C@S Nanofiber as a Freestanding Cathode

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Biomimetic Root-Like TiN/C@S Nanofiber as A Freestanding Cathode with High Sulfur Loading for Lithium-Sulfur Batteries Yaqi Liao, Jingwei Xiang, Lixia Yuan, Zhangxiang Hao, Junfang Gu, Xin Chen, Kai Yuan, Pramod K. Kalambate, and Yunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11118 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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ACS Applied Materials & Interfaces

Biomimetic Root-Like TiN/C@S Nanofiber as A Freestanding Cathode with High Sulfur Loading for Lithium-Sulfur Batteries Yaqi Liao,‡a Jingwei Xiang,‡a Lixia Yuan,a* Zhangxiang Hao,a Junfang Gu,a Xin Chen,a Kai Yuan,a Pramod K. Kalambate,a Yunhui Huanga,b* a State

Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science

and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China. Email: [email protected] (Lixia Yuan), [email protected] (Yunhui Huang). b

Institute of New Energy for Vehicles, School of Automotive Sduties, Tongji University, Shanghai 201804,

PR China. ‡ Yaqi Liao and Jingwei Xiang contributed equally to this work.

KEYWORDS: lithium-sulfur batteries, free-standing cathode, high sulfur loading, rootlike nanofiber, titanium nitride ABSTRACT: It is a tough issue to achieve high electrochemical performance and high sulfur loading simultaneously, which is of important significance for the practical Li-S batteries applications. Inspired by the transportation system of the plant root in nature, a biomimetic root-like carbon/titanium nitride (TiN/C) composite nanofiber is designed as a freestanding current collector for the high sulfur loading cathode. Like the plant root which absorbs water and oxygen from soil and transfers them to the trunk and branches, the rootlike TiN/C matrix provides high-efficiency polysulfide, electron and electrolyte transfer for the redox reactions via its 3D-porous interconnected structure. And in the meantime, TiN can not only anchor the polysulfides via the polar Ti-S and N-S bond but also further facilitate the redox reaction due to its high catalytic effect. With 4 mg cm-2 sulfur loading, the TiN/C@S cathode delivers a high initial discharge capacity of 983 mAh g-1 at 0.2C current density; after 300 charge/discharge cycles, the discharge capacity remains 685 mAh g-1, corresponding to a capacity decay rate of ~0.1%. Even when the sulfur loading is increased to 10.5 mg cm-2, the cell still delivers a high capacity of 790 mAh g-1 and a decent 1

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cycle life. We believe that this novel biomimetic root-like structure can provide some inspiration for the rational structure design of the high energy lithium-sulfur batteries and other composite electrode materials.

INTRODUCTION State-of-the-art lithium-ion batteries (LIBs) have achieved great success in the electronic market in the past two decades.1 Up to now, the large demands for the rechargeable battery with higher energy density have been aroused along with the rapid development of the electric car industry. Conventional LIBs based on the intercalation type electrode cannot meet the long-term requirements of the electric cars due to its limited theoretical specific energy.2 On the other hand, lithium-sulfur (Li-S) batteries have been attracting increasing attention due to their unprecedented theoretical energy density (2600 Wh kg-1).3-5 Furthermore, sulfur has the advantages of natural abundance, low cost and environmental friendliness, which make the Li-S batteries more competitive to the conventional LIBs. Nevertheless, the practical applications of the Li-S batteries are still restricted by the inherent defects of the Li-S system. The reaction between S and Li undergoes a multistep process with the generation of a series of soluble intermediate polysulfides (Li2Sn, 4≤n≤8). The dissolution of the intermediates leads to loss of the energy-bearing material and the knotty “shuttle” phenomenon; the repeated “solid↔liquid” phase transition and the incident volume variation (over 80%) between S and Li2S both pose a tough issue to the electrode structure stability. Moreover, the destructible electrode structure encounters a big challenge for realizing a high-performance sulfur cathode with high areal sulfur loading, which has the vital practical significance for the full cell system. The low electronic conductivity of the sulfur species further aggravates the situation.6-8 In order to address the issues, many efforts have been made. The most popular strategy is combining sulfur with various porous structured carbonaceous materials, which can 2


