Journal of Power Sources 356 (2017) 172e180

Contents lists available at ScienceDirect

Journal of Power Sources

journal homepage: www.elsevier.com/locate/jpowsour

Free-standing host based on titanium-dioxide-modified porous- nanofibers for lithium-sulfur batteries

** Xiong Song a, Tuo Gao a, Suqing Wang a, b, , Yue Bao a, Guoping Chen a, Liang-Xin Ding a, * Haihui Wang a, a School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Institute of Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, NSW 2522, Australia highlights graphical abstract

A flexible porous carbon nanofiber film was fabricated by electrospinning. Ultrafine and gra- phene were adopted to modify the nanofibers. The sulfur cathode film exhibits good flexibility and foldability. The flexible film cathode shows excellent electrochemical performance.

article info abstract

Article history: Lithium-sulfur (Li-S) batteries are regarded as a promising next-generation electrical-energy-storage Received 27 November 2016 technology due to their low cost and high theoretical energy density. Furthermore, flexible and wearable Received in revised form electronics urgently requires their power sources to be mechanically robust and flexible. However, the 24 April 2017 effective progress of high-performance, flexible Li-S batteries is still hindered by the poor conductivity of Accepted 26 April 2017 sulfur cathodes and the dissolution of lithium polysulfides as well as the weak mechanical properties of sulfur cathodes. Herein, a new type of flexible porous carbon nanofiber film modified with and ultrafine polar TiO2 is designed as a sulfur host, in which the artful structure enabled the Keywords: fi fi Flexible highly ef cient dispersion of sulfur for a high capacity and a strong con nement capability of lithium fi fi Free-standing polysul des, resulting in prolonged cycle life. Thus, the cathode shows an extremely high initial speci c 1 1 Titanium dioxide discharge capacity of 1501 mA h g at 0.1 C and an excellent rate capability of 668 mA h g at 5 C as Carbon nanofiber well as prolonged cycling stability. The artful design provides a facile method to fabricate high- Lithium-sulfur batteries performance, flexible sulfur cathodes for Li-S batteries. © 2017 Elsevier B.V. All rights reserved.

