a scanning rate of 0.1 mVs-1. Figure S5: Cycling performance of H-TiOx@S/PPy cathode with a high sulfur loading of 2.4 mg/cm2 at 1 C. Figure S6: TG curve for PPy@S, charge/discharge profiles for the first cycle of PPy@S cathode, CV profiles of PPy@S cathode with a voltage window of 1.7-3.0 V at a scanning rate of 0.1 mVs-1 and cycling performance at 0.5 C of cells with H-TiOx@S/PPy, H-TiOx@S, TiO2@S, CNT@S and PPy@S cathodes, respectively. Figure S7: High-resolution XPS of N 1s spectra of Li2S4 soaked H-TiOx/PPy and Li 1S spectra of pure Li2S4. Table S1: Comparison of electrochemical performance of Li-S batteries with reported composite sulfur cathodes.

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Graphical abstract


Polypyrrole as a

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