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improve the conductivity of the cathode and alleviate the energy-bearing material loss due to the high conductivity and physical adsorption capability of carbonaceous materials.9-17 However, the interactions between the nonpolar carbon and polar polysulfides are weak, which cannot support long-term cycle stability of the sulfur cathode. Recently, some polar materials, such as MnO218, SnO219, TiO220-21 and so on22-25, have been introduced to the sulfur system to anchor the sulfur species by strong chemical bond. But, many metal oxides usually are nonconductor or semiconductor, which is against the conversion reaction from the chemically adsorbed polysulfides to the solid S/Li2S, resulting in lower sulfur utilization. Therefore many researchers choose to combine the carbon and the metal oxides to enhance the electronic conductivity of the hybrid sulfur cathode. 26-30 More recently, the highly conductive TiN is reported as a high-efficiency polysulfide immobilizer.31-34 Manthiram’s group firstly introduced TiN into the Li-S system, which achieved largely improved cycle stability over 500 cycles.35 Our group further reported that the strong surface affinity of TiN for polysulfides comes from the N-S and the Ti-S bond formed in the electrode process.36 Even so, the significant challenge still remains for the Li-S system, especially to obtain the cathode with high sulfur loading that can meet the demand of the practical applications (usually >6.0 mg cm-2). In many previous papers, the areal sulfur loadings reported is ≤2 mg cm-2. Thus, there is an urgent demand for the rational design of the sulfur cathode that can achieve high electrochemical performance and high sulfur loading simultaneously.37 On the other hand, the conventional preparation route of the sulfur cathode relying on the heavy current collector (Al foil) is not helpful to the resolution of this issue. In contrast, the freestanding cathode independent of binder and metal foil collector has natural advantage to fabricate the thick electrodes benefited from its abundant electron and ion channels.38-41

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Figure 1 Schematic illustration of the electrode process for the root-like TiN/C@S cathode.

Plant root, the natural hierarchical interconnected framework, contains bulky main root, big later root, and tiny fibrous root, by which water, oxygen and nutriments are transported to the plant body. For Li-S batteries, in addition to absorb the dissolved polysulfide ions, the ideal hosts should also play a role in accelerating electron/Li-ion transportation and thus facilitate the redox reaction of the polysulfides. Inspired by the interconnected root framework, the biomimetic root-like TiN/C nanofibers are designed in this work. Via electrospinning route, the biomimetic root-like TiN/C nanofibers construct a freestanding current collector, which can realize a thick sulfur cathode with high sulfur loading without any inactive binder addition. Even with a 10.5 mgsulfur cm-2 loading, the TiN/C@S can still deliver a high reversible capacity of 790 mAh g-1 at 0.2C and decent cycle stability. The excellent electrochemical performance demonstrates that the root-like structure can be a rational choice for the thick sulfur cathode with high sulfur loading. EXPERIMENTAL SECTION 4


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Characterization Morphologies of TiO2/C, TiN/C and TiN/C@S were observed by FESEM (SIRION-200) and TEM (JEOL2100). XRD patterns of the composiTE POWDERS WERE RECORDED USING PANalytical B.V. Holland. XPS analysis (VG Multilab 2000 Instruments) was carried out to analyze the surface chemistry of the samples with a monochromatic Al Kα (20°~80°), using the C 1s peak at 284.8 eV to standardization. TGA was tested in air atmosphere from 20 °C to 800 °C with a heating rate of 10 °C min-1 on a Pyrisl TGA (Perkin Elmer Instruments). BET (specific surface area) analysis was performed by nitrogen adsorption at 77 K on Micromeritics ASAP 2010 analyzer. Synthesis of TiN/C nanofibers The TiN/C nanofibers were obtained from the TiO2/C precursor. At first, 0.6 g polyvinyl pyrrolidone (PVP, MW ≈ 1300000) and 0.25 g hexadecyl trimethyl ammonium bromide (CTAB) were dissolved in a beaker containing 7.0 g ethanol and 1.0 g acetic acid, then 3.0 g tetrabutyl titanate (TBOT) was added slowly to the above solution and stirred for 2 h. Next, 2.0 g paraffin oil was dropped slowly and was stirred for another 2 h to form the yellow precursor solution. The obtained precursor solution was loaded into a 20 mL syringe equipped with a single needle and the flowing speed was set as 1.0 mL h-1. A high voltage power (15 kV) supply was connected to the single needle and a piece of Al foil that covered the stainless steel plate was used as the collector, which was kept 15 cm away from the needle. The obtained TiO2 precursor was calcinated in at 500 °C in Ar atmosphere for 2 h. Then the TiO2/C nanofibers were annealed at 800 °C in NH3 for 2 h. After cooling, the TiN/C nanofibers were obtained. Synthesis of hollow carbon nanofibers The hollow carbon nanofibers (HCNF) were used as a control sample to verify the function of TiN. It was also prepared via the electrospinning process. Typically, 0.8 g PAN and 0.5 g PMMA were dissolved in 20 mL DMF and the solution was loaded into 20 mL syringe for the electrospinning operation. The specific parameters for the electrospinning process 5



