Elsevier

Journal of Power Sources

Volume 356, 15 July 2017, Pages 172-180
Journal of Power Sources

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

https://doi.org/10.1016/j.jpowsour.2017.04.093Get rights and content

Highlights

  • A flexible porous carbon nanofiber film was fabricated by electrospinning.

  • Ultrafine titanium dioxide and graphene were adopted to modify the nanofibers.

  • The sulfur cathode film exhibits good flexibility and foldability.

  • The flexible film cathode shows excellent electrochemical performance.

Abstract

Lithium-sulfur (Li-S) batteries are regarded as a promising next-generation electrical-energy-storage technology due to their low cost and high theoretical energy density. Furthermore, flexible and wearable electronics urgently requires their power sources to be mechanically robust and flexible. However, the effective progress of high-performance, flexible Li-S batteries is still hindered by the poor conductivity of 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 highly efficient dispersion of sulfur for a high capacity and a strong confinement capability of lithium polysulfides, resulting in prolonged cycle life. Thus, the cathode shows an extremely high initial specific discharge capacity of 1501 mA h g−1 at 0.1 C and an excellent rate capability of 668 mA h g−1 at 5 C as well as prolonged cycling stability. The artful design provides a facile method to fabricate high-performance, flexible sulfur cathodes for Li-S batteries.

Introduction

Rechargeable battery systems with high capacities and energy densities are essential to the development of electrical vehicles (EV), portable electronic devices and grid energy storage. Among various energy storage systems, lithium sulfur (Li-S) batteries have attracted much attention due to their high theoretical specific energy density, low cost, and benign environmental effects [1], [2]. However, several severe problems still impede the commercialization of Li-S batteries. The first issue is the low utilization of sulfur due to its intrinsic poor electronic conductivity as well as its end discharge products, Li2S/Li2S2 [3]. The second problem is that the intermediate lithium polysulfides (LiPSs) formed in the discharge process are soluble in organic electrolytes, and the Li2S/Li2S2 products will deposit on the surface of the sulfur cathode and lithium metal anode, leading to shuttle effects and an irreversible loss of active material [4], [5], [6]. The last issue is the large volumetric expansion (80%) during the discharge process, where sulfur is converted into Li2S, resulting in damage to the electrode structural integrity and a loss of electrical contact within the electrode [4]. These issues will result in low Coulombic efficiency, fast capacity decay, inferior rate performance and safety concerns.

In the past few decades, various strategies have been developed to address the above issues. Among these attempts, strenuous efforts have been devoted to designing novel nanostructured sulfur/carbonaceous material composite electrodes due to the intrinsically good conductivity and nanostructured diversity of carbonaceous materials [7], [8], [9], [10], [11], [12]. For example, the groundbreaking work reported by Nazar et al. reported a high specific capacity cathode by impregnating sulfur into a porous nanostructured carbon (CMK-3) [9]. After modifying with hydrophilic polyethylene glycol, the sulfur cathode showed a high reversible capacity of 1320 mA h g−1. After that, many other conductive carbon materials have been reported to encapsulate sulfur. However, the nonpolar carbon materials possess only weak physical adsorption with polar LiPSs, which results in limited cycling performance [13]. Recently, it has been demonstrated that polar metal oxides have stronger chemical absorbability with LiPSs than carbon materials [14], [15], [16], [17], [18]. For example, Cui et al. designed a S/TiO2 yolk-shell nanostructured composite as a sulfur cathode [16]. Impressively, the cycle life of the electrode was significantly prolonged to 1000 cycles with a capacity decay of only 0.033% per cycle. This work proposed a new concept to enhance the electrochemical performance of sulfur cathodes. Since then, many studies on TiO2 as a sulfur host for Li-S batteries have been reported [19], [20], [21]. However, the high mass ratios of bulk TiO2 added to the sulfur host result in low sulfur utilization and poor rate performances due to its poor conductivity. More recently, some researchers have combined the above two strategies of structural restriction and chemical absorption to encapsulate sulfur, resulting in impressively improved cycling performances [22], [23], [24]. Nazar's group fabricated a S/MnO2 core-shell nanostructured composite via a very simple reaction process as a sulfur cathode for Li-S batteries, where MnO2 provided both physical obstruction and chemical absorption to LiPSs [24]. Therefore, designing sulfur cathodes with novel structures is a promising strategy to accelerate the practical application of Li-S batteries.