1. Introduction

Rechargeable battery systems with high capacities and energy densities are essential to the development of electrical vehicles * Corresponding author. ** Corresponding author. School of Chemistry & Chemical Engineering, South (EV), portable electronic devices and grid energy storage. Among China University of Technology, Guangzhou 510640, China. various energy storage systems, lithium sulfur (Li-S) batteries have E-mail addresses: [email protected] (S. Wang), [email protected] attracted much attention due to their high theoretical specific (H. Wang). http://dx.doi.org/10.1016/j.jpowsour.2017.04.093 0378-7753/© 2017 Elsevier B.V. All rights reserved. X. Song et al. / Journal of Power Sources 356 (2017) 172e180 173 energy density, low cost, and benign environmental effects [1,2]. [13], (b) the rigid sulfur filling the micropores of the carbon However, several severe problems still impede the commerciali- nanofibers limits the flexibility, and (c) the introduction of most zation of Li-S batteries. The first issue is the low utilization of sulfur polar materials into the carbon nanofibers will also damage the due to its intrinsic poor electronic conductivity as well as its end flexibility. Therefore, it is still a great challenge to fabricate a stable discharge products, Li2S/Li2S2 [3]. The second problem is that the flexible cathode with a high S loading and an outstanding elec- intermediate lithium polysulfides (LiPSs) formed in the discharge trochemical performance for Li-S batteries. process are soluble in organic electrolytes, and the Li2S/Li2S2 Herein, we designed novel flexible nitrogen-doped porous car- products will deposit on the surface of the sulfur cathode and bon nanofibers (NPCFs) mixed with ultrafine polar TiO2 nano- lithium metal anode, leading to shuttle effects and an irreversible particles as a sulfur host. This artful structure not only well blends loss of active material [4e6]. The last issue is the large volumetric the excellent conductivity of a carbon matrix with the outstanding expansion (80%) during the discharge process, where sulfur is absorption ability of polar metal oxides but also well maintains the converted into Li2S, resulting in damage to the electrode structural flexibility of the carbon nanofiber film. These unique characteristics integrity and a loss of electrical contact within the electrode [4]. make allow for great potential to enhance the sulfur cathode per- These issues will result in low Coulombic efficiency, fast capacity formance. Thus, after sulfur loading, the flexible S/TiO2/G/NPCFs decay, inferior rate performance and safety concerns. film exhibited an excellent initial discharge capacity of In the past few decades, various strategies have been developed 1501 mA h g 1 at 0.1 C, prominent rate capability of 668 mA h g 1 at to address the above issues. Among these attempts, strenuous ef- 5 C, and good cycling performance as a cathode for flexible Li-S forts have been devoted to designing novel nanostructured sulfur/ batteries. carbonaceous material composite electrodes due to the intrinsically good conductivity and nanostructured diversity of carbonaceous materials [7e12]. For example, the groundbreaking work reported 2. Experimental by Nazar et al. reported a high specific capacity cathode by impregnating sulfur into a porous nanostructured carbon (CMK-3) 2.1. Materials synthesis [9]. After modifying with hydrophilic polyethylene glycol, the sulfur fl fi cathode showed a high reversible capacity of 1320 mA h g 1. After 2.1.1. Preparation of exible TEOS/TTIP/GO/PAN nano bers that, many other conductive carbon materials have been reported The graphene oxide (GO) used in this work was synthesized by to encapsulate sulfur. However, the nonpolar carbon materials Hummer's method, and the detailed processes can be found in our ¼ possess only weak physical adsorption with polar LiPSs, which re- previous work [38]. Polyacrylonitrile (PAN, Mw 150,000, Sigma- sults in limited cycling performance [13]. Recently, it has been Aldrich) and N,N-dimethylformamide (DMF, Aladdin Co. Ltd., demonstrated that polar metal oxides have stronger chemical China) were used as the carbon precursor and solvent, respectively. absorbability with LiPSs than carbon materials [14e18]. For Tetraethoxysilane (TEOS, Aladdin Co. Ltd., China) was used as the pore-forming agent. Titanium isopropoxide (TTIP, Aladdin Co. Ltd., example, Cui et al. designed a S/TiO2 yolk-shell nanostructured composite as a sulfur cathode [16]. Impressively, the cycle life of the China) was used as the metal source. First, the as-prepared GO electrode was significantly prolonged to 1000 cycles with a capacity powder (0.05 g), TEOS (0.75 g) and TTIP (0.1 g) were dispersed in decay of only 0.033% per cycle. This work proposed a new concept 10 mL DMF solution and sonicated for 4 h. Second, 1 g PAN was to enhance the electrochemical performance of sulfur cathodes. added into the above solutions, followed by constant stirring at 50 C for at least 12 h. Finally, the above solution was loaded into a Since then, many studies on TiO2 as a sulfur host for Li-S batteries have been reported [19e21]. However, the high mass ratios of bulk 10 mL syringe with a 20-gauge blunt tip. The electrospinning pro- e fl TiO added to the sulfur host result in low sulfur utilization and cess was carried out at an applied voltage of 12 13 kV. The ow 2 fi 1 poor rate performances due to its poor conductivity. More recently, rate and tip collector distance were xed at 1.2 mL h and 14 cm, some researchers have combined the above two strategies of respectively. structural restriction and chemical absorption to encapsulate sul- fur, resulting in impressively improved cycling performances 2.1.2. Preparation of flexible S/TiO /G/NPCFs e 2 [22 24]. Nazar's group fabricated a S/MnO2 core-shell nano- The flexible TEOS/TTIP/GO/PAN nanofibers were first stabilized structured composite via a very simple reaction process as a sulfur at 250 C in air for 5 h, followed by carbonization at 1000 C for 2 h cathode for Li-S batteries, where MnO2 provided both physical under an Ar/H2 atmosphere. The carbonized nanofibers were obstruction and chemical absorption to LiPSs [24]. Therefore, further immersed in sodium hydroxide solution (NaOH, 2 M, designing sulfur cathodes with novel structures is a promising Aladdin Co. Ltd., China) to remove the SiO2 particles. The sublimed strategy to accelerate the practical application of Li-S batteries. sulfur was dissolved in CS2 solvent with a concentration of In addition to the optimization of the electrode materials, 40 mg mL 1 of sulfur solution. Then, the as-prepared G/NPCFs designing free-standing electrodes has been proven to be another composite was cut into small pieces (2.5 cm 5 cm), and then way to enhance the energy density of energy-storage devices due to immersed in the sulfur solution for 10 min and dried at 60 Cinan not having the need for a current collector, insulating polymer oven for 4 h. Finally, the above sulfur/nanofiber composite was fl binder, and carbon additive. In addition, exible energy-storage transferred to an autoclave under argon atmosphere and heated to devices are considered to possess a good perspective due to the 155 C for 10 h to obtain the S/TiO2/G/NPCFs. rapid development of flexible and wearable electronics [25e27]. Vacuum filtration and electrospinning are the most common techniques to fabricate flexible and free-standing electrodes in the 2.1.3. Preparation of Li2S6 solution field of sulfur cathodes [28e37]. Compared with vacuum filtration, The solution of Li2S6 was synthesized according to the previous electrospinning is more extensively investigated for preparing free- literature [39]. In a typical synthesis, the sulfur (S, 99.5%, Aladdin standing porous carbon nanofiber films due to its simplicity, low Co. Ltd., China) and lithium sulfide (Li2S, 99.9%, Sigma-Aldrich) with cost and scale-up potential [29e31,33,35]. However, some issues a molar ratio of 5:1 were added into the 1, 2-dimethoxyethane still exist in using sulfur/carbon nanofiber composite as flexible (DME) and 1,3-dioxolane (DOL) (1:1 by volume) solution by electrodes: (a) the nonpolar carbon among the porous carbon magnetically stirring in an argon-filled glove box to form Li2S6 nanofiber film shows weak physical adsorption with the polar LiPSs (0.1 M) solution. 174 X. Song et al. / Journal of Power Sources 356 (2017) 172e180