were the same as the above process for TiO2/C sample. Finally, the obtained nanofibers were annealed at 800 °C in Ar for 2 h and the HCNF sample was obtained. Preparation of TiN/C@S and HCNF@S The sulfur was loaded via traditional melting-diffusion method. Typically, 100 mg sulfur powder was dissolved in 1 mL CS2 to form a yellow solution. Then a certain amount of the solution was dropped into the TiN/C and HCNF sheets. Next, the sulfur loading sheets were dried for about 2 h in the air, then transferred and sealed in a stainless steel autoclave, and heated at 155 °C for 12 h. Preparation of Li2S6 adsorption sample All the experiments were performed in an Ar-filled glove box. At first, 5 mM Li2S6 solution was synthesized via the reaction of S8 and Li2S in the mixed solvents of 1, 2Dimethoxyethane (DME) and 1, 3-dioxolane (DOL) with the equal volume ratio at 60 °C for 72 h. Then 20 mg HCNF and TiN/C powders were added to 1 mL solution, respectively. Experiment about the nucleation of lithium sulfide Typically, 0.4 M Li2S6 was prepared by a reaction of stoichiometric amounts of sulfur and lithium sulfide in DOL/DME under stirring. TiN/C and HCNF were used as collectors. A piece of lithium sheet was used as anode. Then, 30 μL Li2S6 (0.4 M) was added to the collectors and additional 20 μL electrolyte without Li2S6 was added into the anode side. The assembled coin cell was discharged to 2.09 V under a certain current of 0.1C. Then, the cell was discharged until the current below 0.02 mA, while the voltage was kept at 2.07 V. According to Faraday’s law, the capacity of Li2S deposition was calculated from curve area.

RESULTS AND DISCUSSION The root-like TiN/C nanofibers are designed as freestanding host to anchor the dissolved polysulfide and to facilitate the further redox reactions. Figure 1 illustrates the 6


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electrode process of the root-like TiN/C cathode. Similar to the case that the tree root transports mass and energy, the hierarchical interconnected TiN/C matrix can transport polysulfide ions, electrons and Li-ions efficiently. The root-like TiN/C nanofibers are obtained from the TiO2/C precursor. The TiO2/C precursor nanofibers are prepared via the electrospinning method assisted with a simple post heat treatment. The electrospinning process builds the fiber structure and following annealing process eliminates the PVP and paraffin oil templates and generates plenty of mesopores consequently. As shown in Figure S1, the TiO2 in the TiO2/C hybrid shows typical anatase phase (Figure S1a), and the selfstanding TiO2/C sheet has mesoporous structures (2–10 nm). Figure S2 shows the XPS analysis for the Ti 2p of TiO2. It can be seen that the Ti 2p1/2 and Ti 2p3/2 spin-orbital splitting photoelectrons for TiO2 are located at binding energies of 464.3 eV and 458.3 eV. After annealing at 800 °C under NH3 atmosphere, TiO2 is transferred to TiN. The TiN particles embed in the carbon evenly. The XRD spectra in Figure 2d clearly shows five main diffraction peaks of TiN corresponded to (111), (200), (220), (311), (222) diffractions; and the HR-TEM image in Figure S3 further demonstrates the well-resolved lattice fringes of 0.24 and 0.21 nm corresponding to the d-spacing values of (111) and (200) planes of TiN. The hierarchical porous fiber structure is well maintained during the annealing process from the TiO2 phase to the TiN phase, as shown in Figure 2a, 2b and Figure S4a. The average diameter of the TiN/C fibers is ~ 500 nm. The TEM image in Figure 2e gives the fine structure of the TiN/C nanofiber, which demonstrates a hierarchical multicavity structure. Then the optical photographs show the macro profile of the “free-standing” electrode (Figure S5-6). Figure S5 clearly shows the free-standing paper of TiN/C. Even after high loading of sulfur infiltration (~ 8 mg cm-2), the structure of TiN/C@S still remains the stable (Figure S6), which demonstrates the fine free-standing structure of TiN/C@S electrode. From TiO2 to TiN, the specific area increases from 84.6 m2 g-1 (Figure S1b) to 119.2 m2 g-1 (Figure S7). In contrast to most of the electrospinning nanofibers which always show 7



solid structure or simple hollow tube structure, here the interconnected multi-cavity structure reserves/transfers the sulfur/polysulfides in its numerous separate but interconnected cavities, which can boost the mass transfer and further suppress the loss of the dissolved polysulfides more efficiently. The TG curve in Figure S8 identifies that the carbon content in TiN/C nanofibers is ~30 wt%.

