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Free-Standing Sulfur Host Based on Titanium-Dioxide-Modified Porous

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Free-Standing Sulfur Host Based on Titanium-Dioxide-Modified Porous 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 sulfur host based on titanium-dioxide-modified porous- carbon 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 titanium dioxide 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 graphene and ultrafine polar TiO2 nanoparticles 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 À1 À1 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 À À 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 À 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 À1 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 À 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.
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