2.2. Structural characterization shown in Scheme 1. The precursor film was prepared by electro- spinning, in which TEOS and TTIP were used as a pore-forming The crystal structure and morphology of as-prepared products agent and metal source, respectively. After carbonization and were characterized by a Bruker D8 Advance X-ray diffractometer template etching of the as-collected precursor film, a flexible using filtered Cu-Ka radiation and field emission scanning electron porous carbon nanofiber film mixed with polar TiO2 nanoparticles microscopy (SEM, NOVA NANOSEM 430). The microstructures of was successfully fabricated. Then, it was directly used as a free- the as-prepared products were observed by transmission electron standing sulfur host for Li-S batteries. As shown in Fig. 1a, the microscopy (TEM, JEM-2010HR). The X-ray photoelectron spec- final S/TiO2/G/NPCFs composite film can completely recover to its troscopy (XPS) analysis was performed on an ESCALAB 250 X-ray initial state after repeated folding, bending, or rolling into different photoelectron spectrometer using an Al Ka X-ray source. All XPS shapes, which demonstrates its high flexibility and foldability. Such spectra were calibrated using the C 1s line at 284.6 eV. The specific superior flexibility of the S/TiO2/G/NPCFs gives it great advantage surface area was measured using the Brunauer-Emmett-Teller for its application in flexible batteries. The excellent flexibility of (BET) method (Micromeritics analyzer ASAP 2460 (USA)) at liquid the S/TiO2/G/NPCFs film is mainly ascribed to the following rea- nitrogen temperature, pore size distribution (PSD) curves were sons: (i) the amorphous SiO2 and graphene uniformly embedded in obtained using the density functional theory (DFT) method the nanofiber contribute to stabilize the skeleton structure of the (Micromeritics). Thermogravimetric (TG) curves of the precursors nanofiber during thermal treatment, and (ii) the introduction of were collected on a NETZSCH STA44C in the temperature range mesopores within the nanofiber buffers the mechanical stress from 25 to 900 C in nitrogen (N2) atmosphere with a heating rate caused by the loaded sulfur. SEM and TEM were adopted to of 10 C min 1. investigate the morphologies and structures of the as-prepared materials. The as-spun nanofiber (TEOS/TTIP/GO/PAN) shows a 2.3. Electrochemical measurements smooth surface and an average size of 900 nm (Fig. 1b). After carbonization at 1000 C, as shown in Fig. 1c, the diameter of the fi e The flexible and free-freestanding S/TiO2/G/NPCFs film was SiO2/TiO2/G/NPCFs nano ber shrinks to 400 500 nm, and many directly used as working electrode. The sulfur loading in the cath- nano-SiO2 particles emerge on the surface of the nanofiber. The as- ode is ~1.2 mg cm 2. The electrochemical performance was spun mat shrinks approximately 40% at the macroscopic level, but measured using CR2025 coin cells assembled in an argon-filled it still maintains a stable smooth surface without any pulverization glove box. 1 M lithium bis(trifluoromethanesulfonyl)imide (Fig. S1). The reasons for the shrinkage of the nanofiber before and (LITFSI) in a mixture of DOL/DME (1:1 by volume) containing after carbonization are primarily ascribed to the decomposition of lithium nitrate (LiNO3, 0.1 M) was used as the electrolyte, Celgard- TEOS, TTIP, and PAN at high temperature. After templates etching of 2400 as the separator, and Li metal foil as the counter and reference the SiO2/TiO2/G/NPCFs nanofiber, a graphene-modified porous electrode. Cyclic voltammetry (CV) measurements were carried out nanofiber mixed with TiO2 nanoparticles was obtained as the sulfur on an electrochemical workstation (CHI760D, Chenhua Instrument host (Fig. 1d). The TiO2/G/NPCFs nanofiber was further character- Company, Shanghai, China) over the potential range of 1.7e2.6 V vs. ized by TEM (Fig. 1e, f). The TiO2 particles are ultrafine and uni- þ Li/Li at a scanning rate of 0.2 mV s 1. The galvanostatic dis- formly dispersed among the nanofiber. Moreover, several chargeecharge performance, the self-discharge test, and the rate mesopores marked with white dotted lines can be clearly observed performance were tested using a Battery Testing System (Neware in the HRTEM image (Fig. S3). The column diagram inserted in Electronic Co., China) from 1.7 to 2.6 V. Fig. 1f demonstrates that the average size of the TiO2 particles is 5 nm. Notably, the superfine polar grains possess more active sites 3. Results and discussion to bond with polar LiPSs compared with bulk particles. To obtain the high conductive porous carbon nanofiber as a The fabrication procedure of the S/TiO2/G/NPCFs composite is sulfur host, the temperature as high as possible (1000 C) was