Figure 2 (a, b) SEM images of TiN/C, (c) SEM image of TiN/C@S, (d) XRD pattern of TiN/C, (e) TEM image of TiN/C, (f) TEM image of TiN/C@S, (g-j) Element mapping of the red rectangle zone. Scale bars, 100 nm.

According to the TG analysis in Figure S9, the sulfur content in TiN/C@S and HCNF@S is 56% and 54%, respectively. After sulfur loading, the inner void pores of TiN/C fiber become black (Figure 2f) but the overall fiber morphology is well maintained 8


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(Figure 2c). No aggregated sulfur was observed. These observations imply that the sulfur is successfully infiltrated into the inner cavities of the TiN/C nanofibers. The element mapping in Figure 2g-j further verifies the homogeneous distribution of sulfur in the TiN/C@S nanofibers. The freestanding TiN/C@S sheets are directly used as cathode without any conductive agent and binder additive to assemble coin cells and the crosssectional SEM images of TiN/C after the melt sulfur infiltration are shown in Figure S10. It can be clearly seen that there is no massive sulfur filled in the space of TiN/C electrode which implies that sulfur contributes within the TiN/C matrix evenly. As for TiN/S electrode (Figure S10c), without the 3D carbon as skeleton, the electrode is too fragile to maintain the structure which is unsuitable as electrodes. Figure S11 shows the CV curves of the TiN/C@S cathode, which demonstrate two typical cathodic peaks at ~2.29 V and ~2.00 V corresponding to the reaction from sulfur to the dissolved long-chain polysulfides and then to the insoluble Li2S/Li2S2. In the anodic process, two oxidative peaks at 2.40 V and 2.42 V appeared, corresponding to the oxidation reactions from lithium sulfides to polysulfides and finally to the element sulfur, respectively. Figure 3a gives the rate capability of the TiN/C@S cathode with 4.0 mgsulfur cm-2 areal loading, and Figure 3b is the detailed discharge-charge curves. It can be seen that the TiN/C@S electrode (4.0 mgsulfur cm-2) delivers discharge capacities of 1361, 922, 860, 793, 764 and 737 mAh g-1 at 0.05, 0.1, 0.2, 0.5, 0.75, and 1C rate, respectively. When the rate is set back to 0.5C, a high capacity of 826 mAh g-1 can be recovered. For the control HCNF@S cathode (without TiN), the capacities are 1238, 790, 660, 635, 620 and 560 mAh g-1, respectively. Figure 3c shows the long cycling performance of the TiN/C@S cathode with 4.0 mg cm-2 sulfur loading at 0.2C. After the several active cycles in the beginning, the freestanding TiN/C@S cathode delivers discharge capacities of 983, 895 and 685 mAh g-1 at the initial cycle, the 100th cycle and the 300th cycle, respectively. The capacity retention of the 300th cycle is ~70%; the average capacity decay ratio is ~ 0.1% per cycle. As for the HCNF@S, a much lower discharge capacity of 804 mAh g-1 is delivered at 100th cycle and 524 mAh g-1 at 9



300th cycle. Obviously, the TiN/C@S achieves significant improvements in the sulfur utilization, rate capability and cycle stability. Figure S12 shows the charge-discharge behavior of the TiN/C cathode. Obviously, the root-like TiN/C matrix boosts the discharge ability and reversibility of the sulfur species, but TiN/C itself does not contribute to the overall capacity.

Figure 3 Electrochemical performance of the TiN/C@S cathodes with different sulfur loading. (a) Rate capability and (b) voltage curves of the TiN/C@S cathode with 4.0 mg cm-2 sulfur loading. Cycle life and Coulombic efficiency of the TiN/C@S electrodes with (c) 4.0 mg cm-2 sulfur loading and (d) 6.0 , 8.0 and 10.5 mg cm-2 sulfur loading. (e) Areal capacities and Coulombic efficiency of the TiN/C@S cathodes with 10.5 mg cm-2 sulfur loading during 100 cycles. (f) Electrochemical performance comparison for the composite sulfur cathode based on TiN.