In addition to the optimization of the electrode materials, designing free-standing electrodes has been proven to be another way to enhance the energy density of energy-storage devices due to not having the need for a current collector, insulating polymer binder, and carbon additive. In addition, flexible energy-storage devices are considered to possess a good perspective due to the rapid development of flexible and wearable electronics [25], [26], [27]. Vacuum filtration and electrospinning are the most common techniques to fabricate flexible and free-standing electrodes in the field of sulfur cathodes [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. Compared with vacuum filtration, electrospinning is more extensively investigated for preparing free-standing porous carbon nanofiber films due to its simplicity, low cost and scale-up potential [29], [30], [31], [33], [35]. However, some issues still exist in using sulfur/carbon nanofiber composite as flexible electrodes: (a) the nonpolar carbon among the porous carbon nanofiber film shows weak physical adsorption with the polar LiPSs [13], (b) the rigid sulfur filling the micropores of the carbon nanofibers limits the flexibility, and (c) the introduction of most polar materials into the carbon nanofibers will also damage the flexibility. Therefore, it is still a great challenge to fabricate a stable flexible cathode with a high S loading and an outstanding electrochemical performance for Li-S batteries.

Herein, we designed novel flexible nitrogen-doped porous carbon nanofibers (NPCFs) mixed with ultrafine polar TiO2 nanoparticles as a sulfur host. This artful structure not only well blends the excellent conductivity of a carbon matrix with the outstanding absorption ability of polar metal oxides but also well maintains the flexibility of the carbon nanofiber film. These unique characteristics make allow for great potential to enhance the sulfur cathode performance. Thus, after sulfur loading, the flexible S/TiO2/G/NPCFs film exhibited an excellent initial discharge capacity of 1501 mA h g−1 at 0.1 C, prominent rate capability of 668 mA h g−1 at 5 C, and good cycling performance as a cathode for flexible Li-S batteries.

Section snippets

Preparation of flexible TEOS/TTIP/GO/PAN nanofibers

The graphene oxide (GO) used in this work was synthesized by Hummer's method, and the detailed processes can be found in our previous work [38]. Polyacrylonitrile (PAN, Mw = 150,000, Sigma-Aldrich) and N,N-dimethylformamide (DMF, Aladdin Co. Ltd., China) were used as the carbon precursor and solvent, respectively. Tetraethoxysilane (TEOS, Aladdin Co. Ltd., China) was used as the pore-forming agent. Titanium isopropoxide (TTIP, Aladdin Co. Ltd., China) was used as the metal source. First, the

Results and discussion

The fabrication procedure of the S/TiO2/G/NPCFs composite is shown in Scheme 1. The precursor film was prepared by electrospinning, in which TEOS and TTIP were used as a pore-forming agent and metal source, respectively. After carbonization and template etching of the as-collected precursor film, a flexible porous carbon nanofiber film mixed with polar TiO2 nanoparticles was successfully fabricated. Then, it was directly used as a free-standing sulfur host for Li-S batteries. As shown in Fig. 1

Conclusions

In summary, a novel, flexible and free-standing porous S/TiO2/G/NPCFs film modified by graphene and ultrafine polar TiO2 nanoparticles was successfully designed as a cathode for Li-S batteries. This delicate structure not only well combined the excellent conductivity of a carbon matrix with the outstanding absorption ability of polar metal oxides but also well maintained the flexibility of the carbon nanofiber film. As a result, the S/TiO2/G/NPCFs cathode exhibited significantly improved

Acknowledgments

The authors greatly acknowledge the financial support by The National Key Research and Development Program of China (2016YFA0202601), National Science Fund for Distinguished Young Scholars of China (No. 21225625), Natural Science Foundation of China (No. 21576100), Nature Science Foundation of Guangdong (2014A030312007).

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