Scheme 1. Schematic illustration of the fabrication process for the S/TiO2/G/NPCFs composite. X. Song et al. / Journal of Power Sources 356 (2017) 172e180 175

Fig. 1. a) The photographs of flexible S/TiO2/G/NPCFs film with different shapes; b, c, d) Typical SEM images of TEOS/TTIP/GO/PAN, SiO2/TiO2/G/NPCFs, and TiO2/G/NPCFs, respectively; e, f) TEM images of TiO2/G/NPCFs (inset is the column diagram of the TiO2 particles' size distribution).

adopted to carbonize the nanofiber. Many macro SiO2 particles nanofiber are fused together. Moreover, many of the TiO2 nano- agglomerated on the surface of the nanofiber when the calcination particles escaped from the nanofiber and aggregated into large temperature was elevated to 1100 C(Fig. 2a). Thus, the pore- flakes, as identified by the TEM images of the TiO2/NPCFs nanofiber forming agent could not form pores in the nanofiber to load sul- (Fig. 2c). The SAED image and elemental mapping further prove fur after the templates were removed. Therefore, a suitable calci- that the flakes on the surface of the nanofiber are TiO2 (Fig. 2d, e). nation temperature was chosen to control the size of the pore- After sulfuring, the morphology and phase composition of the forming templates. Moreover, it should be emphasized that the as-prepared S/TiO2/G/NPCFs composite were investigated. The SEM graphene nanosheets in the nanofiber have at least two main ad- images of the S/TiO2/G/NPCFs show that no bulk sulfur particles are vantages: (i) improving the conductivity of the nanofiber, and (ii) found on the surface of the S/TiO2/G/NPCFs, showing a homoge- giving a nanoconfined effect on the formation of the particles neous distribution of S inside the carbon fiber framework (Fig. 3a). within the nanofiber. To verify the influences of the graphene The XRD spectrum of the S/TiO2/G/NPCFs composite, shown in nanosheets on the formation of TiO2 and SiO2 particles, the SiO2/ Fig. 3b, exhibits a broad and flat peak accompanying with some TiO2/NPCFs nanofiber without added graphene was also prepared. sharp diffraction peaks, which correspond to the (002) lattice fringe As shown in Fig. 2b, the SiO2 particles on the surface of the of carbon with a low degree of and crystalline sulfur [40].

Fig. 2. a, b) Typical SEM images of SiO2/TiO2/G/NPCFs calcined at 1100 C and SiO2/TiO2/NPCFs without graphene modified; c) TEM image, d) SAED, and e) elemental mapping of TiO2/NPCFs. 176 X. Song et al. / Journal of Power Sources 356 (2017) 172e180

Fig. 3. a) Typical SEM images of S/TiO2/G/NPCFs; b, c) XRD patterns of S/TiO2/G/NPCFs composite, SiO2/TiO2/G/NPCFs, and TiO2/G/NPCFs, respectively; d) XPS spectrum of S/TiO2/G/ NPCFs; e, f) High resolution spectrums of Ti2p and S2p in S/TiO2/G/NPCFs, respectively; g) TEM and SAED images of S/TiO2/G/NPCFs; h) Elemental mapping of S/TiO2/G/NPCFs.