The areal loading of the sulfur is one of the most important factors for the practical 10


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applications of Li-S batteries. Herein, we improve the sulfur loading via increasing the thickness of the cathode while keeping the same sulfur content. The areal loading of sulfur is increased to 6.0, 8.0 and 10.5 mg cm-2, respectively. As shown in Figure 3d, all cathodes show good cycle stability. The discharge capacities achieved are 874, 855 and 791 mAh g-1 for the 6.0, 8.0 and 10.5 mg cm-2 electrodes, respectively. The capacity retention is 92%, 87% and 88% after 100 cycles, respectively. For the cathode with a sulfur loading of 10.5 mg cm-2, the areal capacity achieved is as high as 8.3 mAh cm−2 , which is two times more than that of the Li-ion batteries (4 mAh cm-2); coulombic efficiency (CE) is also stabilized at >95%. To the best of our knowledge, the biomimetic structured TiN/C@S cathode demonstrates the best electrochemical performances among all the reported S/TiN/C system (Figure 3f, Table. S1, S2). Obviously, the root-like matrix structure, which provides efficient mass transfer, contributes a lot to the performance enhancement of the sulfur cathode. Although the TiN/C@S owns high gravimetric energy density, just like other 3D conductors (as shown in Table S3), the volumetric energy density is limited for the very low compaction density of S and massive pores existed in the 3D conductors. Therefore, for the 3D conductor system, or for the S cathode system, more measures should be taken to improve the volumetric energy density in the further.

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Figure 4 Potentiostatic discharge curves of (a) the TiN/C electrode and (b) the HCNF electrode in the Li2S6 solution. The yellow and gray color represent the precipitation of Li2S. (c) The polarization curves of the TiN/C electrode and the HCNF electrode. (d) EIS plots of the TiN/C@S and HCNF@S cells. (e) Self-discharge curves of the TiN/C@S and HCNF@S cells. (f) The photograph of the Li2S6/DOL/DME solution with HCNF or TiN/C addition.

To further analyze the function mechanism of TiN/C, the Li2S precipitation experiments on the surface of TiN/C or HCNF were conducted. The capacity of Li2S precipitation is calculated based on Faraday’s law (Figure 4a and 4b).42-44 The capacity on the TiN/C surface is much higher (231 mAh g-1) than that of the HCNF system (130 mAh g-1), indicating that TiN markedly accelerates the precipitation of Li2S. Figure 4c displays 12


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the polarization curves of the symmetric TiN/C cell. The voltage window is set as -0.8 V to 0.8 V. When the blank electrolyte (without Li2S6) is adopted, the response current is very low. The current increases significantly with the addition of Li2S6. Furthermore, the increase in current of the TiN/C system is much higher than that of the HCNF system, implying that TiN boosts a faster kinetics for the redox reaction of Li2S6. The electrochemical impedance spectroscopy (EIS) measurement also gives the same result. As shown in Figure 4d, both impedance curves of TiN/C and HCNF are composed of one depressed semicircle in high and a short inclined line in low frequency regions. The semicircle in the high frequency can reflect the electron transfer process of the sulfur species at the redox site provided by carbon and TiN. Obviously, the TiN/C@S electrode shows a much lower Rct than that of the HCNF/S cell. After cycling (Figure S13), the differences between the Rcts of TiN/C@S and HCNF/S electrode become much more enlarged because the decline of the TiN/C@S cathode is much more significantly than that of the HCNF/S cathode. The huge difference in the Rcts should be caused by the highly conductive TiN in the TiN/C@S system which can not only anchor the dissolved polysulfides but also further facilitate the redox reactions of the polysulfides. Figure 4e shows the OCV of the fresh cells of TiN/C@S and HCNF@S. The drop of the open circuit voltage (OCV) can give some information about the loss of active sulfur and degradation of electrodes. After 30 days, the OCV of TiN/C@S maintains at 2.28 V, but the OCV of HCNF@S is only 2.19 V. Obviously, the voltage drop of the HCNF cell is much faster than that of the TiN/C cell, especially during the long-term storage. The alleviation of the self-discharge should be benefited from the strong surface adsorption of TiN for polysulfides, as shown in Figure 4f. The Li2S6 solution fades markedly with the addition of TiN/C, while no color change was observed in the sample with HCNF. Figure 5 further gives the XPS analysis for the interaction between TiN and polysulfides. Due to the interaction with the sulfur atoms of polysulfides, a small shift was observed in the Ti 2p spectrum: the Ti-N peaks in the Ti 2p spectrum shift to the lower binding energy slightly, 13