Although the diffraction peaks of TiO2 are inconspicuous due to the pattern further confirm that the phase of the nanoparticles is rutile existence of crystalline sulfur, the XRD patterns of the SiO2/TiO2/G/ TiO2 with a polycrystalline structure. The elemental mapping of a NPCFs and TiO2/G/NPCFs clearly exhibit the diffraction peaks of single S/TiO2/G/NPCFs nanofiber (Fig. 3h) shows that the TiO2 TiO2 corresponding to a rutile structure (Fig. 3c, JCPDS No. 76- nanoparticles and the sulfur are homogeneously distributed among 0323). In addition, there are still no peaks of SiO2, proving that the the carbon nanofiber. SiO2 nanoparticles exist in an amorphous form. The XPS spectrum N2 adsorption-desorption analysis was adopted to investigate of the S/TiO2/G/NPCFs only shows five peaks of S 2p (164.1 eV), C 1s the porous structure of the as-prepared materials (Fig. 4aed and (284.8 eV), N 1s (401.4 eV), Ti 2p (459.3 eV), and O 1s (531.1 eV) Table S1). The specific surface area of the SiO2/TiO2/G/NPCFs (Fig. 3d) [41e43]. The high-resolution scan of Ti 2p1/2 and Ti 2p3/2 nanofiber calculated by the Brunauer-Emmett-Teller (BET) method shows peaks at 464.9 eV and 459.3 eV, respectively, which dem- was 157.1 m2 g 1, and the pores mainly consisted of micropores onstrates the impregnation of TiO2 in the nanofiber (Fig. 3e) [42]. (Fig. 4a). After etching of the SiO2 template, the BET area of the TiO2/ 2 1 The S 2p spectrum of the S/TiO2/G/NPCFs can be fitted into three G/NPCFs increased to 341.5 m g with a total pore volume of 3 1 peaks (Fig. 3f), which correspond to S 2p3/2 at 163.9 eV, S 2p1/2 at 0.309 cm g , and the pore size distribution (PSD) curve showed 165.1 eV, and sulfate species at 168.6 eV [43,44]. The high- that the pores among the nanofiber were mainly composed of resolution scan of C 1s, N 1s, and O 1s are shown in Fig. S3 [45]. mesopores with an average size of 3.6 nm. This demonstrates that The microstructure and crystal structure of the S/TiO2/G/NPCFs the TEOS agent was successfully implemented to create mesopores composite were further characterized by TEM and HRTEM (Fig. 3g). within the nanofiber. The surface area of the S/TiO2/G/NPCFs 2 1 The TEM image of a single nanofiber reveals that the TiO2 nano- sharply decreases to 7.2 m g after sulfur impregnation (Fig. 4c). particles and sulfur are uniformly impregnated within the nano- Due to the existence of many mesopores, the S loading in the S/ fiber and that a certain number of pores still exist, which is TiO2/G/NPCFs reaches 55 wt%, as calculated by TG measurement beneficial to electrolyte permeation, acceleration of ion diffusion, (Fig. 4d). In addition, the weight ratio of TiO2 nanoparticles among and alleviation of large volume changes during the dis- the S/TiO2/G/NPCFs composite was approximately 8.8 wt.%. chargeecharge process. The HRTEM image of the S/TiO2/G/NPCFs Since the S/TiO2/G/NPCFs composite film is flexible, free- further demonstrates that the TiO2 nanocrystals (circled with white standing, and conductive, it can be directly used as a cathode for dotted lines) have an average size of 5 nm. The lattice fringes Li-S batteries. Notably, the binder and conductive carbon as well as enlarged from the HRTEM image and the corresponding SAED current collector are unnecessary here. The CV curves of the S/TiO2/ X. Song et al. / Journal of Power Sources 356 (2017) 172e180 177

Fig. 4. a, b, c) Nitrogen adsorption-desorption isotherms of SiO2/TiO2/G/NPCFs, TiO2/G/NPCFs, and S/TiO2/G/NPCFs, respectively; d) TG curve of S/TiO2/G/NPCFs (in air).