from 461.98 eV and 456.60 eV (Figure 5a) to 461.70 eV and 455.39 eV (Figure 5c). In addition, a new Ti-S bond appeared at 456.75 eV, which indicates TiN absorbs the polysulfides via chemical interaction. Furthermore, in the Li2S6 surroundings, a new N-S bond also appeared at 401 eV in N 1s XPS spectra of TiN (Figure 5b and d). Thus, the Ti-S bond and the N-S bond both contribute to the strong adsorption capability of TiN, which well agrees well with the reported literature. 26, 30-31

Figure 5 (a, b) Ti 2p and N 1s XPS spectra of TiN, (c, d) Ti 2p and N 1s XPS spectra of TiN/Li2S6

CONCLUSION Biomimetic root-like TiN/C nanofibers are designed as a freestanding host for the sulfur cathode. Based on this ideal architecture, the chain procedures of the sulfur cathode involving the transfer, capture, and conversion of the polysulfides are significantly boosted and therefore a high areal sulfur loading cathode is realized. As a result, the cathodes with sulfur loading from 4.0 to 10.5 mg cm-2 demonstrate high sulfur utilization, excellent rate 14


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capability and good cycle stability. For the TiN/C@S cathode with 10.5 mg cm-2 sulfur loading, a high reversible capacity of 790 mAh g-1 is obtained at 0.2C. The corresponding areal capacity is as high as 8.3 mAh cm-2, which is the double of the current Li-ion batteries (4 mAh cm-2). The performance improvements are benefited from the ideal hierarchical architecture of the TiN/C@S host: (i) the root-like interconnected multi-cavity network provides a highly efficient polysulfides/electron/Li-ion transfer and the highly conductive TiN also contributes to the electron transfer; (ii) the abundant void space in the root-like structure provides reservoirs to relieve the volume expansion and stabilize the electrode structure; (iii) TiN can not only anchor the polysulfides via strong polar chemical bond but also boost the Li2S precipitation reaction. The unique material structure design is expected to provide an effective method to achieve a high performance sulfur cathode with high sulfur loading, which is important significance for the practical applications of the Li-S technology.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.H. Huang) *E-mail: [email protected] (L.Yuan) ORCID Yunhui Huang: 0000-0003-1687-1938 E-mail addresses for all authors Yaqi Liao: [email protected] Jingwei Xiang: [email protected] Zhangxiang Hao: [email protected] Junfang Gu: [email protected] Xin Chen: [email protected] Kai Yuan: [email protected] Pramod K. Kalambate: [email protected] 15



Lixia Yuan: [email protected] Yunhui Huang: [email protected] Author Contributions Y. Q. Liao and J. W. Xiang contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y.L. and J.X. have contributed equally to this work. This work was supported by the National Science Foundation of China (Grant Nos.21773077 and 51532005), the 973 program (Grant No. 2015CB258400), and the Project Funded by China Postdoctoral Science Foundation (2016T90689). The authors acknowledge the Analytical and Testing Center of HUST for XRD, XPS, FESEM, FTEM measurement, and the State Key Laboratory of Materials Processing and Die & Mould Technology of HUST for TG and BET tests.

ASSOCIATED CONTENT Supporting Information XRD patterns and BET curves of TiO2/C; XPS spectra of TiO2; HRTEM image of TiN/C; SEM and TEM images of TiO2/C; The optical photograph of TiN, TiN/C and TiN/C@S; BET curves of TiN/C; TG curves of TiN/C, TiN/C@S and HCNF@S; SEM images of TiN/C@S; CV curves of TiN/C@S; cycling performance of blank (TiN/C) as a cathode; nyquist plots of the TiN/C@S and HCNF@S; performance comparison of various cathodes of Li-S batteries. 16


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diffusion–conversion of polysulfides towards ultralong life lithium–sulfur batteries. Energy Environ. Sci., 2017, 10, 1694-1703.

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C@S Nanofiber as a Freestanding Cathode

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