G/NPCFs cathode are shown in Fig. 5a. On the first cathodic scan, shown in Fig. S5, on the 11th discharge process, the cell was the two reduction peaks observed at 2.06 V and 2.31 V are ascribed interrupted at 2.15 V for 72 h and then continuously tested for 10 to the reduction of sulfur to long-chain LiPSs and short-chain cycles. The 10th and 11th dischargeecharge profiles exhibit a insoluble lithium sulfides, respectively [7,22,23]. On the reverse discharge capacity decay of only 8.0% and a corresponding charge anodic scan, there are two peaks side-by-side at 2.32 V and 2.38 V, capacity decay of 21.5% (Fig. 5e), which are much lower than the which correspond to the conversion of lithium sulfides to poly- pure carbon materials [45]. This result demonstrates the strong sulfides, and the further oxidation of polysulfides to sulfur. Notably, adsorption capability of the TiO2/G/NPCFs film on lithium poly- the oxidation peaks are more distinct than those in previously re- sulfides. By comparison with other similar previously reported ported results, demonstrating a more complete oxidation reaction cathodes for Li-S batteries, it is reasonable to conclude that the [22]. The CV curves in the following two cycles exhibit similar flexible and free-standing S/TiO2/G/NPCFs cathode of this work shapes, indicating good cycling stability. The first galvanostatic shows excellent rate capabilities and high cycling stability (Fig. 5f dischargeecharge curves at 0.1 C (1 C ¼ 1675 mA g 1) shown in and Table S2). The prolonged cycling performance is significant for Fig. 5b are well consistent with the CV curves. It shows an the practical application of Li-S batteries. Fig. 5g shows the cycling 1 extremely high specific discharge capacity of 1501 mA h g with a performance of the S/TiO2/G/NPCFs cathode at 1 C for 500 cycles. Coulombic efficiency of 98.7%, which is close to the theoretical Here, an initial capacity of 987 mA h g 1 and a sustained capacity of specific capacity (1675 mA h g 1) of sulfur. The rate capability of the 618 mA h g 1 are observed, corresponding to a capacity retention of S/TiO2/G/NPCFs electrode as evaluated at various current densities 62.6% of its initial value and a low average capacity decay rate of (Fig. 5c). The specific capacity of the S/TiO2/G/NPCFs film gradually 0.074% per cycle. The good cycling capability further confirms the decreased with an increase of the current density. However, the excellent conductivity of the TiO2/G/NPCFs host, the highly efficient 1 capacity at 5 C still sustained 668 mA h g , indicating an excellent dispersion of sulfur among the porous TiO2/G/NPCFs nanofiber, and rate performance. The dischargeecharge voltage profiles at the strong adsorption capability of LiPSs by the ultrafine TiO2 different current densities are shown in Fig. S4. The dis- particles. chargeecharge plateaus can be clearly observed even at high cur- To investigate the performance of a S/TiO2/G/NPCFs cathode rent rates of 5 C. Moreover, the overpotential calculated by the with a higher areal sulfur loading, layer-by-layer structured S/TiO2/ dischargeecharge voltage plateaus at different rates exhibits a low G/NPCFs cathodes were fabricated by stacking S/TiO2/G/NPCFs polarization (Fig. 5d). The results suggest that the S/TiO2/G/NPCFs films from two layers to four layers. As shown in Fig. 6a, the cath- cathode possesses excellent conductivity and fast redox-reaction odes with different areal sulfur loading (1.2, 2.4, 3.6, and kinetics as well as high efficient sulfur utilization [23]. These 4.8 mg cm 2) exhibited initial capacities of 987, 967, 890, and characteristics are ascribed to the delicate design of the cathode 814 mA h g 1 at a current rate of 1 C, and reversible capacities of structure, in which the size of the sulfur in the mes- 846, 734, 617, and 490 mA h g 1 after 100 cycles, respectively. The opores is less than 5 nm and the TiO2 nanoparticles can provide corresponding initial dischargeecharge voltage profiles are shown strong adsorption capability to the LiPSs. In addition, a self- in Fig. S6. In addition, the TiO2/G/NPCFs film was loaded with a discharge test was further adopted to examine the adsorption higher sulfur content of 64% (Fig. S7). The S/TiO2/G/NPCFs cathode capability of the TiO2/G/NPCFs film on lithium polysulfides ac- with a sulfur content of 64% delivered an initial discharge capacity cording to the method reported by Nazar et al. [45] (Fig. 5e). As of 783 mA h g 1, and sustained a reversible capacity of 178 X. Song et al. / Journal of Power Sources 356 (2017) 172e180

1 Fig. 5. a) CV curves for the first three cycles of S/TiO2/G/NPCFs electrode tested at a scan rate of 0.2 mV s in the potential range from 1.7 to 2.6 V; b) The initial dischargeecharge curves of S/TiO2/G/NPCFs at 0.1 C; c) Rate capability of S/TiO2/G/NPCFs recorded at the current rates of 0.5 C, 1 C, 2 C, 3 C, and 5 C; d) The over potential between the charge and discharge plateaus at different current density of S/TiO2/G/NPCFs electrode; e) The typical dischargeecharge voltage profiles showing the self-discharge behavior; f) Rate capabilities comparisons of this work with some other sulfur cathode; g) Long-term cycling performance of S/TiO2/G/NPCFs electrode at 1 C [46,47].

Fig. 6. a) Cycling performance of S/TiO2/G/NPCFs electrode with different specific areal sulfur loading at the current rate of 1 C; b) Cycling performance of S/TiO2/G/NPCFs electrode with different sulfur content at the current rate of 1 C.

1 612 mA h g at 1C after 100 cycles (Fig. 6b). These results indicate Moreover, the adsorption ability of the TiO2/G/NPCFs host on LiPSs the potential of the S/TiO2/G/NPCFs film for use in high-energy- was evaluated (Fig. 7b). After the TiO2/G/NPCFs (15 mg) host was density Li-S batteries. added into a Li2S6 solution (5 mL, 5 mM), the color of the solution In Fig. 7a, the SEM image of the S/TiO2/G/NPCFs electrode after changed from yellow to transparent after standing for 10 h, con- testing at 1 C for 500 cycles shows that the nanofiber sustains its firming that the TiO2/G/NPCFs host can effectively trap the LiPSs original morphology and structure, indicating that the mesopores during the dischargeecharge process. To more clearly illustrate the within the nanofiber are sufficient to store sulfur and accommodate above results, a schematic of the S/TiO2/G/NPCFs nanofiber before the volume changes during the dischargeecharge processes. and after discharge is described in Fig. 7c. In addition, as shown in X. Song et al. / Journal of Power Sources 356 (2017) 172e180 179

Fig. 7. a) SEM image of S/TiO2/G/NPCFs after 500 cycles at 1 C; b) Photograph of sealed vitals of Li2S6/DOL/DME solutions before and after soaking with TiO2/G/NPCFs for 10 h; c) Schematic illustration of the functions of the mesopores and ultrafine TiO2 particles with the nanofiber during lithiation; d) Photographs of LED device lighten by the flexible battery during folding.

Fig. 7d, the flexible pouch cell assembled with the S/TiO2/G/NPCFs Appendix A. Supplementary data film can be used to drive a light-emitting diode device (LED) and even endure a bending process, confirming the potential applica- Supplementary data related to this article can be found at http:// tion of the S/TiO2/G/NPCFs film for flexible Li-S batteries. dx.doi.org/10.1016/j.jpowsour.2017.04.093.

References

4. Conclusions [1] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Nat. Mater. 11 (2012) 19e29. [2] J. Wang, Y.S. He, J. Yang, Adv. Mater. 27 (2015) 569e575. In summary, a novel, flexible and free-standing porous S/TiO2/G/ [3] Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 42 (2013) 3018e3032. NPCFs film modified by graphene and ultrafine polar TiO2 nano- [4] A. Manthiram, Y. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Chem. Rev. 114 (2014) e particles was successfully designed as a cathode for Li-S batteries. 11751 11787. [5] Y.-S. Su, A. Manthiram, Nat. Commun. 3 (2012) 1166. This delicate structure not only well combined the excellent con- [6] D.-W. Wang, Q. Zeng, G. Zhou, L. Yin, F. Li, H.-M. Cheng, I.R. Gentle, G.Q.M. Lu, ductivity of a carbon matrix with the outstanding absorption ability J. Mater. Chem. A 1 (2013) 9382e9394. fl [7] J. Cao, C. Chen, Q. Zhao, N. Zhang, Q. Lu, X. Wang, Z. Niu, J. Chen, Adv. Mater. 28 of polar metal oxides but also well maintained the exibility of the e fi fi (2016) 9629 9636. carbon nano ber lm. As a result, the S/TiO2/G/NPCFs cathode [8] Z. Li, H.B. Wu, X.W.D. Lou, Energy Environ. Sci. 9 (2016) 3061e3070. exhibited significantly improved electrochemical performance. The [9] X. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500e506. free-standing cathode with 55 wt% sulfur in the whole electrode [10] J. Zhang, C.P. Yang, Y.X. Yin, L.J. Wan, Y.G. Guo, Adv. Mater. 28 (2016) e fi 9539 9544. showed an extremely high initial speci c discharge capacity of [11] L. Yuan, H. Yuan, X. Qiu, L. Chen, W. Zhu, J. Power Sources 189 (2009) 1 1501 mA h g with a high Coulombic efficiency of 98.7% at the 1141e1146. current density of 0.1 C. An excellent rate capability of 668 mA h g 1 [12] N. Jayaprakash, J. Shen, S.S. Moganty, A. Corona, L.A. Archer, Angew. Chem. Int. Ed. 50 (2011) 5904e5908. at 5 C as well as a prolonged cycling of up to 500 cycles at 1 C was [13] Q. Pang, X. Liang, C. Kwok, L.F. Nazar, J. Electrochem. Soc. 162 (2015) also achieved. The present work demonstrates a facile process, low- A2567eA2576. cost and large-area strategy to fabricate flexible multifunctional [14] J. Jiang, J. Zhu, W. Ai, X. Wang, Y. Wang, C. Zou, W. Huang, T. Yu, Nat. Commun. 6 (2015) 8622. sulfur cathodes for high-performance Li-S batteries. [15] X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss, L.F. Nazar, Nat. Commun. 6 (2015) 5682. [16] Z.W. Seh, W. Li, J.J. Cha, G. Zheng, Y. Yang, M.T. McDowell, P.-C. Hsu, Y. Cui, Nat. Commun. 4 (2013) 1331. [17] X. Wang, G. Li, J. Li, Y. Zhang, A. Wook, A. Yu, Z. Chen, Energy Environ. Sci. 9 Acknowledgments (2016) 2533e2538. [18] J. Zhang, H. Hu, Z. Li, X.W.D. Lou, Angew. Chem. Int. Ed. 55 (2016) 3982e3986. e The authors greatly acknowledge the financial support by The [19] Y. Zhao, W. Zhu, G.Z. Chen, E.J. Cairns, J. Power Sources 327 (2016) 447 456. [20] J.-Y. Hwang, H.M. Kim, S.-K. Lee, J.-H. Lee, A. Abouimrane, M.A. Khaleel, National Key Research and Development Program of China I. Belharouak, A. Manthiram, Y.-K. Sun, Adv. Energy Mater. 6 (2016) (2016YFA0202601), National Science Fund for Distinguished Young 1501480e1501486. Scholars of China (No. 21225625), Natural Science Foundation of [21] H. Fan, Q. Tang, X. Chen, B. Fan, S. Chen, A. Hu, Chem. Asian J. 11 (2016) 2911e2917. China (No. 21576100), Nature Science Foundation of Guangdong [22] Y.-J. Li, J.-M. Fan, M.-S. Zheng, Q.-F. Dong, Energy Environ. Sci. 9 (2016) (2014A030312007). 1998e2004. 180 X. Song et al. / Journal of Power Sources 356 (2017) 172e180

[23] Z. Li, J. Zhang, B. Guan, D. Wang, L.-M. Liu, Nat. Commun. 7 (2016) 13065. [37] G. Zhou, L. Li, D.W. Wang, X.Y. Shan, S. Pei, F. Li, H.M. Cheng, Adv. Mater. 27 [24] X. Liang, L.F. Nazar, ACS Nano 10 (2016) 4192e4198. (2015) 641e647. [25] T. Cheng, Y. Zhang, W.Y. Lai, W. Huang, Adv. Mater. 27 (2015) 3349e3376. [38] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochim. Acta 55 (2010) [26] M.J. Cima, Nat. Biotechnol. 32 (2014) 642e643. 3909e3914. [27] G. Zhou, F. Li, H.-M. Cheng, Energy Environ. Sci. 7 (2014) 1307e1338. [39] Y. Zhao, M. Liu, W. Lv, Y.-B. He, C. Wang, Q. Yun, B. Li, F. Kang, Q.-H. Yang, Nano [28] H. Chen, C. Wang, Y. Dai, S. Qiu, J. Yang, W. Lu, L. Chen, Nano Lett. 15 (2015) Energy 30 (2016) 1e8. 5443e5448. [40] R. Andrews, D. Jacques, D. Qian, E. Dickey, Carbon 39 (2001) 1681e1687. [29] L. Ji, M. Rao, S. Aloni, L. Wang, E.J. Cairns, Y. Zhang, Energy Environ. Sci. 4 [41] Z. Cui, C. Zu, W. Zhou, A. Manthiram, J.B. Goodenough, Adv. Mater. 28 (2016) (2011) 5053e5059. 6926e6931. [30] L. Zeng, Y. Jiang, J. Xu, M. Wang, W. Li, Y. Yu, Nanoscale 7 (2015) [42] C. Choi, K.-J. Hwang, Y.J. Kim, G. Kim, J.-Y. Park, S. Jin, Nano Energy 20 (2016) 10940e10949. 76e83. [31] L. Zeng, F. Pan, W. Li, Y. Jiang, X. Zhong, Y. Yu, Nanoscale 6 (2014) 9579e9587. [43] G. Zhou, L.-C. Yin, D.-W. Wang, L. Li, S. Pei, I.R. Gentle, F. Li, H.-M. Cheng, ACS [32] H.S. Kang, Y.K. Sun, Adv. Funct. Mater. 26 (2016) 1225e1232. Nano 7 (2013) 5367e5375. [33] D. Nan, Z.-H. Huang, R. Lv, L. Yang, J.-G. Wang, W. Shen, Y. Lin, X. Yu, L. Ye, [44] C. Wang, X. Wang, Y. Wang, J. Chen, H. Zhou, Y. Huang, Nano Energy 11 (2015) H. Sun, J. Mater. Chem. A 2 (2014) 19678e19684. 678e686. [34] L. Sun, D. Wang, Y. Luo, K. Wang, W. Kong, Y. Wu, L. Zhang, K. Jiang, Q. Li, [45] C.J. Hart, M. Cuisinier, X. Liang, D. Kundu, A. Garsuch, L.F. Nazar, Chem. Comm. Y. Zhang, ACS Nano 10 (2015) 1300e1308. 51 (2015) 2308e2311. [35] H.-G. Wang, S. Yuan, D.-L. Ma, X.-B. Zhang, J.-M. Yan, Energy Environ. Sci. 8 [46] B. Li, S. Li, J. Liu, B. Wang, S. Yang, Nano Lett. 15 (2015) 3073e3079. (2015) 1660e1681. [47] W. Zhou, B. Guo, H. Gao, J.B. Goodenough, Adv. Energy Mater. 6 (2015) [36] Z. Yuan, H.J. Peng, J.Q. Huang, X.Y. Liu, D.W. Wang, X.B. Cheng, Q. Zhang, Adv. 1502059e1502065. Funct. Mater. 24 (2014) 6105e6112.